Arsène Chemin

Amanda Ross

Sylvain Hermelin

Patrick Crozet

Vincent Motto-Ros

Gilles Ledoux

Christophe Dujardin

David Amans
email: david.amans@univ-lyon1.fr

On the use of laser-induced fluorescence to probe the thermodynamic equilibrium in laser-generated plasmas

Keywords: Laser-Induced Fluorescence, collisionally induced fluorescence, rovibronic emission, rotation-vibration relaxation, AlO, laser-induced plasma, thermodynamic equilibrium, rotational temperature

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Introduction

This paper explores just one of the several avenues of study of molecular emission from laser ablation plasmas discussed in the review by Di Giacomo and Hermann [START_REF] De Giacomo | Laser-induced plasma emission: from atomic to molecular spectra[END_REF], exploring non-equilibrium energy distributions in rotational and vibrational degrees of freedom [START_REF] Harilal | Optical diagnostics of laser-produced plasmas[END_REF]. Measurement of temperatures in laser-generated plasma is a useful diagnostic for the data analysis in laserinduced breakdown spectroscopy (LIBS) [START_REF] Hermann | Analysis of multi-elemental thin films via calibration-free laser-induced breakdown spectroscopy[END_REF][START_REF] Tognoni | Calibrationfree laser-induced breakdown spectroscopy: state of the art[END_REF] and to model nanoparticle growth in the framework of laser-generation of particles such as pulsed laser deposition (PLD) and ablation in air [START_REF] Itina | Laser-generated plasma plume expansion: Combined continuous-microscopic modeling[END_REF], cluster sources [START_REF] Chemin | Microcanonical nucleation theory for anisotropic materials validated on alumina clusters[END_REF] and laser ablation in liquids (LAL) [START_REF] Itina | On nanoparticle formation by laser ablation in liquids[END_REF][START_REF] Lam | Atomistic mechanisms for the nucleation of aluminum oxide nanoparticles[END_REF]. Regarding elemental analysis, diatomic molecules sometimes offer possible advantages in LIBS diagnostics when molecular signatures fall in a more convenient spectral region than the strongest atomic transitions. A particularly striking practical illustration was the diagnostic for fluorine content in minerals via CaF bands, proposed by Alvarez et al. [START_REF] Alvarez | Quantification of fluorite masscontent in powdered ores using a laser-induced breakdown spectroscopy method based on the detection of minor elements and caf molecular bands[END_REF] in 2014, and successfully applied in the analysis of Martian soil with ChemCam one year later [START_REF] Forni | First detection of fluorine on mars: Implications for gale crater's geochemistry[END_REF].

In this article, we are addressing temperature measurement in AlO molecules, one of the molecular species mentioned in the context of diatomics offering possible advantages in LIBS. We developed a fitting procedure on partiallyresolved laser induced fluorescence (LIF) spectra in order to handle separately direct fluorescence (resonant emission, DLIF) and laser-then-collisionally induced fluorescence (LCIF). Using DLIF, we have probed the population of the rotational levels in the ground electronic state XΣ + . The rotational temperature of the ground electronic state is then deduced. Using LCIF, We have focused on energy redistribution in the excited B 2 Σ + state of AlO.

Regarding the energy redistribution in the excited electronic state, it is well established that there are no selection rules governing collisional energy transfer in a non-symmetric molecule, and a single collision can result in large changes in ∆N and ∆J, if ∆E commensurate with k B T , even with unreactive collision partners [START_REF] Polanyi | Mechanism of rotational relaxation[END_REF][START_REF] Broida | Rotational, vibrational, and electronic energy transfer in fluorescence of nitric oxide[END_REF]. Changes up to ∆J = 70 have been reported in alkali dimers [START_REF] Jones | Rotationally inelastic collisions of excited nak and nacs molecules with noble gas and alkali atom perturbers[END_REF], under controlled conditions of temperature and pressure. Here, more collisional partners are possible, including Al or O atoms, as well as atmospheric oxygen or nitrogen. Spectra reveal extensive rotational energy transfer up to ∆J = -100.

A second issue deals with the measurement of the kinetic temperature of the heavy species in laser generated plasma. An established strategy consists of measuring the rotational temperatures of diatomic molecules from emission spectroscopy assuming an equilibrium between the internal degrees of freedom of the molecules and the kinetic temperature of the species in the plasma. This is true when molecules are excited by collision with the electrons in the plasma. In that case, the population of the rotational levels in the excited state maps the population of the rotational levels in the ground electronic state, which is supposed to be at equilibrium with the kinetic temperature of the species in the plasma. If molecules are formed in the excited state by chemical reactions, equilibrium may not be achieved if emission lifetimes of the excited state of the molecule are shorter than the characteristic time of collision between the species in the plasma. In such a case, the emission from newly produced diatomic molecules will contribute to the emission spectrum. Hence, the kinetic temperature in transient lasergenerated plasma might not match the rotational temperature deduced from the rovibronic emission. Indeed, the imaging of the spatial distribution of AlO emission following the ablation of an Al target or an Al 2 O 3 target in air shows a spatial localisation of the emission restricted to the edge of the plasma plume. This observation suggests that the AlO emission comes from the newly-produced molecules at the interface between the plasma plume and the ambient air [START_REF] Lam | Investigation of local thermodynamic equilibrium in laserinduced plasmas: Measurements of rotational and excitation temperatures at long time scales[END_REF][START_REF] Ran | Molecule formation induced by non-uniform plume-air interactions in laser induced plasma[END_REF][START_REF] Wainwright | Effect of sample morphology on the spectral and spatiotemporal characteristics of laser-induced plasmas from aluminum[END_REF][START_REF] Harilal | Shock wave mediated plume chemistry for molecular formation in laser ablation plasmas[END_REF]. To determine if the rovibronic emission from diatomic molecules can be used to assess the kinetic temperature of the species in the plasma, one needs to check the consistency between the temperatures deduced from the rovibronic emission (from plasma emission), characterising the populations in the excited electronic state, and a direct measurement of the rotational temperature (from DLIF) characterising the population of the rotational levels in the ground electronic state.

