Michel Broquier

Satchin Soorkia

Gustavo Pino

Claude Dedonder-Lardeux

Christophe Jouvet

Gilles Grégoire

Excited State Dynamics of Cold Protonated Cytosine Tautomers: Characterization of Charge Transfer, Inter System Crossing and Internal Conversion Processes

Charge transfer reactions are ubiquitous in chemical reactivity and often viewed as ultrafast processes. For DNA, femtochemistry has undeniably revealed the primary stage of the deactivation dynamics of the locally excited state following electronic excitation. We here demonstrate that the full time scale excited state dynamics can be followed up to milliseconds through an original pump-probe photodissociation scheme applied to cryogenic ion spectroscopy. Protonated cytosine is chosen as a benchmark system in which the locally excited 1 * state decays in the femtosecond range towards long-lived charge transfer and triplet states with lifetimes ranging from microseconds to milliseconds, respectively. A three-step mechanism ( 1 * → 1 CT → 3 *) is proposed where internal conversion from each state can occur leading ultimately to fragmentation in the ground electronic state.

INTRODUCTION

Photochemical damage to DNA by UV absorption is one of the main causes of possible mutagenic effect affecting the genetic material. [START_REF] Schreier | Early Events of DNA Photodamage[END_REF] It has been suggested that excited state dynamics of these heterocycles has played a decisive role in the natural selection of the building blocks of life. [START_REF] Chen | From Underwear to Non-Equilibrium Thermodynamics: Physical Chemistry Informs the Origin of Life[END_REF] The excited state dynamics of DNA have indeed received much attention in the past thanks to the continuous advances in ultrafast spectroscopic techniques [START_REF] Zewail | Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond †[END_REF] and computational methodologies. [START_REF] Crespo-Hernández | Ultrafast Excited-State Dynamics in Nucleic Acids[END_REF][START_REF] Improta | Quantum Mechanical Studies on the Photophysics and the Photochemistry of Nucleic Acids and Nucleobases[END_REF] We shall refer the reader to the recent review articles (ref. 4 and 5) to get an exhaustive list of condensed-and gas-phase studies on DNA photochemistry. The excited state photo physics of single nucleosides and nucleotides have been investigated in aqueous solution, pointing out to an ultrafast, sub-picosecond deactivation dynamics. [START_REF] Peon | DNA/RNA Nucleotides and Nucleosides: Direct Measurement of Excited-State Lifetimes by Femtosecond Fluorescence up-Conversion[END_REF][START_REF] Pecourt | DNA Excited-State Dynamics: Ultrafast Internal Conversion and Vibrational Cooling in a Series of Nucleosides[END_REF] Such ultrafast process is totally consistent with the very low fluorescence quantum yield of DNA nucleosides, about 10 -4 , measured through steadystate spectroscopy. [START_REF] Daniels | Fluorescence of the Purine and Pyrimidine Bases of the Nucleic Acids in Neutral Aqueous Solution at 300 K[END_REF] For cytosine (Cyt), the initially accepted mechanism is ascribed to direct relaxation to the ground electronic state through [START_REF] Schreier | Early Events of DNA Photodamage[END_REF] */S0 conical intersection (CI) with low triplet quantum yield (< 0.01). [START_REF] Cadet | Bioorganic Photochemistry[END_REF] Howev-er, significant solvent effects have been questioned in the radiationless decay by nucleobases, including relative stabilization of the bright 1 * state compared to "dark" charge transfer (CT) states and fast intermolecular energy transfer to the solvent. [START_REF] Crespo-Hernández | Ultrafast Excited-State Dynamics in Nucleic Acids[END_REF][START_REF] Improta | Quantum Mechanical Studies on the Photophysics and the Photochemistry of Nucleic Acids and Nucleobases[END_REF][START_REF] Pecourt | DNA Excited-State Dynamics: Ultrafast Internal Conversion and Vibrational Cooling in a Series of Nucleosides[END_REF][START_REF] He | Decay Pathways of Thymine and Methyl-Substituted Uracil and Thymine in the Gas Phase[END_REF] Besides, in single-and double-stranded oligonucleotides, lifetimes ranging from tens of picosecond to nanoseconds have been reported. [START_REF] Georghiou | Large-Amplitude Picosecond Anisotropy Decay of the Intrinsic Fluorescence of Double-Stranded DNA[END_REF] These long-lived excited states have generally charge transfer character with very low fluorescence yield, making them difficult to detect in condensed phase through ultrafast fluorescence up-conversion technique. [START_REF] Crespo-Hernández | Base Stacking Controls Excited-State Dynamics in A•T DNA[END_REF] From these seminal works, many gas phase studies have then been devoted to address the possible mechanism responsible for the ultrafast nonradiative process in DNA. Femtosecond time-resolved photoelectron [START_REF] Ullrich | Electronic Relaxation Dynamics in DNA and RNA Bases Studied by Time-Resolved Photoelectron Spectroscopy[END_REF] and massselected ionization spectroscopy [START_REF] Kang | Intrinsic Lifetimes of the Excited State of DNA and RNA Bases[END_REF][START_REF] Canuel | Excited States Dynamics of DNA and RNA Bases: Characterization of a Stepwise Deactivation Pathway in the Gas Phase[END_REF][START_REF] Kosma | Excited-State Dynamics of Cytosine Tautomers[END_REF][START_REF] Ho | Disentangling Intrinsic Ultrafast Excited-State Dynamics of Cytosine Tautomers[END_REF] have been conducted on bare cytosine using different pump (from 300 nm to 260 nm) and probe wavelengths (800 nm, 400 nm or 200 nm). Because of the lack of tautomer selection and the large excess energy brought by the fixed pump wavelength, the femtochemistry of Cyt was somehow difficult to rationalize. In overall, the femtosecond experiments have revealed multiexponential decay components with different weights and lifetimes ranging from 50 fs to few ps plus a possible long time component that cannot be firmly assigned within the experimental time window. Besides, charge transfer and triplet states require smaller probe wavelength than the one used for ionization from the [START_REF] Schreier | Early Events of DNA Photodamage[END_REF] *, so the recorded ionic transients might be blind to the dynamics towards these low-lying states.

An active debate is still ongoing to decipher which reaction path should be the most important to access the ground state, either through direct 2-fold (*/S0) or 3fold (*/n*/S0) degeneracy. [START_REF] Barbatti | Photodynamical Simulations of Cytosine: Characterization of the Ultrafast Bi-Exponential UV Deactivation[END_REF][START_REF] Nakayama | Photophysics of Cytosine Tautomers: New Insights into the Nonradiative Decay Mechanisms from MS-CASPT2 Potential Energy Calculations and Excited-State Molecular Dynamics Simulations[END_REF] The picture is getting even more obscure when looking at the results of nonadiabatic dynamics [START_REF] Hudock | Excited-State Dynamics of Cytosine Reveal Multiple Intrinsic Subpicosecond Pathways[END_REF][START_REF] Lan | Photoinduced Nonadiabatic Dynamics of Pyrimidine Nucleobases: On-the-Fly Surface-Hopping Study with Semiempirical Methods[END_REF][START_REF] Barbatti | Photoinduced Phenomena in Nucleid Acids[END_REF] with explicit account of spin-orbit coupling (SOC). [START_REF] Mai | Singlet and Triplet Excited-State Dynamics Study of the Keto and Enol Tautomers of Cytosine[END_REF] In almost all cases, the time constants extracted from the dynamical studies emphasize an ultrafast (subpicosecond) relaxation both to the ground state (independently of the 2 or 3 steps mechanism) and to triplet states. These dynamics are always started following vertical excitation from the ground state, i.e. in the high energy region (about 0.5 eV of excess energy) of the PES. So the calculated time constants and deactivation paths are relevant for comparison with the fs experiments (in which the excess energy in large), but do not apply to the deactivation from the vibrationless 1 * minimum structure. Very recently, S. Blaser et al. [START_REF] Blaser | Gas-Phase Cytosine and Cytosine-N 1 -Derivatives Have 0.1-1 Ns Lifetimes Near the S 1 State Minimum[END_REF] indeed measured the dependence of the S1 ( 1 *) state vibronic level lifetimes of keto Cyt through a picosecond pump-probe ionization scheme, revealing a lifetime of 730 ps at the band origin (BO), i.e. a thousand time longer than at higher energy. They also estimated the IC and ISC rates for keto Cyt for the low-lying vibronic levels of the 1 * state. [START_REF] Lobsiger | Intersystem Crossing Rates of S 1 State Keto-Amino Cytosine at Low Excess Energy[END_REF] IC rate constants are 20-50 times faster than the corresponding ISC rates that are predicted at about 10 9 s -1 , but still way smaller than the ones calculated at larger excess energy. ISC was previously suggested for Cyt by E. Nir et al. [START_REF] Nir | Spectroscopy of Cytosine[END_REF] All these experimental and theoretical results emphasize the need to study photo chemical reaction in well controlled conditions. Cryogenic ion spectroscopy indeed fulfills such requirement. [START_REF] Rizzo | Spectroscopic Studies of Cold, Gas-Phase Biomolecular Ions[END_REF][START_REF] Wolk | Cryogenic Ion Chemistry and Spectroscopy[END_REF] These spectroscopic studies rely on action spectroscopy, in which the absorption of UV photon promotes the system in the excited state that ultimately leads to fragmentation. [START_REF] Gregoire | Non-Statistical Deactivation Pathways in the UV Photo-Fragmentation of Protonated Tryptophan-Leucine Dipeptide[END_REF][START_REF] Féraud | Photofragmentation Spectroscopy of Cold Protonated Aromatic Amines in the Gas Phase[END_REF][START_REF] Féraud | Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation[END_REF] Cryogenic ion spectroscopy allows deciphering the lowest energy conformers through their energy-resolved vibronic spectra. Besides, cold ion traps offer the unique opportunity of extending the experimental time window for timeresolved dynamics over seconds as compared to what can be done for neutral molecules streamed in supersonic expansion. [START_REF] Gregoire | Non-Statistical Deactivation Pathways in the UV Photo-Fragmentation of Protonated Tryptophan-Leucine Dipeptide[END_REF][START_REF] Féraud | Photofragmentation Spectroscopy of Cold Protonated Aromatic Amines in the Gas Phase[END_REF][START_REF] Féraud | Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation[END_REF] Although the deactivation mechanisms in protonated nucleobases cannot be directly compared to those expected in neutrals, in both cases, the locally excited 1 * state lies in close proximity to low-lying CT and triplet states. Since protonation occurs at the nitrogen site, the photodynamical process should be simplified as compared to neutral Cyt due to the absence of nN* state in keto CytH + (Chart 1). It is worth mentioning that the energy of the adiabatic transition to the bright 1 * state of Cyt is not really affected by protonation, as reported by Berdakin et al. [START_REF] Berdakin | Excited States of Protonated DNA/RNA Bases[END_REF] In the present paper, we report the full timescale excited state dynamics of protonated keto and enol cytosine (CytH + ) through pump-probe photofragmentation spectroscopy. The time-resolved dynamics has been followed over ten decades, providing the complete deactivation process following electronic excitation. The lifetimes of the locally excited 1 * state, charge transfer 1 n* states and triplet 3 * states have been determined for both tautomers in the 100 fs, µs and ms time ranges, respectively. Geometry optimization and frequency calculations of the singlet and triplet states have been performed at the coupled-cluster CC2 level to assign the adiabatic excited state structures and to propose a general mechanism of deactivation to the ground state leading ultimately to fragmentation.