The synthetic computation of emission spectra for the B-X system of AlO is well described in literature [START_REF] Parigger | Computation of AlO B 2 Σ + → X 2 Σ + emission spectra[END_REF][START_REF] Kovacs | Rotational Structure in the Spectra of the Diatomic Molecules[END_REF] and spectroscopic temperature determination of aluminium monoxide in laser ablation was reported many years ago [START_REF] Dors | Spectroscopic temperature determination of aluminum monoxide in laser ablation with 266-nm radiation[END_REF]. Although packages exist to simulate electronic spectra of diatomics, for example PGOPHER [START_REF] Western | Pgopher: A program for simulating rotational, vibrational and electronic spectra[END_REF], ExoMol or programs from Parigger's group [START_REF] Parigger | Computation of diatomic molecular spectra for selected transitions of aluminum monoxide, cyanide, diatomic carbon, and titanium monoxide[END_REF], we have produced out own routines and developed in house C++ library to compute the spectra (see supplementary materials) and a Matlab program for the fitting procedure.

Experimental Set-up

The experimental set-up is described in figure 1. The fourth harmonic of a pulsed Nd:YAG laser (266 nm, 5 ns, 20 Hz, 30 mJ per pulse) is focused vertically by a UV lens (f = 50 mm) on a mono-crystal of alumina, creating a plasma. The plasma is re-excited by a second laser pulse generated by an NT230-50 Optical Parametric Oscillator (OPO) with SH/SF extension from EKSPLA (NT230 series) using non-linear mixing of an Nd:YAG laser in order to produce broad wavelength coverage (220-1500 nm, 3.2 ns, 20 Hz). A shortpass filter (FES0500, Thorlabs) is introduced on the excitation line to remove any contribution above 500 nm. In the excitation range of the AlO molecules used in this work (483 -496 nm) the laser line-width is 66 pm FWHM (2.6 cm -1 ). It was measured using a DEMON spectrometer (Lasertechnik Berlin) with a resolution of about 1 pm. The energy per pulse 17 ± 2 mJ was measured with a pyroelectric head on a Nova II energymeter (Ophir). A few percent of the laser beam is sent to a wavemeter WaveMaster TM from Coherent, measuring the excitation wavelength with an accuracy of 5 pm for each LIF spectrum. The beam is focused on the centre of the plasma, about 1 mm above the target surface. Emission from this part of the plasma is collected perpendicularly to the OPO beam path by a 1/2-inch achromatic lens (f=25 mm) imaging about 1 mm of the plasma on a bundle of 19 optical fibres (core diameter of 200 µm) in a 2f /2f configuration.

Light is diffracted in an Andor Shamrock 500i spectrometer (Czerny-Turner optical design) using a 2400 l/mm (blaze 240 nm) grating and an entrance slit of 10 µm. The spectrometer is equipped with DHT340-25F-03 intensified CCD camera from ANDOR with full vertical binning. The spectrometer is : Time dependence of the normalised integrated intensity of the bandheads of three vibronic B -X transitions (v ′ , v ′′′ ): red dots correspond to the transition (0, 1), blue dots to (1, 2), and yellow dots to [START_REF] Harilal | Optical diagnostics of laser-produced plasmas[END_REF][START_REF] Hermann | Analysis of multi-elemental thin films via calibration-free laser-induced breakdown spectroscopy[END_REF]. Bandheads are integrated over 0.5 nm. The delay 0 is arbitrary. Solid lines correspond to the fitting procedure described in the text and taking into account the Gaussian excitation pulse, with a pulse width of 3.2 ns FWHM, and a time gate of 5 ns. The deduced emission lifetime for each v ′ -v ′′′ transition is displayed in the inset. calibrated before each measurement using scattered light from the OPO at different wavelengths. 22 spectra are recorded by shifting the OPO line by a step of 0.5 nm. For each spectrum, the peak position is fitted by a Lorentzian peak function. The resulting set of measured peak positions is then associated with the set OPO wavelengths and a 3 rd order polynomial fit is used for the calibration. The overall resolution of the acquisition system is 0.05 nm.

The gate width is set to 5 ns to reduce the background signal with respect to the LiF signal. The delays between the ablation pulse, the excitation pulse and the measurement are controlled using a digital delay/pulse generator DG645 from Stanford Research Systems. During measurement, the target is moved in the horizontal plane by the translation stage with a precision of about 10 µm and moving at about 1 mm/s. This ensures ablation from a clean surface and provides repeatable ablation conditions and plasma characteristics. The spectral sensitivity response of the system is characterised by measuring a calibrated black body lamp in the exact configuration of the measurement. The measured spectrum is fitted by a 4 th degree polynomial curve and divided by the tabulated spectrum. The resulting correction function is then applied to all the measurements.