EXPERIMENTAL AND THEORETICAL METHODS

The experiments have been conducted at the Centre Laser of the Université Paris Sud (CLUPS) in Orsay. The experimental setup has been previously presented, [START_REF] Broquier | A Comprehensive Study of Cold Protonated Tyramine: UV Photodissociation Experiments and Ab Initio Calculations[END_REF] which comprises an electrospray ion source (ESI), a 3Dquadrupole ion trap QIT (Jordan TOF, Inc.) mounted on a cold head of a compressed helium cryostat that maintains the temperature around 10-15 K and a linear time-of-flight mass spectrometer. The stored ions are thermalized through collisions with helium buffer gas introduced in the QIT by a pulsed valve (General valve, series 9) triggered a few ms before. The photodissociation UV laser, operated at 10 Hz, is shined into the trap after 40 ms to ensure complete cooling of the ions and pumping of the buffer gas. All the ionic fragments and parent molecules are extracted and accelerated for mass-analysis in a linear time-of-flight mass spectrometer and detected with micro channel plates (Z-Gap, Jordan TOF, Inc.) The resonant absorption of the UV laser is reflected by the observation of ionic fragments at lower masses. Action spectra are recorded by scanning the laser frequency and monitoring each ionic fragment signals normalized by the intensity of laser and the parent ion signal. Three types of electronic spectroscopy have been performed: (i) the vibrationally-resolved spectroscopy of the locally excited state through 1-color excitation scheme (UV only) (ii) the fragmentation kinetics of these fragment ions (iii) the time-resolved dynamics continuously monitored from picosecond to millisecond time scales through a 2-color pump-probe excitation scheme. The combination of these experiments done on the same machine allows us to follow the complete dynamics in the excited and ground states on more than ten decades:

In the ground state, the fragmentation kinetics following internal conversion is followed from one hundred of nanoseconds to tens of milliseconds by delaying the extraction of the fragment ions from the photodissociation UV laser. The time resolution, in the order of one hundred of ns, is limited by the rise time of the high voltage pulser.

In the excited state: (1) the sub-picosecond lifetime is deduced from the linewidth broadening of the vibronic spectrum (1-color). This is a perfectly effective measurement since (i) the inhomogeneous broadening and the laser bandwidth are much smaller than the linewidth (ii) the bare ions are cold without internal energy, avoiding spectral congestion due to low vibrational modes. (2) the picosecond timescale is investigated through a picosecond pump-probe photodissociation scheme using an optical delay line (3) the longer time scale from ten nanoseconds continuously up to tens of milliseconds is achieved in a pump-probe photodissociation scheme with two lasers electronically synchronized. More detail on the laser systems can be found in Supporting Information.

Ab initio calculations have been done with the Turbomole program package 37 (v6.6) making use of the resolution-ofidentity approximation. [START_REF] Ahlrichs | Efficient Evaluation of Three-Center Two-Electron Integrals over Gaussian Functions[END_REF] Equilibrium geometries and vibrational frequencies in the ground electronic state S0, singlet and triplet excited states have been calculated at the coupled-cluster CC2 level [START_REF] Köhn | Analytic Gradients for Excited States in the Coupled-Cluster Model CC2 Employing the Resolution-of-the-Identity Approximation[END_REF] with correlationconsistent polarized double zeta aug-cc-pVDZ basis set augmented with diffuse functions. [START_REF] Kendall | Electron Affinities of the First-Row Atoms Revisited. Systematic Basis Sets and Wave Functions[END_REF] True minima of the potential energy surface are systematically checked by looking at possible imaginary frequencies. In that case, in order to escape from the transition state minimum, the structure is distorted along the normal mode having a negative frequency (screwer module in Turbomole) and further optimized, leading to the stable structure without imaginary frequency.

RESULTS

Vibronic spectrum and fragmentation kinetics through 1-color excitation scheme

The UV photodissociation mass spectrum of protonated cytosine CytH + (m/z 112) is reported in figure 1a along with its electronic spectrum from 306 nm to 250 nm (Figure 1b). The electronic spectrum, averaged on all fragments (m/z 95, 94 and 69), is similar to the one obtained with nanosecond laser by Berdakin et al. [START_REF] Soorkia | Conformer-and Mode-Specific Excited State Lifetimes of Cold Protonated Tyrosine Ions[END_REF] This electronic spectrum consists of two sets of vibronic progression reflecting the presence of two tautomers which have been previously assigned to the excitation of the locally excited * state that bears the oscillator strength for UV excitation of the keto and enol tautomers of CytH + . The first intense transition at 32940 cm -1 is the band origin (BO) of the * state of the keto form while the enol tautomer has its BO at 37300 cm -1 , i.e. 0.54 eV to the blue. Broadening of the vibronic transition is clearly observed with full width at half maximum (FWHM) of 40 cm -1 for the keto form and 55 cm -1 for the enol tautomer, reflecting a short excited state lifetime in the range of 130 and 100 fs, respectively. [START_REF] Berdakin | Excited States of Protonated DNA/RNA Bases[END_REF][START_REF] Pino | Non-Radiative Processes in Protonated Diazines, Pyrimidine Bases and an Aromatic Azine[END_REF] Three main fragmentation channels are detected at m/z 95 (NH3 loss), m/z 94 (H2O loss) and m/z 69 (HNCO loss) for both CytH + tautomers. Their kinetics of fragmentation is the same for all ions. The evolution of the fragmentation time as a function of the excitation energy (UV) is reported in Supporting Information (Figure SI1). The kinetics of fragmentation is 27 ± 2 µs at the BO of the keto tautomer and continuously decreases as the excitation energy increases down to less than 200 ns at the BO of the enol tautomer. Above this excess energy, the fragmentation kinetics is too fast to be accurately measured with our time resolution. These primary fragments are also observed through low-energy collision-induceddissociation. [START_REF] Qian | Ammonia Elimination from Protonated Nucleobases and Related Synthetic Substrates[END_REF] Theoretical calculations indeed predict that ammonia loss and HNCO loss are produced after ring cleavage reaction of the N3-C2 bond [START_REF] Yao | Proton and Hydrogen Atom Adducts to Cytosine. An Experimental and Computational Study[END_REF] (see chart 1 for atom labeling) through a transition state (TS8 in ref [START_REF] Yao | Proton and Hydrogen Atom Adducts to Cytosine. An Experimental and Computational Study[END_REF] ) which is the rate determining step of the fragmentation process. We have calculated at the B3LYP/6-311++G(d,p) level the energetics following this path and have estimated its kinetics through RRKM modeling. The result reported in Figure SI1 closely matches the measured fragmentation times of CytH + . Such simple modeling should be taken with care for a quantitative prediction, it nevertheless provides a qualitative picture of the kinetics of fragmentation. The main outcome is that photofragmentation must occur in the ground electronic state after internal conversion.

Photodissociation excited state dynamics through 2-color excitation scheme

In such experiment, [START_REF] Soorkia | Conformer-and Mode-Specific Excited State Lifetimes of Cold Protonated Tyrosine Ions[END_REF][START_REF] Broquier | A Comprehensive Study of Cold Protonated Tyramine: UV Photodissociation Experiments and Ab Initio Calculations[END_REF] the pump laser is tuned to a vibronic transition of the electronic spectrum, creating a population in a specific vibrational level of the locally excited state (*) that evolves in time leading ultimately to dissociation of the parent molecule. It is worth mentioning that a special attention has to be paid to the choice of the probe photon wavelength. The 355 nm probe laser used in this experiment fulfills the two follow-ing conditions: (i) absorption can only occur from an electronic excited state of the parent ion to promote the initial population into a higher electronic excited state before internal conversion to the ground state occurs. It implies that "hot" ground electronic state does not absorb at 355 nm. This has been checked by using different probe wavelengths of the ps OPO laser from 450 to 600 nm without notable changes in the recorded transients (ii) The probe laser cannot be absorbed by the primary fragment issued from the UV excitation. For CytH + , all the primary fragment (m/z 95, 94 and 69) are closed-shell species with electronic transition in the UV range below 260 nm.

The time evolution of the initially excited state population (pump) can be monitored through absorption of the probe photon delayed in time as long as it induces a change in the fragmentation branching ratio. The variation of the fragmentation branching ratio upon absorption of the probe laser is related to the change in the fragmentation efficiency or/and opening of new fragmentation channels of the parent ions. The broadening of the vibronic bands of CytH + reveals a sub-picosecond lifetime of the locally excited * state, which precludes to measure its dynamics with our picosecond laser (16 ps time resolution). Interestingly, we do observe a 2-color photofragmentation signal, with a decrease of the fragmentation yield on the primary fragment ions at m/z 95, 94 and 69 along with the concomitant increase of the fragmentation signal detected on secondary fragment channels at m/z 68, 40 and a net increase of the fragmentation yield at m/z 42 that was barely seen through 1-color excitation (pump only). The absorption of a probe photon leads to an increase of the internal energy of the parent ion that can then undergoes secondary fragmentation.

The fragmentation yield of CytH + through 2-color is the same as through 1-color. This implies that the excited population (pump) that would initially decay to form the primary fragments is promoted to higher excited states (probe) leading to new fragments. The 2-color signal has been recorded over ten decades of pump-probe delays. Besides, the amplitude of the depleted signal on the primary fragments (UV only) is at least 50% of the total fragmentation yield. This definitively indicates that more than half of the initially created excited state population responsible for the 1-color fragmentation signal evolves in the excited state potential energy surface (PES) for milliseconds. The analysis of the transients will reveal the lifetime of the electronic excited states responsible for the deactivation process following electronic excitation in CytH + . Independently of the time scale, the transients S(t) recorded on each fragmentation channel are fitted by the sum of exponential decay and growth functions, starting at zero delay time t0, with the same time constant  but with different weights. As stated before, these transients do not contain contribution from hot electronic ground state populated after IC. This model fitting function indeed depicts a simple kinetics scheme in which the population of state 1 (P1) decays with a time constant to a second excited state 2 (population P2). Each population Pi absorbs a 355 nm probe photon with an absorption cross section i that leads to specific ionic fragments (with Ai = i × Pi). This leads to eqn (1):

) exp 1 ( exp ) ( ) ( 2 ) ( 1 0 0 0   t t t t A A A t S           (1)
in which A0 is the background signal before time t0 (UV photofragmentation only). Finally, this function is convoluted by a Gaussian function of width l0 to account for the finite temporal profile of the laser beams (cross correlation of 16 ps for the picosecond lasers and 10 ns when using the nanosecond probe laser). For a given excitation energy associated with a time constant , the only adjustable parameters are the amplitudes Ai.

The time evolution of the fragmentation yield on each fragmentation channel has been recorded for a set of pump wavelengths resonantly tuned to specific vibronic transitions for both keto (Figures 23) and enol (Figures 4 andSI2) tautomers. In all cases, the transients reported for the primary fragments are average of the signals recorded at m/z 95, 94 and 69, while the transients reported for the secondary fragments are average of the signals recorded at m/z 68, 42 and 40. In figure 2, the transient signals at several excitation wavelengths of the keto tautomer are monitored during the first 1.5 ns, the maximum time delay that can be reached with the optical delay line used with the two picosecond beams. It can be readily checked that the associated decays decrease with the excess energy, are in the nanosecond range but longer than the maximum time delay, therefore the time constants cannot be deduced confidently.

In figure 3, the same transients are reported for a much longer time delay. Up to 100 µs (Figure 3a), the observed decay can be confidently fitted by eqn (1) (red line) with a time constant of 20 ± 2 µs at the BO of the CytH + keto tautomer plus a long-lived component with lower intensity (green line) that stays constant on the 100 µs time scale. The evolution of the excited state lifetime (logarithm scale) with the excess energy above the BO of the keto conformer is reported in Figure 3b and exhibits a linear decrease down to 4 µs at 2500 cm -1 of excess energy. Because of the constant signal recorded at the µs time scale, the time-resolved photofragmentation yield has been monitored on the millisecond time range (Figure 3c) and clearly exhibits a monoexponential decay signal with a time constant in the range of 14 ms. Here, the amplitude of the 2-color signal is low and can only be detected on the background-free secondary fragments. For the enol tautomer, the same procedure has been applied. The transients recorded on the 1.5 ns time scale do not exhibit any time evolution (step-like function, see Figure SI2) whatever the excitation wavelengths. On the µs time scale (Figure 4a), a time constant of 1.5 µs has been deduced from eqn (1) at the BO plus a long-lived component (green line). The evolution of the excited state lifetime with the excess energy from the BO is reported in Figure 4b and shows a linear decrease (logarithm scale) as the excess energy increases down to 800 ns at 2000 cm -1 above the BO. The transients recorded on the millisecond time scale (Figure 4c) show a monoexponential decay with a time constant of 10 ms at the BO and 5 ms 2000 cm -1 above. For the enol tautomer, the amplitude of the 2-color signal for the long delays is larger, which allows following the excited state lifetime on both the primary and secondary fragmentation channels.

The overall excited state dynamics of the keto and enol CytH + tautomers share similarities. The locally excited * state decays in the sub-picosecond time scale, according to the spectral band broadening, towards a second excited state. For keto CytH + , the lifetime of this excited state is in the range of 20 µs to 4 µs depending on the excess energy imparted in the * state. For enol CytH + , the lifetime is reduced to 1.5 µs at the BO down to 800 ns 2000 cm -1 above. As it will be explained below, the states are assigned to charge transfer (CT) states of different electronic characters for the two tautomers. For both tautomers, a long-lived component with time constant in the 10 ms range is clearly seen, and will be assigned to the lifetime of the triplet state following intersystem crossing (ISC).

DISCUSSION 4.1. Vertical excited state calculations

Ab initio coupled-cluster (CC2/aug-cc-pVDZ) calculations have been performed on the electronic excited states of the keto and enol tautomers of CytH + . Although many theoretical papers have been devoted to neutral cytosine, 5 a very few have been undertaken for the protonated analogue. [START_REF] Berdakin | Excited States of Protonated DNA/RNA Bases[END_REF][START_REF] Pino | Non-Radiative Processes in Protonated Diazines, Pyrimidine Bases and an Aromatic Azine[END_REF] In the ground state, at this theoretical level, the most stable tautomer is the enol form while the keto tautomer is calculated 0.2 kcal/mol higher (E+ZPE). As already reported, both tautomers are detected through cryogenic ion spectroscopy with clearly distinguished band origins at 4.08 eV and 4.62 eV for the keto and enol tautomers, respectively. The assignment of the electronic transitions has been partially performed elsewhere. [START_REF] Pino | Non-Radiative Processes in Protonated Diazines, Pyrimidine Bases and an Aromatic Azine[END_REF] In the present study, geometry optimizations and frequency calculations of the different electronic excited states, including the * state, charge transfer states and triplet states, have been performed. The overall very good accuracy of the CC2 method to predict the adiabatic transition energies has already been discussed in detail [START_REF] Féraud | Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation[END_REF][START_REF] Winter | Benchmarks for 0-0 Transitions of Aromatic Organic Molecules: DFT/B3LYP, ADC(2), CC2, SOS-CC2 and SCS-CC2 Compared to High-Resolution Gas-Phase Data[END_REF] with an averaged error of about ± 50 meV, which definitively allows a firm assignment of the electronic configuration of the different excited states and their optimized structures. However, such mono configurational method cannot, by definition, treat the electronic couplings and curve crossings. The present calculations are thus relevant to provide the energies of the different minima of the excited state potential energy surface (PES).