Quantum numbers and notation

The observed bands belong to the electronic B 2 Σ + -X 2 Σ + system of AlO. We use Hund's case b) notation [START_REF] Kovacs | Rotational Structure in the Spectra of the Diatomic Molecules[END_REF], designating the excited state B 2 Σ + by a prime symbol (n ′ , v ′ , N ′ , J ′ ) and the ground state X 2 Σ + by a double prime symbols (n ′′ , v ′′ , N ′′ , J ′′ ) in absorption. Triple prime symbols (n ′′′ , v ′′′ , N ′′′ , J ′′′ ) indicate ground state populations following the B -X rovibronic emission. n stands for the electronic levels. v is the vibrational quantum number and J stands for the quantum number associated with the total angular momentum. J = N ± 1 /2 for the 2 Σ + electronic states, while N corresponds to the quantum number associated with the rotational angular momentum of the nuclei. The variation of each quantum number A is always given following ∆A = A upper -A lower (for example, ∆J = J ′ -J ′′ or J ′ -J ′′′ ), likewise we write (v ′ , v ′′ ) for excitation and (v ′ , v ′′′ ) for emission.

Both X 2 Σ + and B 2 Σ + states have been extensively studied; assignments were straightforward using parameters from Saksena et al. [START_REF] Saksena | Fourier transform spectral study of B 2 Σ + -X 2 Σ + system of AlO[END_REF].

Emission lifetime measurements

The lifetime of the B 2 Σ + state of AlO molecules is measured 10 µs after the creation of the plasma. This delay was chosen as a good compromise between maximizing the LIF signal and minimizing the plasma emission since the plasma already cooled down significantly. The OPO wavelength is set to 487 nm corresponding to the resonant absorption of the (1, 1) bandhead.

The acquisition gating is set to its minimum (5 ns). The delay between the OPO and the acquisition is changed by 1 ns steps and starts about 5 ns before the OPO pulse. Each measurement represents an averaged signal over 100 pulses. Figure 2 displays LIF intensity corresponding to bandheads at 507.9 nm (0,1), 510.2 nm (1, 2), and 512.3 nm (2, 3) integrated over 0.5 nm and normalised with respect to the maximum intensity observed. The excitation pulse and detection gate duration being similar to the deexcitation time of the molecules, their shapes have to be taken into account to fit the emission spectra display in Figure 2. Assuming a non depleted ground state, the excitation rate of the molecules remains proportional to the instantaneous intensity during the whole laser pulse duration. Excitation intensity evolves as a Gaussian function G(t) with a pulse width of 3.2 ns FWHM.

The emission intensity is given by the convolution product of the OPO pulse intensity and an exponential decay:

I(t) = G(t) ⊛ e -t τ H(t) , (1)
with H(t) the Heaviside step function. During the measurement, the emission is integrated over a gate Π(t) of 5 ns time duration. The signal recorded S(t)

is the convolution product of the Rectangular function Π(t) with the emission

I(t): S(t) = Π(t) ⊛ I(t). (2)
The solid lines correspond to fits of the data by the function described above.

It provides a measurement of the emission lifetime for the three (v ′ , v ′′′ ) transitions considered: τ 0→1 = 2.8 ns, τ 1→2 = 2.6 ns, τ 2→3 = 2.8 ns. These values are very short compared to the radiative lifetime tabulated by the National Institute of Standards and Technology (NIST), with values from 100 ns [START_REF] Dagdigian | Laser fluorescence study of alo formed in the reaction Al+O 2 : Product state distribution, dissociation energy, and radiative lifetime[END_REF] to 130 ns [START_REF] Johnson | Laser Excited Fluorescence and Radiative Lifetimes of AlO (B 2 Σ + -X 2 Σ + )[END_REF] for v ′ = 0, 1, 2. Indeed, the typical lifetime from NIST has been measured at low pressure (20 Pa) while our measurements are made at atmospheric pressure. Lifetime of a few nanoseconds has been previously discussed by Nagli et al. [START_REF] Nagli | Combining laser-induced breakdown spectroscopy with molecular laser-induced fluorescence[END_REF]. They assume that the shorter lifetime is due to collisional quenchings. Knowing the true effective value is important when using this emission lifetime to determine the ratio of species in the plasma from emission ratios [START_REF] Lam | Atomistic mechanisms for the nucleation of aluminum oxide nanoparticles[END_REF][START_REF] Lam | γ-Al 2 O 3 nanoparticles synthesised by pulsed laser ablation in liquids: a plasma analysis[END_REF], so care is taken to measure the effective emission lifetime in similar conditions.

Excitation Emission maps

Emission-excitation maps of atoms and molecules in the laser generated plasma are obtained by changing the excitation wavelength and measuring the emission of the plasma. The delay between the ablation and the OPO pulses is set to 30 µs. The acquisition starts just after the OPO pulse, and the gate width is 5 ns. The wavelength of the OPO is changed from 483 nm to 492 nm in steps of 0.05 nm. This value is similar to the FWHM of the laser. The background spectrum of the plasma emission acquired in the same conditions but without the OPO excitation pulse is subtracted from each spectrum. All the spectra are corrected for the spectral sensitivity of the optical and detection system using the black body calibration. A background contribution determined from the average intensity for λ < 507.5 nm is also subtracted. The resulting 2D map is presented on figure 3 (c). The largest

(c) (d) Excitation (Δv = 0) v''=0 → v'=0 Emission (Δv = -1) v'=0 → v'''=1 v''→ v'→v''' : 0 → 0 → 1 1 → 1 → 2 2 → 2 → 3 3 → 3 → 4
Schematic drawing starting from v'' = 0 intensities correspond to the the R-branch heads in absorption and emission.