The vertical excited state energies are reported in Table SI1 along with the corresponding molecular orbital (MO) representation. These vertical transitions and the ordering of the excited states are somehow meaningless for comparison with the experimental band origins. It nevertheless gives a rough picture of the electronic configuration of the excited states at the ground state optimized structure that will be used for further optimizations. In the ground electronic state, both keto and enol tautomers belong to the Cs point group in a planar geometry. For both tautomers, the first excited state corresponds to * transition with high oscillator strength for optical excitation (A'). In the keto form, S2 is a nO * transition of A'' symmetry, 0.78 eV above the S1 state, with very weak oscillator strength as expected for a charge transfer state. For the enol tautomer, the second excited state is also a * state while the first charge transfer state (S3) is calculated 0.73 eV above the bright S1 state, and corresponds to a transition from the nN3 lone pair orbital to the antibonding * orbital of the aromatic ring.

Adiabatic excited state calculations at the Franck-Condon region (Cs symmetry)

Geometry optimizations and frequency calculations of the locally excited * state and energy calculations of the charge transfer states (nO* for keto CytH + and nN* for enol CytH + ) at the optimized geometry of the * state have first been performed while keeping the Cs symmetry in order to stay close to the Franck-Condon active region (Table 1). For both CytH + tautomers, these excited state structures are saddle points of the PES since one or more imaginary frequencies have been found. For the * state, it is totally consistent with the short excited state lifetimes deduced from the line broadening of the vibronic spectra. These modes are out-of-plane bending motions of the NH2 and CH groups with calculated frequencies in the range of -40 to -400 cm -1 (see Figure SI3). Therefore, the vibrationless adiabatic transitions cannot be accurately calculated, but can be estimated by taking an average of the difference in zero point energy between the ground and excited states (-0.15 eV) that we have deduced in previous studies on related systems. [START_REF] Féraud | Photofragmentation Spectroscopy of Cold Protonated Aromatic Amines in the Gas Phase[END_REF][START_REF]TURBOMOLE V6.6, a Development of University of Karlsruhe and[END_REF] For both tautomers, the adiabatic transitions of the bright * state are then predicted within 80 meV or less to the experimental band origins, which is in line with the expected error of the CC2 method. It should be stressed here that both keto and enol tautomers can be univocally assigned through their calculated adiabatic energy transitions. Besides, there are some striking differences between the two tautomers. 5). We have also calculated the energy of the triplet states at this structure (Table 1). The lowest triplet state is the 3 * state, and T2 is the 3 nO*, calculated 0.12 eV below the 1 * state. Due to the unpaired electrons of the oxygen lone pair and orbital of the ring, the energies of the singlet and triplet charge transfer states are almost degenerated. Besides, we have optimized the structure of the 1 nO* state while keeping the Cs symmetry. The state is further stabilized and lies 0.4 eV below the locally excited state.

For enol CytH + , the situation is different since at the optimized structure of the singlet 1 * (Cs symmetry), the corresponding 3 * triplet state is the only state which lies below, at 0.9 eV. Since the optimized 1 nN* charge transfer state (S3 state, lowest A'' state) lies 0.38 eV above the 1 *, it is not expected to play a role in the deactivation process in enol CytH + . However, it should be pointed out that for the 1 * state, there is a non-negligible electron density on the  orbital of the amino group, i.e. on the lone pair orbital of the nitrogen (Figure 6). This is exactly what is found in the case of protonated paminopyridine that undergoes a twisted intramolecular charge transfer (TICT) following electronic excitation of the 1 * state. [START_REF] Broquier | Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine[END_REF] As we will show below, the same process is proposed for enol CytH + .

Excited state lifetimes of the charge transfer states and triplet states

In order to escape from these transition state structures, geometry optimizations without symmetry constraint have been performed (Table 2). In each case, the molecule is first distorted along the imaginary out-of-plane mode before being optimized. For keto CytH + , this leads to the adiabatic minimum with CT nO* electronic character, 0.56 eV below the calculated * transition at the FC active region (Cs symmetry).

This CT state is the adiabatic S1 state without imaginary frequency. The molecule has undergone a large structural change with a twist of the amino group out of the plane of the aromatic ring (Figure 5). The amino nitrogen has now a sp 3 hybridization with its lone pair pointing toward the protonated nitrogen. In this structure, the S2 state is a CO* transition, the electron density of the occupied orbital being mostly centered on the  orbital of the carbonyl group, with much less electron density in the  orbital of the aromatic ring. S2 however closely resembles the initial * with a partial charge transfer configuration. Geometry optimization of the S2 CO* state does not lead to large structural change as compared to the optimized structure of the nO* state. S2 CO* is also a minimum of the PES and its adiabatic energy (corrected by the ZPE) is calculated 0.09 eV below the adiabatic energy of the locally excited * state at the FC geometry. Finally, the 3 * triplet state adiabatic energy is 0.2 eV below the singlet and the triplet nO* states that are degenerated. The optimized structure of the T1 3 * state leads to a slightly bent cytosine ring (along the N3-C6 axis) with the amino group twisted back to the plane of the aromatic ring (Figure 5) while the T2 3 nO* retains the structure of the singlet state. The barrierless process is totally consistent with the fs lifetime of the locally excited 1 * state. In the course of the CT reaction, a two-step deactivation of the * state might occur. Experimentally, the nanosecond transient recorded for keto CytH + (Figure 2) might be assigned to the excited state lifetime of the S2 CO* state that decays to the adiabatic S1 nO* state with lifetimes in the µs time range. The millisecond transient recorded for keto CytH + is assigned to the excited state lifetime of the 3 * triplet state which is energetically accessible from the adiabatic minimum of the singlet PES ( 1 no*).

For the enol tautomer, the adiabatic minimum structure of the singlet excited state corresponds to the TICT state with the amino group twisted by 90° and bent out of the plane of the aromatic ring by 14° (Figure 6). This structure is a true minimum of the PES. This state has a charge transfer character, characterized by a transfer of the electronic density from the lone pair orbital of the NH2 group to the * orbital of the aromatic ring. The adiabatic transition energy of the 1 nNH2* state is calculated 0.42 eV below the calculated * band origin (Table 2). After UV excitation in the FC active region (Cs symmetry), the locally excited * state evolves into the CT 1 nNH2* in the course of the twist motion. It is worth mentioning that no other low-lying singlet excited state is predicted by the CC2 calculation whereas low-lying triplet states are. The lowest triplet state is a 3 * state, 0.7 eV below the degenerated singlet and triplet CT states.

The barrier free twisting motion leading to the TICT is compatible with the fs dynamics deduced from the broadening of the UV spectrum. The 1.5 µs decay recorded for enol CytH + (Figure 4) is assigned to the lifetime of this TICT state. The millisecond transient recorded for enol CytH + is assigned to the excited state lifetime of the triplet [START_REF] Zewail | Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond †[END_REF] * state which is accessible from the adiabatic minimum of the PES (TICT state 1 nNH2*). It should be stressed that the lifetimes of the TICT state and triplet state in enol CytH + are similar to those measured in protonated p-aminopyridine that also evolves to the TICT 1 nNH2* and 3 * states. [START_REF] Broquier | Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine[END_REF]

Deactivation processes in the excited state potential energy surface

Both CytH + tautomers undergo ISC following initial electronic excitation of the 1 * transition state which has sub-picosecond lifetime. Direct ISC from the locally excited 1 * state to the lowest triplet 3 * state would thus require very high singlet-triplet interactions in order to be effective with kISC≈ 10 13 s -1 . Such high ISC rate constant for El-Sayed forbidden transition is unrealistic. [START_REF] Tatchen | Intersystem Crossing Driven by Vibronic Spin-orbit Coupling: A Case Study on Psoralen[END_REF] However, ISC is energetically and symmetry allowed according to El-Sayed rules [START_REF] El-Sayed | Spin-Orbit Coupling and the Radiationless Processes in Nitrogen Heterocyclics[END_REF] from CT states to the 3 * state.