J P ' J R ' ~ 485.3 nm Zoom 0 → 0 → 1 J = N + 1 / 2 J = N -1 / 2 P 1 P 2 R 1 R 2 P 1 P 2 R 1 R 2
A bandhead is characterised by a large density of nearly-overlapping rovibronic transitions (See Fortrat diagram 3 (a)). In the excitation-emission map, a large density of transitions leads to intense emissions at 507.9 nm, corresponding to the bandhead of the (0,1) R-branch, following an excitation at 484.2 nm, corresponding to the bandhead of the (0,0) R-branch. In the same way, an intense emission is observed at 510.2 nm following an excitation at 486.7 nm (bandhead of (1,2) and (1,1) transitions), as well as at 512.3 nm following an excitation at 488.9 nm (bandhead of (2,3) and (2,2) transitions). Furthermore, the transitions involving low vibrational levels, namely (0,0) excitation and (0,1) de-excitation, have a larger intensity, reflecting enhanced populations, and large Franck-Condon factors ( for (0,0) at least).

The curved branches on figures 3 (c) and (d) correspond to direct fluorescence from the rotational states J ′ populated by the resonant OPO pulse excitation according to transition enabled by selection rules. For AlO B 2 Σ + -X 2 Σ + system, these include ∆N = ∆J = ±1 corresponding to the four branches R 1 , R 2 and P 1 , P 2 represented in blue on figure 3 band-head and so on. For an excitation wavelength at a bandhead, the light absorption is drastically increased because many transitions are excited simultaneously, populating multiple (v ′ , N ′ , J ′ ) levels, and then the collisionally induced fluorescence signal increases accordingly. Horizontal lines correspond to bandhead exaltation with ∆v = -1. We observe that regardless of the excitation wavelength, the collisionally induced fluorescence is concentrated around bandhead wavelengths simply because of a larger density of lines. In particular, one can notice horizontal lines corresponding to emission bandheads at 507.9nm for the (0,1) transitions, 510.2 nm for the (1,2) transitions, 512.3 nm for the (2,3) transitions and so on. Such Excitation/Emission maps show that, despite dense plasmas and large collision rates, one can observe emission from partially thermalised molecules exhibiting clear patterns according to selection rules of transitions. These 2-D patterns resemble Fortrat diagrams and could be used to determine the validity of molecular constants in hot states (above 3000 K). Our measurements match well the tabulated values for AlO within the precision of the presented measurements.

The lowest horizontal line corresponding to (0,1) bandhead emission at 507.8 nm appears following excitation anywhere out to 496 nm, where the laser is resonant with J ′′ > 120 transitions in the (0,0) band, and lower J ′′ transitions in (1,1), (2,2) or (3,3) hot bands. Collisional energy transfer is responsible for transferring these excited state populations down to v ′ = 0, J ′ ≈ 20, with ∆J ′ as much as -100, and ∆v ′ = -1, -2, -3. Gain in energy is evident, too, as illustrated by the vertical line formed following (0,0) laser excitation at 484.2 nm, extending to 512.3 nm. In this case, the emission signal comes from molecules in v ′ = 0 vibrational state with J values larger than 60, implying rotational energy transfer up to ∆J = +40, or from molecules with v up to 2, corresponding to vibrational energy transfer up to ∆v = +2. Horizontal lines for excitation wavelengths shorter than an excitation bandhead ∆v = 0 reveal vibrational energy transfer. As an example, emission at 514.3 nm in the bandhead for the [START_REF] Hermann | Analysis of multi-elemental thin films via calibration-free laser-induced breakdown spectroscopy[END_REF][START_REF] Tognoni | Calibrationfree laser-induced breakdown spectroscopy: state of the art[END_REF] transitions after excitation at 485 nm is only possible if there is a collisional energy transfer from v ′ = 0 to v ′ = 3. There are no selection rules governing collisional energy transfer in a non-symmetric molecules. These experiments with selective excitation and detection give direct evidence for collisional energy transfer propensities in AlO.

LIF spectrum simulation

In order to calculate the LIF spectrum, one has to consider both the excitation and de-excitation processes between the X 2 Σ + (n ′′ , n ′′′ ) and B 2 Σ + (n ′ ) electronic states. The LIF spectrum can be described as the superposition of direct fluorescence (DLIF), where the excited state (n ′ , v ′ , J ′ , N ′ ) involved in the absorption and the emission is the same (resonant fluorescence), and laser-then-collisionally induced fluorescence (LCIF) after collisional population transfer in the excited state (ro-vibrationally relaxed fluorescence). The population redistribution can be approximated by independant vibrational and rotational Boltzmann distributions. By fitting spectral profiles to a linear combination of both combributions to LIF spectra, direct fluorescence and collisionally induced fluorescence, we are able to determine effective temperatures describing the populations in the rotational and vibrational manifolds of X and B states independently. We are following formalisms available in standard textbooks [START_REF] Kovacs | Rotational Structure in the Spectra of the Diatomic Molecules[END_REF][START_REF] Herzberg | Molecular Spectra and Molecular Structure, Volume I -Spectra of Diatomic Molecules[END_REF][START_REF] Bernath | Spectra of Atoms and Molecules[END_REF].

To calculate the direct fluorescence spectrum, we consider the following excitation/de-excitation channel:

(n ′′ , v ′′ , N ′′ , J ′′ ) → (n ′ , v ′ , N ′ , J ′ ) → (n ′′′ , v ′′′ , N ′′′ , J ′′′ ) (3)
with B l ′ l ′′ the Einstein coefficient for absorption and ρ(ν) [J.s.m -3 ] the radiance of the OPO pulse at the resonant wavelength. ν stands for the wavenumber of the transition. The population of the upper state is given by the integration of this transition rate over the duration of the OPO pulse.

The radiance ρ(ν) is assumed undepleted on the whole plasma plume. The number of molecules in the ground state being large compared to the number of molecules excited by the pulse, one can consider that N l ′′ remains constant.

The Einstein coefficient for absorption can be related to the Einstein coefficient for spontaneous emission A n ′ ,v ′ n ′′ ,v ′′ through the following relation:

B l ′ l ′′ = (2J ′ + 1) (2J ′′ + 1)
c 3 8πhν 3 A n ′ ,v ′ n ′′ ,v ′′ S J ′ J ′′ 2J ′ + 1 (8)
S J ′ J ′′ denotes the Hönl-London coefficient [START_REF] Kovacs | Rotational Structure in the Spectra of the Diatomic Molecules[END_REF]. Combining equation 6, 7 and 8 we obtain the number of molecules in the excited state after the pulse absorption:

N l ′ ∝ ρ(ν) ν 3 • A n ′ ,v ′ n ′′ ,v ′′ • S J ′ J ′′ • B n ′′ ,v ′′ k B T ′′ rot • exp - hc • F n ′′ ,v ′′ ,i (J ′′ ) k B • T ′′ rot • exp - hc • G n ′′ (v ′′ ) k B • T ′′ vib (9)
The emission intensity from the state (n ′ , v ′ , N ′ , J ′ ) to (n ′′′ , v ′′′ , N ′′′ , J ′′′ ) is thus proportional to the population of the excited state multiplied by the Einstein coefficient for spontaneous emission A n ′ ,v ′ n ′′′ ,v ′′′ :

I l ′ l ′′′ ∝ A n ′ ,v ′ n ′′′ ,v ′′′ • S J ′ J ′′′ (2J ′ + 1) • N n ′ ,v ′ ,J ′ I l ′ l ′′′ ∝ ρ(ν) ν 3 • A n ′ ,v ′ n ′′′ ,v ′′′ • A n ′ ,v ′ n ′′ ,v ′′ • S J ′ J ′′′ • S J ′ J ′′ (2J ′ + 1) B n ′′ ,v ′′ k B T ′′ rot × exp - hc • F n ′′ ,v ′′ ,i (J ′′ ) k B • T ′′ rot • exp - hc • G n ′′ (v ′′ ) k B • T ′′ vib ( 10
)
Within the effective excited state life-time, the excited state

(n ′ , v ′ , N ′ , J ′ )
population is partially redistributed over other quantum levels. Because of the limited experimental resolution, the population redistribution of excited states cannot be determined directly from spectral analysis. Here we chose to model the population after relaxation using two Boltzmann distributions with effective temperatures T ′ vib and T ′ rot for vibration and rotation. One should consider these temperatures as fitting parameters. The higher these parameters, the larger the population in states with high quantum levels.

The intensity of each transition is proportional to:

I l ′ l ′′′ ∝ A n ′ ,v ′ n ′′′ ,v ′′′ • S J ′ J ′′′ • B n ′ ,v ′ k B T ′ rot • exp - hc • F n ′ ,v ′ ,i (J ′ ) k B • T ′ rot •exp - hc • G n ′ (v ′ ) k B • T ′ vib (11)
The energy of each state, including the electronic energy, the vibrational energy G n (v) and the rotational energy F n ′ ,v ′ ,i (J ′ ), is computed using the data from Saksena et al. [START_REF] Saksena | Fourier transform spectral study of B 2 Σ + -X 2 Σ + system of AlO[END_REF] (See supplementary materials). The rotational constant B n,v are also taken from Saksena et al. [START_REF] Saksena | Fourier transform spectral study of B 2 Σ + -X 2 Σ + system of AlO[END_REF]. Figures 3 (a) and 5 (a) show Fortrat diagrams calculated for electric-dipole allowed transitions between these states, verifying predicted wavenumbers with the lines lists given by Launila and Berg [START_REF] Launila | Spectroscopy of AlO: Combined analysis of the A 2 Π i → X 2 Σ + and B 2 Σ + → X 2 Σ + transitions[END_REF]. Each spectrum is calculated by summing the contribution of all allowed transition convoluted by a Gaussian function of width σ, and using the Einstein coefficient

A n ′ ,v ′ n ′′ ,v ′′ (and A n ′ ,v ′ n ′′′ ,v ′′′ ) from Hebert
and Nicholls [START_REF] Hebert | Absolute band strengths for the AlO blue-green (B 2 Σ + -X 2 Σ + ) band system[END_REF]. Both direct fluorescence (DLIF) spectrum and laser-thencollisionally induced fluorescence (LCIF) spectrum are normalised by their integral over the wavelength range. The final spectrum is the sum of both contributions, weighted by coefficient x:

I = x × I LCIF I LCIF + (1 -x) I DLIF I DLIF (12)
x is the fraction of the collisionally induced fluorescence with respect to the total emission. The closer the value of x to one, the more important is relaxation in the excited electronic state.

LIF spectrum fit and results

Band contour fits have been performed for each excitation wavelength λ e corresponding to the measured wavelength of the OPO pulse. The curvefitting is performed using functions available in Matlab to solve nonlinear least-squares regression. The radiance of the OPO pulse ρ(ν) is taken proportional to a Lorentz function centered on λ e with a line-width of 66 pm FWHM. The fitting parameters are T X 2 Σ + describing the population in the X 2 Σ + state, T ′ rot and T ′ vib describing the population in the B 2 Σ + relaxed state, the width σ of the Gaussian function used to compute the synthetic spectra and the ratio x defined in equation 12. The average value of the fitted σ is 0.084 nm (FWHM) with a standard deviation of 0.006 nm. T ′′ rot = T ′′ vib = T X 2 Σ + is assumed in the fitting process as we consider the equilibrium for the molecules in the ground state to be reached. Leaving both parameters free in the optimisation process is not relevant as T ′′ vib has little effect on the shape of the emission for the ∆v = 0 transitions we measured here. In order to fit both temperatures of the ground state, one should consider exciting other transitions such as ∆v = ±1. Furthermore, for excitation wavelengths lower than 486.5 nm, v" = 0 is the only excited state and the fit does not depend on T ′′ vib .