For enol CytH + , at the FC region, the two lowest excited states are the 1 * and 3 * states, 0.9 eV below, so direct ISC is not thought to occur within the 100 fs lifetime of the locally excited state. The adiabatic singlet state, 0.4 eV below the 00 0 transition, corresponds to the 1 nNH2* TICT state which has a 1.5 µs lifetime at the 00 0 ( It should be stressed that an experimental evidence for ISC process in neutral keto Cyt has been already reported by de Vries et al. [START_REF] Nir | Spectroscopy of Cytosine[END_REF] and Leutwyler et al. [START_REF] Lobsiger | Intersystem Crossing Rates of S 1 State Keto-Amino Cytosine at Low Excess Energy[END_REF][START_REF] Röttger | Probing the Excited State Relaxation Dynamics of Pyrimidine Nucleosides in Chloroform Solution[END_REF] In the latter studies, the authors quantify the rate constant kISC≈ 10 9 s -1 for the direct 1 * (S1) → 3 * (T1) process and a larger one, in agreement with El-Sayed rules, for the symmetry allowed mechanism kISC≈ 10 10 s -1 for 1 * (S1) → 3 n* (T2). The unexpected high ISC rate for symmetry-forbidden transition (S1-T1) is caused by a substantial mixture of 1 nO* character into the adiabatic 1 * state with has a 730 ps lifetime at the band origin. [START_REF] Blaser | Gas-Phase Cytosine and Cytosine-N 1 -Derivatives Have 0.1-1 Ns Lifetimes Near the S 1 State Minimum[END_REF] In CytH + , the subpicosecond 1 * lifetime clearly precludes direct ISC process from the locally excited state to occur with a substantial quantum yield.

Deactivation pathways to the ground state

In CytH + , fragmentation following UV excitation is energetically open only in the ground electronic state after internal conversion. There is no direct dissociation in the excited state as found in protonated tryptophan [START_REF] Gregoire | UV Photoinduced Dynamics in Protonated Aromatic Amino Acid[END_REF][START_REF] Gregoire | Ab Initio Study of the Excited-State Deactivation Pathways of Protonated Tryptophan and Tyrosine[END_REF] and aromatic amine [START_REF] Féraud | Photofragmentation Spectroscopy of Cold Protonated Aromatic Amines in the Gas Phase[END_REF] for instance where a dissociative * excited state is coupled to the locally excited state. So the main questions are (i) from which states IC to the ground state occurs and (ii) can we quantify their rates? Three different excited states are in play in the photofragmentation process of CytH + (see Figure 7): the 1 * state which bears the strength for optical excitation and two dark CT and triplet 3 * states. Each of these three states have quite different lifetimes, from sub-picosecond to µs up to ms, respectively, which allows unravelling their specific contributions in the deactivation process. Experimentally, the amplitude of the 2-color signals indeed provides a qualitative information on the IC yield.

When looking to the dynamics on the CT states (µs time scale), the depletion of the primary fragmentation yield through the absorption of the probe photon accounts for about half the UV only fragmentation signal. The absolute cross section absorption of the probe photon cannot be easily estimated, but it is unlikely that 100 % of the initial excited state population being promoted to higher excited states in the 2-color scheme. Even within this hypothesis, we can confidently assume that at least, the quantum yield for deactivation of the 1 * to the CT states is larger than 0.5 for both tautomers.

For enol CytH + , at the band origin, the fragmentation time recorded on the primary fragments are faster by almost one order of magnitude than the excited state lifetime of the CT state. This clearly indicates that part of the initially excited * state population returns to the ground state within 100 fs. For keto CytH + , independently of the excess energy brought by the UV excitation, the recorded fragmentation times closely match the corresponding excited state dynamics, i.e. in the 20-4 µs timescale. RRKM calculations predict a similar timescale for the fragmentation in the ground state after internal conversion, so we cannot disentangle if the recorded kinetics of fragmentation is strictly the time-resolved dissociation or the convolution of the deactivation process from the CT state to the ground state leading to the dissociation.

Interestingly, a long time fragmentation kinetics has been recorded for both tautomers (Figure SI4), with a time constant in the range of 20 ms that is similar to the excited state lifetime of the triplet states. Because the fragmentation channels are the same independently of the time scale, the measured long fragmentation times simply reflects the deactivation dynamics of the triplet state through ISC to the ground state which thus dissociated within few µs. The amplitude of the fragmentation yield at long time scale is at least twice the one recorded at the µs time scale, which reveals that more than 50 % of the CT state population goes to the triplet state which then decays through ISC to the ground state leading to the observed long time fragmentation events (Figure SI4). This proposed mechanism indeed suggests that for enol CytH + , ground state repopulation from both the 1 *, CT and triplet 3 * states occurs. For keto CytH + , ISC from the triplet 3 * to the ground state accounts for 50% of the primary fragmentation signal. However, we cannot disentangle the IC process directly from the 1 * state or from the CT state.

CONCLUSIONS

Using a combination of cryogenic ion photofragmentation spectroscopy and time-resolved excited state dynamics, the full time scale dynamics of excited protonated cytosine have been deciphered and the deactivation processes have been assigned through comparison with ab initio calculations at the CC2/aug-cc-pVDZ level including singlet, triplet excited state geometry optimizations and frequency calculations. For the keto and enol tautomers, the locally excited * state is a saddle point of the PES, in total agreement with the sub-picosecond lifetime (about 150 fs) deduced from the line broadening of the vibronic spectrum. Although the deactivation mechanisms in protonated nucleobases cannot be directly compared to those expected in neutrals, in both cases, the locally excited 1 * state lies in close proximity to CT and triplet states. It is worth mentioning that the energy of the adiabatic transition to the bright 1 * state is not really affected by protonation, but its excited state lifetime is strongly reduced by three orders of magnitude from 700 ps to about 150 fs in keto Cyt. It is proposed that the * state decays without barrier towards low-lying charge transfer states. These CT states, 1 nO* for keto CytH + and 1 nNH2* for enol CytH + , are the adiabatic minima of the excited state PES with 20 µs and 1.5 µs lifetimes at the BO of the 1 * state of the keto and enol tautomers, respectively. A much longer lived component has been evidenced which has been assigned to the relaxation of the CT states through intersystem crossing to the lowest 3 * triplet state with lifetime in the order of 10 ms for both tautomers. For both CytH + tautomers, the fragmentation signals following UV excitation proceeds from repopulation of the ground electronic state. In enol CytH + , direct IC from the locally excited 1 * has been evidenced and accounts for about half of the fragmentation signal. For keto CytH + , IC to the ground state from either the * state or from the CT state cannot be disentangled from the experimental data, but ISC from the triplet 3 * states accounts for about 50 % of the fragmentation signal.

These results provide benchmark experimental data for the complete dynamics of protonated nucleobases followed over more than 10 decades which could thus be directly compared to multi configurational ab initio calculations in which vibronic couplings and singlet-triplet interactions can be calculated. Because protonated molecules have the same closed-shell electronic configuration than in neutral analogues, these high-level calculations could be performed as well at the same cost. We believe that these experimental results could foster theoretical works to be done on protonated molecules. The energyand time-resolved photodissociation experiment that we have developed provides the full time scale dynamics from the locally excited state towards the charge transfer states, the triplet states and the ground state that ultimately leads to fragmentation. This unique experiment opens new areas for the precise knowledge of the photo physical processes in well-controlled molecular systems.

The present results have focused on protonated cytosine and will be extended in the near future to the other nucleobases and nucleotides.

  who observed a long time component of 290 ns in the 2-color photoionization signal. Chart 1. Chemical structure and heavy atom numbering of the keto CytH + (protonation on N3) and enol CytH + (protonation on N1).

Figure 1 .

 1 Figure 1. (a) UV Photodissociation (1-color) mass spectrum of CytH + ; (b) vibronic spectrum of CytH + from 306 nm to 250 nm averaged on the sum of the primary fragments. The two arrows indicates the band origin (BO) of the keto and enol tautomers.

Figure 2 .

 2 Figure 2. Photodissociation excited state dynamics averaged on the primary (top) and secondary (bottom) fragmentation channels as a function of the excess energy above the BO for keto CytH + for the first 1.5 ns.

Figure 3 .

 3 Figure 3. Photodissociation excited state dynamics for keto CytH + (a) at the BO for the first 100 µs. Full fit (red), exponential decay (blue) and exponential growth (green) functions with same time constant . (b) Evolution of the time constant  recorded as a function of the excess energy above the BO. (c) same as (a) for the long time constant up to 20 ms.

Figure 4 .

 4 Figure 4. Photodissociation excited state dynamics for enol CytH + (a) at the BO for the first 7µs. Full fit (red), exponential decay (blue) and exponential growth (green) functions with same time constant . (b) Evolution of the time constant  recorded as a function of the excess energy above the BO. (c) same as (a) for the long time constant up to 20 ms.

Figure 5 .

 5 Figure 5. Excited state optimized structures of keto CytH + along with the corresponding frontier molecular orbitals involved in the different excited states.

Figure 6 .

 6 Figure 6. Excited state optimized structures of enol CytH + along with the corresponding frontier molecular orbitals involved in the different excited states.