Figures 4 (a Between 486.7 nm and 488.8 nm, both (0, 0) and (1, 1) transitions are excited, leading to a higher T ′ vib . At 486.7 nm, the density of (1, 1) transition is large (bandhead) and most of the excited molecules end in this vibrational state explaining the peak in the vibrational temperature. On the contrary, the rotational temperature first drops before increasing for the same reason as presented for pure (0, 0) transitions. The amplitude of the variation is weaker as there are also contibutions from the high-J resonances in the (0, 0) band, giving more "strating point" for collisional energy transfers. This pattern for T ′ rot and T ′ vib repeats for each of the excited (v ′ , v ′′ ) bands.

Figure 5 (c) shows the fitted temperatures T X 2 Σ + for the ground state. For each OPO pulse centred on an excitation wavelength away from the bandhead, at least two sets of J" values are excited, one belonging to R-branch and the other to the P-branch. The large difference between these two sets of J" numbers allows obtaining a value of T X 2 Σ + for each excitation wavelength longer than 485 nm. For wavelengths longer than 490 nm, the direct fluorescence signal is too weak to achieve convergence and deduce T X 2 Σ + .

Figure 5 (c) shows that T X 2 Σ + remains stable as it depends on the plasma thermodynamic state only, and not on the excitation process. One can notice a slight peak for the (1, 1) bandhead, but also for the (2, 2) bandhead.

In this region, large resonant intensity and self-absorption might impact the fitting procedure, but the possible consequences of an optically thick sample are not considered here. To determine the temperature of the ground state, we average T X 2 Σ + values from excitation wavelength between 485 and 486.5 nm (shaded area in figure 5 (c)). In that range, the emission shape does not depend on T ′′ vib since only one vibrational state is involved, and we then assume T X 2 Σ + = T ′′ rot . The histogram for the T X 2 Σ + distribution from the shaded area is plotted in figure 5 standard deviation is 552 K and the temperature interval for a confidence level of 70% (two-sided) is ±67 K.

This temperature can be compared to the vibrational and rotational temperatures of the B 2 Σ + excited state measured from emission spectroscopy.

Emission of AlO molecules has been measured before and after the 2D scanning without OPO excitation and fitted as presented in the supplementary materials following the method presented in Lam et al. [START_REF] Lam | γ-Al 2 O 3 nanoparticles synthesised by pulsed laser ablation in liquids: a plasma analysis[END_REF]. The value obtained for the vibrational temperature is 5620 ± 330 K (confidence level of 70%) at a delay of 30 µs. The value obtained for the rotational temperature is 3130 ± 100 K. The rotational temperature of the excited state T B 2 Σ + rot is in good agreement with the rotational temperature of the ground state T X 2 Σ + measured by LIF. These measurements confirm that after 30 µs, the measurement of the rotational temperature of the excited state from the emission of the plasma is a good probe of the rotational temperature of the ground state.

In order to test it for shorter delay time in the plasma, the same experiment was run with a delay of 5 µs. The average value of T X 2 Σ + is 3700 K, the standard deviation is 530 K and the temperature interval for a confidence level of 70% (two-sided) is ±64 K. T X 2 Σ + at 5 µs is larger than after 30 µs as These temperatures are consistent with the ones reported for AlO molecular temperatures for the same delay in laser-generated plumes from metals [START_REF] Kautz | Oxidation in laser-generated metal plumes[END_REF].

Conclusion

We used time resolved 2D LIF to study the out-of-equilibrium properties of excited AlO molecules formed in a laser-generated plasma. Laser induced plasmas are usually characterised using emission spectroscopy to determine temperature and composition. This approach probes only the excited state of the molecules that are likely to be directly formed in the plasma. Groundstate distributions are likely to be different. By tuning a laser to chosen molecular transitions, we can probe rovibrational distribution in the ground state as well.

LIF was used to measure the effective lifetime τ = 2.7 ± 0.2 ns of the B 2 Σ + state of AlO molecules at atmospheric pressure. This lifetime is much shorter than tabulated values in vacuum because of collisional quenching in the plasma. Effective lifetimes are key parameters for the calculation of molecular concentrations in LIBS.

By fitting both direct and collisionally induced fluorescence contributions to spectra of AlO molecules 5 µs and 30 µs after ablation, we were able to determine the temperature of the ground electronic state of AlO(X 2 Σ + ) and the effective temperatures describing the relaxed state of the molecules partially thermalised. Their population appears to be well described by Boltzmann populations using effective rotational and vibrational temperatures.

These temperatures are different, and depend on excitation channel. Clearly molecules do not reach full internal equilibrium, or equilibrium with the plasma kinetic temperature. In such a plasma, collisions are not sufficient to ensure equilibrium of the excited molecule population from an initial state strongly out of equilibrium.

On the contrary, the rotational temperature deduced from standard plasma emission, without re-excitation of the plasma, is consistent with the rotational temperature of the ground state deduced from LIF, both after 5 µs and 30 µs. The AlO molecules formed in the excited state B 2 Σ + in the plasma reach the equilibrium before emission. This might be because they are not formed far from equilibrium, unlike the molecules excited by LIF. The measurement of the temperature in the plasma considering the rotational temperature of the excited state of the diatomic molecules is confirmed to be valid.

Figure 1 :

 1 Figure 1: Scheme of the laser induced fluorescence (LIF) experimental set-up.

Figure 2

 2 Figure2: Time dependence of the normalised integrated intensity of the bandheads of three vibronic B -X transitions (v ′ , v ′′′ ): red dots correspond to the transition (0, 1), blue dots to (1, 2), and yellow dots to[START_REF] Harilal | Optical diagnostics of laser-produced plasmas[END_REF][START_REF] Hermann | Analysis of multi-elemental thin films via calibration-free laser-induced breakdown spectroscopy[END_REF]. Bandheads are integrated over 0.5 nm. The delay 0 is arbitrary. Solid lines correspond to the fitting procedure described in the text and taking into account the Gaussian excitation pulse, with a pulse width of 3.2 ns FWHM, and a time gate of 5 ns. The deduced emission lifetime for each v ′ -v ′′′ transition is displayed in the inset.

Figure 3 :

 3 Figure 3: (a) Computed Fortrat diagram for the vibronic B -X transition (0, 0) in blue and (0, 1) in red. Only the main branches characterised by ∆J = ∆N = ±1 are displayed, i.e. the R 1 -branch, R 2 -branch, P 1 -branch, and P 2 -branch. Indexes 1 and 2 correspond to the spin splitting. 1 stands for J = N + 1 /2 while 2 stands for J = N -1 /2. Satellite branches P Q 12 and R Q 21 are not displayed because their intensity is weak for large J ′ values with respect to the intensity of the R-branch and P-branch. A given excitation wavelength can excite a few states (v ′ , N ′ , J ′ ) in the R-branches and P-branches. As an illustration, for an absorption around 485.3 nm (vertical blue line), we highlight two excited states (0, N ′ R , J ′ R = N ′ R + 1 /2) and (0, N ′ P , J ′ P = N ′ P + 1 /2). Each excited state leads to two emission lines (red vertical lines), one corresponding to a the R-branch and one corresponding to a P-branch. The highlight emission lines appear in the LIF spectrum (black). (b) Same example represented on an energy level scheme with arrows displayed for the resonant emission (DLIF). (c) 2D excitation/emission of an AlO molecule for a delay of 30 µs. (d) Superposition of the 2D excitation/emission spectrum with theoretical transitions assuming resonant excitation and emission from the (v ′ , N ′ , J ′ ) state. Satellite branches P Q 12 and R Q 21 are not displayed. The spectrum displayed in (a) corresponds to the white dashed line. Intersection with the four red branches correspond to the four emission peaks highlighted in (a) and (b). Vertical white lines show collisionally induced fluorescence following absorption in the bandheads ∆v = 0.

  photons are highlighted in figure3(a) by the vertical blue lines, one in the R 1 branch and one in the P 1 branch for (v ′ , v ′′ ) = (0, 0). Assuming spin conservation, and looking at the transitions ∆v = -1, each of these two excited states can decay by spontaneous emission through 2 channels following the selection rules ∆N = ∆J = ±1, i.e. one in the R-branch and one in the P -branch. In this example, it leads to 4 transitions indicated by the red vertical lines on figure3 (a). The black spectrum in figure3(a) corresponds to a measured spectrum for an excitation at 485.3 nm. One can observe 4 groups of transitions centred around 507.95, 508.7, 509.1 and

Figure 4 :

 4 Figure 4: Example of fitted LIF spectrum (red) on measured spectrum for a delay of 30 µs (black) for the excitation wavelength 484.80 nm (a) and 488.30 nm (c) considering a single temperature T X 2 Σ + for the electronic ground state X 2 Σ + , and two temperatures, T ′ rot and T ′ vib , for the electronic excited state B 2 Σ + . Fitted values at 484.80 nm are T ′ rot = 4520 K, T ′ vib = 1330 K, and T X 2 Σ + = 2380 K. Fitted values at 488.30 nm are T ′ rot = 4450 K, T ′ vib = 2970 K, and T X 2 Σ + = 2720 K. Panels (b) and (d) show the collisionally induced fluorescence contribution in blue (LCIF) and the direct fluorescence contribution in magenta (DLIF) for each excitation wavelength.

  Figures 4 (a) and (c) represent the calculated LIF spectrum (in red) fitted on the LIF spectrum measured (in black) for an excitation of 484.80 nm and 488.30 nm respectively.Figure 4 (b) and (d) represent the direct fluorescence contribution in magenta and the collisionally induced fluorescence contribution in blue in the calculated spectrum. The calculated LIF spectrum accurately reproduces the shape and intensities of the measured LIF spectrum. On this spectral range, direct fluorescence represents only 6 to 16% of the integrated intensity signal. However, direct fluorescence remains easy to identify in tight spectral windows. Most of the intensity, about 90%, corresponds to collisionally induced fluorescence. Despite the short lifetime of the excited state (2.7 ns), collisions efficiently redistribute population amongst excited states v ′ and J ′ .

Figure 5 :

 5 Figure 5: (a) Fortrat diagram for the ∆v = 0 excited transitions. (b) Rotational (red) and vibrational (blue) temperatures describing the collisionally induced fluorescence (LCIF) depending on the excitation wavelength. (c) Temperature of the ground state (X 2 Σ + ) depending on the excitation wavelength measured from the direct fluorescence (DLIF).(d) Histogram of the ground state (X 2 Σ + ) temperatures corresponding to the selected excitation range shaded area. The average value is 3150 K, the standard deviation is 552 K and the temperature interval for a confidence level of 70% (two-sided) is ±67 K.

  the plasma cools down. In comparison, the direct fluorescence provides a vibrational temperature of the excited state T B 2 Σ + vib = 6860 ± 350 K (confidence level of 70%) and a rotational temperature of T B 2 Σ + rot = 3850 ± 100 K. Here again, the values are in agreement considering the error of the measurement.