Figure 7 .

 7 Figure 7. Schematic PES, excited state lifetimes and proposed deactivation mechanisms to the ground state for keto and enol CytH + .

Table 1 . Calculated energy transitions of singlet and triplet states of keto and enol CytH + (CC2/aug-cc- pVDZ) at the Cs 1 * optimized structure (Franck- Condon active region). In bold, calculated 1 * adia- batic energy compared to the experimental BO of the

 1 For keto CytH + , at the optimized structure of the bright * state (Cs symmetry), the nO* charge transfer state becomes the lowest energy minimum, 0.10 eV below (Figure

	Molecules	States	Energy	00 0 exp
		T1 : 3 * (A')	3.69
	Keto CytH +	T2 : 3 nO* (A'') S1 : 1 nO* (A'')	4.03 4.05
		S2 : 1 * (A')	4.15 (4.00) a	4.08
		T1 : 3 * (A')	3.89
		S1 : 1 * (A')	4.79 (4.64) a	4.62
	Enol CytH +	T2 : 3 * (A')	5.02
		S2 : 1 * (A')	5.87
		S3 : nN* (A'')	6.27

1 * state. All values in eV. a Adiabatic 00 0 energy corrected by ZPE = -0.15 eV (see text)

Table 2 . Calculated adiabatic energy transitions of singlet and triplet states of keto and enol CytH + (CC2/aug-cc-pVDZ) for several excited state opti- mized structures. Energies corrected by the differ- ence in zero point energy between the excited and ground states. All value in eV.

 2

	Molecules (spin state)	Optimized States	00 0 calc
	Keto	1 nO*	3.44
	(singlet)	 CO*	3.91
	Keto	 *	3.25
	(triplet)	3 nO*	3.43
	Enol (singlet)	1 nNH2*	4.22
	Enol	 *	3.52
	(triplet)	 *	3.56

  [START_REF] Schreier | Early Events of DNA Photodamage[END_REF] *) excitation energy. Populating the triplet[START_REF] Zewail | Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond †[END_REF] * state from such a longlived CT state does not require high ISC rate. So the pro-posed mechanism for the triplet formation in enol CytH + is a sequential process following 1 * → 1 nNH2* → 3 *.For keto CytH + , we propose a similar sequential mechanism, the only difference being the ordering of the low excited states in the FC region. The bright 1 * state is 0.4 eV above the adiabatic S1 1 nO* state that is degenerated with its triplet state. Although 1 * → 3 nO* intersystem crossing would be allowed according to El-Sayed rules between states of different symmetries, ISC rate should be larger than 10 13 s -1 to compete with the 1 * → 1 nO* process. Here again, since the 1 nO* state has a 20 µs lifetime at the 00 0 ( 1 *) excitation energy, we suggest that the dominant ISC mechanism of keto CytH + is the sequential 1 * → 1 CO* → 1 nO* → 3 * process.

ACKNOWLEDGMENT

This work has been conducted under the International Associated Laboratory LIA/LEMIR with financial supports of CNRS GDR EMIE. We acknowledge the use of the computing facility clusters MésoLUM of the LUMAT federation (FRLUMAT 2764) and MAGI of the University Paris 13.

ASSOCIATED CONTENT

Supporting Information. Additional results, table and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

* E-mail: gilles.gregoire@u-psud.fr / tel: (33) 1 69 15 31 03


Evangelos Christou
email: echristou@ihu.gr

Experiential learning through industrial placement in hospitality education: The meat in the sandwich

Keywords: experiential learning, hospitality education, industrial placement period, tourism education, hospitality internships, work placement JEL Classification: I23, L83, M12, O15, Z30

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Experiential learning is a critically important curriculum component of a degree program in hospitality management. Relevant sandwich degree courses (i.e., degrees that include a period of industrial placement or some type of supervised work experience) are specifically designed to prepare undergraduate students for particular employment fields in the hotel and catering industry. A period of industrial placement is fundamental to the sandwich structure; currently more than 93% of the hospitality and/or tourism management degree programs in North America and 95% of similar courses in Europe require some type of supervised work experience [START_REF] Christou | Experiential learning-internships, externships, co-ops, practicums: a state of the art[END_REF][START_REF] Chatzigeorgiou | Modelling the impact of social media influencers on behavioural intentions of millennials: The case of tourism in rural areas in Greece[END_REF]. Despite the fact that a large number of researchers around the world have investigated the value of supervised work experience or experiential learning through industrial placement (IP) in the past, it appears that this field of research presents a lot of controversy and requires additional study [START_REF] Prinianaki-Tzorakoleftheraki | Tourism and hospitality education in Greece: tertiary public sector, with emphasis on the "tourism industries" course of the Technological Educational Institutions[END_REF][START_REF] Eaton | Hospitality management competencies for graduate trainees: employers' views[END_REF][START_REF] Oh | A self-service technology adoption model in the resort hotel environment[END_REF]. In addition, although much of the relevant research conducted internationally has been centered on the value of the students' industrial placement, studies have not investigated in depth the undergraduates' perspective of the supervised work experience period and the effect on their careers. Undergraduate students at the BA in Tourism and Hospitality Management course at the International Hellenic University (in Greece) have to complete a six-month period of supervised work experience. The industrial placement is an indispensable component of the course and it has been claimed in the past to assist greatly the development of the students' knowledge and skills that were acquired during their studies [START_REF] Christou | Hospitality management education in Greece An exploratory study[END_REF][START_REF] Christou | Exploring the impact of visitor satisfaction on loyalty towards a specific destination[END_REF]. In addition, the IP is expected to influence positively both the continuation of the students' studies after they return to the university as well as to finding a permanent job after graduation [START_REF] Rowe | From classroom to boardroom[END_REF][START_REF] Christou | Exploiting Multimedia for Effective Hospitality Education[END_REF][START_REF] Christou | Professional Development in Hospitality and Tourism Education: A Strategy for the 21st. Century[END_REF]. The study described in this paper was aiming at investigating in depth the characteristics of these two issues. The specific objectives of the survey were: a) to investigate the contribution of supervised work experience to skill and knowledge development of undergraduate students of the BA in Tourism and Hospitality Management course; b) to explore student experience and reaction to IP; and c) to identify existing drawbacks and limitations of the IP period. The IP element of an undergraduate requires significant investment of university resources [START_REF] Fu | Reengineering knowledge for etourism and hospitality curricula[END_REF]. All stakeholders related to these resources (students, host organisations from the hospitality and tourism industry, and educators) need to look for ways in which to achieve a justifiable return on investment [START_REF] Chatzigeorgiou | Destination branding and visitor brand loyalty: Evidence from mature tourism destinations in Greece[END_REF]. Therefore, the assessment of IP outcomes is essential for educators, universities, students and employers. In addition, the outcomes of the exploratory research described here could serve as stimuli for the implementation of more in-depth investigation of IP in the future. Apart from these benefits, the research described here is justified by its great potential in terms of practical use. Its outcomes may provide practical solutions to problems faced by the parties involved in the organisation and execution of the IP period and may provide usable information for further improving undergraduate programs in hospitality management.

THEORETICAL FRAMEFORK

Nature of hospitality management education

What is hospitality management education? Is it an applied or a professional discipline? There is no clear answer to what hospitality management education is; many researchers like [START_REF] Lundberg | A note on targeting hospitality education[END_REF][START_REF] Ferreira | A review of case analysis and simulation for use in hospitality management education[END_REF] and [START_REF] Samenfink | A rebuttal: careerism and general education revisited in the hospitality curriculum[END_REF] have tried to define it by examining its educational nature. Based on the essence of the art of management [START_REF] Laesecke | Meeting the industry needs[END_REF] stated that the role of hospitality educational institutions must be to produce graduates who can think, lead, and solve problems. [START_REF] Casado | Higher education hospitality schools: meeting the needs of the industry[END_REF] and [START_REF] Defranco | Back to the basics: humanism in education[END_REF] suggested that hospitality students must receive similar amount of professional concepts, general business principles, and liberal studies. [START_REF] Pavesic | Hospitality education 2005: curricular and programmatic trends[END_REF] explained that "education must be thought as a journey, not a destination" and [START_REF] Pizam | Who is the customer in hospitality education?[END_REF], following a marketing-based approach to education, suggested that the ultimate customers (or recipients of benefits) in hospitality education are not the students of hotel and catering courses but the society in general and the hospitality and tourism industry. The organisational structure of most hospitality firms is becoming horizontal rather than vertical in its basic shape, due to the widespread adoption of a leaner organizational structure requiring personnel multiskilling [START_REF] Hodge | Organizational Theory[END_REF][START_REF] Christou | Exploring brand confusion through hotel adverts[END_REF]. Hence, the hospitality graduates of the future will have to carry out a multidepartment manager role who bring together multiple competencies and multi-skilling [START_REF] Williams | The challenge of multidepartment management for future hospitality graduates[END_REF][START_REF] Christou | A review of wine tourism research from 1995 to 2010: Analysis of 110 contributions[END_REF]. It cannot be argued that hospitality management education is about the management of hotel and catering operations and organisations. Therefore, hospitality management education is a function of the acquisition of knowledge and skills about management in a way similar to that of other business administration courses; and, the acquisition of knowledge and competencies related to hotel and catering operations in a similar way to craft courses. In addition, higher education in hospitality should assist the student to develop communications and interpersonal "soft" skills and learn how to lead others [START_REF] Goodman | The future of hospitality education: meeting industry's needs[END_REF][START_REF] Eaton | Management competencies for graduate trainees[END_REF]. The development of students' interpersonal skills is considered to be of paramount importance according to the views of the hospitality industry [START_REF] Tas | Teaching future managers[END_REF][START_REF] Baum | Competencies for hotel management: industry expectations of education[END_REF][START_REF] Eaton | Hospitality management competencies for graduate trainees: employers' views[END_REF]2006;[START_REF] Christou | Investigating attitudes towards mobile commerce for travel products[END_REF]Christou & Karamanidis, 1999a[START_REF] Christou | Enhancing competitiveness in the hospitality industry: Revisiting Porter's generic competitive strategies in Greece[END_REF]Christou & Sigala, 2002).

Supervised work experience and industrial placement for hospitality students

A common distinction between education and training is that establishments like universities mainly provide education while employers (organisations) usually provide vocational training. This distinction, which is the cornerstone of the traditional view towards education and training, suggests that education is for professionals and training is for the artisan and the unskilled [START_REF] Baum | Managing Human Resources in the European Hospitality Industry: A Strategic Approach[END_REF]. However, in today's environment, it is not realistic to impose an absolute distinction between education and training because they are not exactly two independent routes. Both the educational establishments [START_REF] Fitzgerald | Learning through a "real world" experience[END_REF][START_REF] Ford | Management education in the USA: how much practical hotel management education is necessary?[END_REF]Chatzigeorgiou et al., 2017;[START_REF] Chatzigeorgiou | Confidence and loyalty for agrotourism brands: The Lesvos paradigm[END_REF] and employers [START_REF] Partlow | Human resources practices of TQM hotels[END_REF]Harris, 1997;[START_REF] Lyons | Cross cultural change, adjustment and culture shock: UK to USA[END_REF] around the world have adopted this approach. The element of training is taken into account seriously by universities, hence the development of "sandwich" undergraduate courses that incorporate a period of industrial placement [START_REF] Hogg | A foretaste of work[END_REF][START_REF] Lebruto | The educational value of "captive hotels[END_REF][START_REF] Christou | On-line buyers' trust in a brand and the relationship with brand loyalty: the case of virtual travel agents[END_REF] aiming at the blending of theory with practical experience through experiential learning [START_REF] Dimicelli | Blending theory & practical experience: a hands-on approach to educating hospitality managers[END_REF][START_REF] Nair | E-learning adoption in hospitality education: An analysis with special focus on Singapore[END_REF]. It should be noted that, what is called in Europe as "industrial placement period" or more often as "supervised work experience" is usually called in North America as "internship". According to [START_REF] Petrillose | An exploratory study of internship practices in hospitality education and industry's perception of the importance of internships in hospitality curriculum[END_REF] the most popular of the experiential learning models in North America are the internships. However, research on internship programs has shown significant gaps in expectations between industry, students and hospitality educators [START_REF] Downey | Hospitality internships: an industry view[END_REF][START_REF] Christou | Examining the adoption of eshopping for travel services: determinants of consumers' perceptions[END_REF][START_REF] Sigala | Enhancing and Complementing the Instruction of Tourism and Hospitality Courses Through the Use of On-line Educational Tools[END_REF][START_REF] Sigala | Exploiting Web 2.0 in open and distance education: Developing personalised and collaborative learning environments[END_REF]. Student internships ranked third in the top twenty-seven components of quality in a hospitality undergraduate program [START_REF] Pavesic | determinants of quality in hospitality undergraduate programs[END_REF][START_REF] Valachis | Assessment of training quality in hospitality industry: an exploratory model[END_REF]2009). The goal of experiential learning through a period of supervised work experience is for students to gain valuable work experience with an appropriate employer in the hospitality industry. Students have the opportunity through internships to develop analytical and leadership skills along with the concept of a productive worker [START_REF] Cargill | Developing the ideal internship: what are your thoughts, part two[END_REF][START_REF] Barron | Education and talent management: Implications for the hospitality industry[END_REF].

In addition, students develop more realistic expectations of the professional world, faculty are up to date on trends and demands of the hospitality industry, and industry professionals are aware of the particular objectives of university education [START_REF] Dresh | Educators must pursue industry relationships[END_REF][START_REF] Benavides | Internships adrift? Anchoring internship programs in collaboration[END_REF]. In fact, IP appears to bridge the gap between classroom or textbook theory and the practical side of applying it in a real hospitality setting [START_REF] Busby | Tourism and hospitality internship experiences overseas: A British perspective[END_REF]. The supervised work experience period can assist students to learn how to cultivate communication, tolerance and interpersonal skills [START_REF] Wisch | Stressing the practical side of training hospitality students through internships[END_REF]Chatzigeorgiou & Simeli, 2017). The hospitality industry can assist in training the next generation of managers by working closely with educational institutions to appropriately blend theory and practice [START_REF] Hill | The co-op opportunity[END_REF][START_REF] Chen | Does employability increase with internship satisfaction? Enhanced employability and internship satisfaction in a hospitality program[END_REF]. Students' satisfaction with their IP in terms of satisfaction with their jobs as learning experiences was measured by [START_REF] Nelson | Hospitality internships: the effects of job dimensions on student satisfaction[END_REF]. At the same time, Emenheiser, Clayton and Tas (1997) empirically determined students' perceptions of the effectiveness of the IP phase. The work described here was aiming at the assessment of the IP period for students of the Department of Organisation Management, Marketing & Tourism of the International Hellenic University, in Greece; students are required to undertake an industrial placement for six months. They have freedom of choice in terms of location and hospitality sector. However, the Department provides assistance to most students to find a host company.

RESEARCH METHODOLOGY

The survey presented here was conducted in Greece between April and October 2018. Its objective was to evaluate the outcome of the supervised work experience, by examining the views of BA in Tourism and Hospitality Management students at the International Hellenic University who completed their IP during the last six months. The data were obtained through personal interviews and were of both qualitative and quantitative nature. The quantitative information was gathered by using a structured questionnaire. To gather qualitative or "rich" information, depth interviews and focus groups were conducted using an informal discussion method with no structured questions. The questionnaire was containing a variety of close-end multiple choice and dichotomous questions. While wording and laying it out, a lot of effort was spent in avoiding bias, using clear and concise words and avoiding ambiguous or leading questions [START_REF] Christou | Research methodology in hospitality industry: the role of the inquiry paradigms[END_REF]. Additional consideration was given to the sequence of the questions, ensuring a logical flow by using the "funnel" technique [START_REF] Chisnal | Marketing Research[END_REF]. A pilot test was carried out in the 10% of the sample. The survey was anonymous, but limited data for the respondent were collected. The chosen form of the depth interviews was that of "free response", giving the respondents a great deal of freedom in discussing their experiences and in answering questions arising from the general points of discussion. All interviews were conducted by adopting the "funnel" technique to the particular needs of qualitative research: the discussion started on the broadest possible level and gradually narrowed down through progressively more restricted channels. A list of points, which the interviews were going to cover, was prepared in advance. These points revolved around the following themes: the skills and knowledge gained from the IP; overall evaluation of the IP; the quality of the educational preparation for the reality of the workplace; other issues that were raised by the respondents. The researcher (without directly influencing the research) guided all interviews, so that all of the above themes were covered adequately. The population under investigation consisted of all the students in the BA in Tourism and Hospitality Management course at the International Hellenic University that had completed their IP during the last six months (134 students in total). The sample was selected by using the census method and it consisted of the 134 students who undertook their IP at different hotels all over Greece. Hence, there was achieved a response rate of 100 percent.

SURVEY FINDINGS AND DISCUSSION

The most significant of the results obtained are presented in tables 1, 2, 3,4 and 5. It is widely accepted that students who have not worked in the hospitality industry may have premade perceptions of a glamorous industry (Woods, 1990); most students start their education with a positive attitude towards the hospitality industry [START_REF] Purcell | Hospitality Management Education and Employment Trajectories[END_REF][START_REF] Sigala | Investigating the impact of ecustomer relationship management on hotels' website service quality[END_REF]. The survey reported here attempted to discover if students became disillusioned and if so, whether they recovered. According to the outcome of the personal interviews, 69 per cent of the students admitted to some period of disillusionment ranging from a couple of weeks to 20 weeks, with an average period of 4.8 weeks. The major reasons suggested for the disillusionment were being away from friends and relatives and experiencing a stressful and exhausting work environment. 48 per cent of the students did recover but 16 per cent did not and therefore completed their IP period in a state of considerable disillusionment. All of the interviewees expressed the view that the industrial placement was, to an extent, an effective way of putting their newly acquired knowledge and skills to a real-life test. The majority of the students (88%) agreed that by the end of the placement phase they had a better understanding of the hospitality and tourism industry and of the potential of their future careers in it. They also added that although prior to their IP they were not psychologically prepared for formal work responsibilities, they gained valuable experience from being in a real-life working environment. The students' opinion was that they managed to improve greatly their practical skills in hotel and catering operations during the placement period. Another area of satisfaction was the development of the students' soft skills; soft skills are human relations associated competencies [START_REF] Cho | Student perspectives on the quality of hotel management internships[END_REF]. The areas that were usually improved were time management, discipline, organisational skills, and team working. They also wished they could have developed further their skills in problem solving, leadership and motivation, and interviewing techniques. The main criticisms about the industrial placement period were focused on working conditions and the lack of development of management skills. It appeared to be a common view that the working conditions provided by the "hosting" organisations were not good enough, especially for those who did their placement in hotels that operate on a seasonal basis. The most widely mentioned sources of disappointment included: underpayment, hours of work were too long and, the feeling that they were seen by companies as cheap labour or as a burden. However, the vast majority of the interviewees (78%) admitted that the structuring of the placement was good regarding hotel operations skills. However, almost all of the respondents remained in entrylevel positions throughout their placements and had no chance of developing their management and administrative skills. Even the development of supervisory skills was kept to a minimum. The students expressed the view that the combination of poor working conditions and the lack of management experience resulted in reduced motivation and low moral. Six of the interviewees mentioned that three to six months after their placement period started, they were anxiously waiting for the completion of the phase and that they were considerably demotivated to pursue a future career in the hospitality industry. These results are supported by the findings of the questionnaire survey (table 4). However, these results seem to contradict the findings of previous studies [START_REF] Leslie | The hospitality industry, industrial placement and personnel management[END_REF]N.S.S.G., 1994;Yilmaz, 2018). In addition, [START_REF] Brennen | Higher Education and the Labor Market, CNAA Graduates: Their Experiences and Views Twelve Months After Graduation[END_REF] and [START_REF] Sigala | Social computing in travel, tourism and hospitality[END_REF] argued that studying a vocational degree greatly increases the chance of employment in that field. The advantages of the supervised work experience to students have internationally been described in the past [START_REF] Rise | Assessment of the Costs and Benefits of Sandwich Education[END_REF]Sigala & Christou, 2002); the results of the present survey tend to conform to the major findings of other authors [START_REF] West | Supervised work experience in graduate employment[END_REF][START_REF] Leslie | The hospitality industry, industrial placement and personnel management[END_REF][START_REF] Christou | Revisiting competencies for hospitality management: Contemporary views of the stakeholders[END_REF]. The IP period seems to have been a beneficial experience for most students as the results at table 5 present. The vast majority of the respondents (eighty-five per cent) described their industrial placement as a very positive or positive experience. However, it would be interesting to investigate in the future the deeper reasons behind the negative responses given by the fifteen per cent of the students. The contribution of the supervised work experience to the development of management competencies seems to have been quite important (table 2). According to the vast majority of the students, the IP period contributed up to a medium or high level to the development of nine key management competencies. This level was considerably higher than that of their period of traditional study at the university (table 1). Therefore, it is reasonable to assume that management competencies are better developed under a real-life work environment than at the classrooms of an educational establishment. This view is further supported by the findings presented at table 3; most of the students that went through the industrial placement period rated as high or medium the contribution of their work experience to the development of eleven key personal skills. In addition, the findings reported here are strongly supported by other studies [START_REF] Hogg | A foretaste of work[END_REF][START_REF] Farmaki | Tourism and hospitality internships: A prologue to career intentions?[END_REF] which emphasised that, in order to ensure the development of the students' skills and competencies, all hospitality management schools or undergraduate programmes have to include some form of practicum or internship. The findings suggested that as a result of the supervised work experience more than half of the students could be lost to the hospitality industry. Perhaps this should be a source for concern for both the education and the industry. It is essential to find solutions for this problem and therefore to optimise the benefits for the students and the industry. The core of the IP period consists by a group of stakeholders: the student, the host organisation and the educational establishment. It seems that each of these three should amend and enhance some parts of their approach in order to produce the maximum benefit for each one.

CONCLUSIONS

The tentative results of the research of a small sample, as described in this exploratory study, indicate some areas for concern and allows for the following conclusions to be drawn:

• The major benefits of the IP period to the students include: a more complete knowledge of the tourism products and of the diversity of the hospitality industry; development of the individual by improving his/hers personal skills; development of professional skills; and, development of key management competencies. However, the development of hospitality operations skills was significantly higher than the development of management skills. • The students had the chance to identify career paths within different sectors of the hospitality industry. In addition, it can be suggested that there is a positive relationship between the sector of the IP and the sector intended to be chosen after graduation for full-time employment. • It may be necessary for the educational establishments in the future to examine carefully the selection process of industrial placement hosting organisations. • There is evidence suggesting that the school must improve the students' psychological preparation for the IP period in a real-work environment. • By critically examining the use of IP period as a teaching and learning tool a better understanding of its advantages, disadvantages and relative value was developed. • The increased personal interaction with a large number of students as a requirement of the research methodology resulted to establishing closer links with them and to a better understanding of their needs, beliefs and attitudes towards their studies and their supervised work experience periods. Finally, alternative courses of action, based on the research findings, can be proposed in order to improve the effectiveness and efficiency of the supervised work experience as an educational tool. These proposals might be based on the future results of a more detailed examination of the issues identified in this survey. The issues for further investigation could include: the requirements and the techniques necessary for better psychological preparation of the students before the supervised work experience; the improvement of the selection process of potential host organisations; ways for establishing closer links between the faculty and the hospitality industry; and ways for improving the relationship and the understanding between the student body and the faculty.

  Notes for tables 1, 2, & 3: a) Respondents could choose more than one alternative answer b) Percentages are rounded to nearest non decimal number c) The competencies examined are considered as most important for hotel manager trainees LeBruto & Murray, 1994). Notes for the tables 4 & 5: a) Respondents could choose only one alternative answer b) Percentages are rounded to nearest non decimal number

Table 1 :

 1 Development of students' key management competencies during their studies (before the IP period)

	A. Key management competencies	Development of competencies during studies
		High	Medium	Low
		(%)	(%)	(%)
	Solving customer	5	20	75
	problems
	Human resource	24	56	20
	management skills
	Oral communication	15	18	67
	Basic management	32	38	30
	principles
	Positive peer relationships	21	57	22
	Professional standards
	Knowledge of ethics	8	13	78
	Written communication	12	16	72
	Professional appearance	48	33	19
	and poise	0	8	92

Table 2 :

 2 Contribution of IP period to the further development of key management competencies

	B. Key management competencies	Contribution of supervised work experience
		High	Medium	Low
		(%)	(%)	(%)
	Solving customer	46	48	6
	problems
	Human resource	25	63	12
	management skills
	Oral communication	64	27	8
	Basic management	27	58	15
	principles
	Positive peer relationships	35	44	21
	Professional standards
	Knowledge of ethics	80	20	0
	Written communication	17	73	10
	Professional appearance	36	57	7
	and poise	7	75	8

Table 3 :

 3 Contribution of IP period to the development of key personal skills

	Key personal skills	Contribution of supervised work
			experience
		High	Medium	Low
		(%)	(%)	(%)
	Personal initiative	32	56	12
	Guest interaction	67	21	12
	Ability to work with	24	73	3
	others
	Linguistic ability	15	47	38
	Self-reliance	28	39	23
	Self-discipline	7	84	8
	Hotel operation skills	14	33	53
	(front office)
	Hotel operation skills	16	36	48
	(housekeeping)
	Food and beverage	65	22	13
	service skills
	Food and beverage	40	29	31
	production skills
	Application of theory to	24	62	14
	practice

Table 4 :

 4 Relationship between IP period & future career preferences

	Future career preferences	Answers (%)
	The supervised work experience period	33
	reinforced my desire to work in the
	hospitality industry after my graduation.
	The supervised work experience period
	made me sceptical about working in the	59
	hospitality industry after my graduation.
	After the supervised work experience
	period I took the decision not to work in the
	hospitality industry after my graduation.	8

Table 5 :

 5 Overall assessment of the IP period

	Future career preferences	Answers (%)
	Very positive experience	22
	Positive experience Negative experience Very negative experience	63 10 5

 Woods, R.C. (1990)The image of the hotel in popular literature: a preliminary statement.