Funding sources

This work was financed by the French region Rhônes Alpes Auvergne (Optolyse, CPER2016).

Data availability

Data will be made available on request.

In the following, the set of quantum numbers (n, v, N, J) is noted l in the equations. The population of the ground state n ′′ (X 2 Σ + ) is described by a rotational and vibrational temperatures T ′′ rot and T ′′ vib such that the density of molecules in the state (n ′′ , v ′′ , N ′′ , J ′′ ) is:

with N n ′′ the total population of the electronic state, Q the partition functions, G the vibrational energy, and F the rotational energy. i corresponds to the spin splitting. i = 1 stands for J = N + 1 /2 and i = 2 stands for

Considering only transitions from the X 2 Σ + electronic state,

N n ′′ and Qvib are assumed constant. Since we have kT rot ≫ B n ′′ ,v ′′ (with

, Qrot is approximated using the rigid rotator model [START_REF] Lam | γ-Al 2 O 3 nanoparticles synthesised by pulsed laser ablation in liquids: a plasma analysis[END_REF]:

and from equation 4,

The population of the upper state (n ′ , v ′ , N ′ , J ′ ) is considered to be zero prior to excitation. This is not strictly true, but remaining plasma emission is very much weaker than LIF signal at the delay considered, and in any case the plasma emission contribution is subtracted from LIF spectra in our data processing. Considering only the absorption of the excitation pulse, the population variation of the upper state N n ′ ,v ′ ,J ′ is:

Disclosures

The authors declare no conflicts of interest.

CRediT authorship contribution statement

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary material

The supplementary materials describes the energy level calculations, including the parameters used, and the transition probability calculation. It also shows the plasma emission spectra at 5 µs and 30 µs with their fits.


Margherita Mussi
email: margherita.mussi@fondazione.uniromal.it

Matthew M Skinner

Rita T Melis

Joaquin Panera

Susana Rubio Jara

Thomas W Davies

Denis Geraads

Hervé Bocherens

Giuseppe Briatico

Adeline Le Cabec

Margherita Mussi1

Matthew M Skinner4

Susana Rubio

Thomas W Davies4

Denis Geraads10

Adeline Le

Agness Gidna16

Raymonde Bonnefille17

Luca Di Bianco2

Eduardo Méndez-Quintas18

Early Homo erectus lived at high altitudes and produced both Oldowan and Acheulean tools

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

ln Africa, the scarcity of hominin remains found in direct association with stone tools has hindered attempts to link Homo habilis and Homo erectus with particular lithic industries. The infant mandible discovered in level E at Garba IV (Melka Kunture) on the highlands of Ethiopia is critical to this issue due to its direct association with an Oldowan lithic industry. Here, we use synchrotron imaging to examine the internal morphology of the unerupted permanent dentition and confirm its identification as Homo erectus. Additionally, we utilize new palaeomagnetic ages to show that (i) the mandible in level E is ca. 2 million-years-old, and represents one of the earliest Homo erectus fossils, and (ii) that overlying level D, ca. 1.95 million-years-old, contains the earliest known Acheulean assemblage.

The Melka Kunture complex consists of a cluster of prehistoric sites that extends over tens of km² in the Upper Awash, on the Ethiopian plateau at 22000 m above sea level (asl) (Fig. 1A). At this high altitude, the Pleistocene climate would have been cooler and wetter than at the lower elevations of the Rift Valley where most fossil evidence for early species of Homo derives. The paleovegetation belonged to the Afromontane complex, ranging from forest to grassland and bushland (1), and the plant species were distinct from those of the African savanna, which developed at lower elevations in drier and warmer environments. The degree of topographic relief of the region meant that the highlands were relatively isolated, leading to the appearance of endemic mammal sub-species (2). In 1981, an infant mandible was discovered in level E of the Garba IV site, though which species of Fig. 1. Location, magnetostratigraphy, and chronostratigraphy of Garba IV. (A) Location map of the Garba IV site (Melka Kunture, Upper Awash, Ethiopia). (B) The stratigraphy of the Garba gully, with both Garba IV and Garba XIII, a middle Acheulean site [START_REF] Gallotti | Upper Awash, Ethiopia): A new Acheulean site of the late Lower Pleistocene[END_REF]. Geomagnetic polarity timescale is from [START_REF] Channell | Timing of Quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives[END_REF], 40 Ar/ 39 Ar dates are from [START_REF] Ma | Ethiopia: Implications for understanding the earliest stages of stone knapping[END_REF], and K/Ar dates are from [START_REF] Schmitt | Initial K/Ar and Paleomagnetic Results of the Melka-Kunturé early-man sites, Ethiopia[END_REF]. (C and D) The stratigraphy of Garba IV and the paleomagnetic sequence of samples with normal (small black squares) and reverse polarity (small white squares) (6) (fig. S3).

Stratigraphy and chronology

The sedimentary sequence of the multi-layered Garba IV site records hominin activity on lag deposits close to shallow ponds and pools that are situated within a floodplain affected by the migrating paleo-Awash channel and associated flood events, and also influenced by the deposition of volcanic ashes. The site is at the mouth of a small gully, the Garba gully, incised by a seasonal tributary of the Awash River (fig. S1). We refer here to three main archaeological levels at the base of the stratigraphic sequence of the gully (Fig. 1B and figs. S1 to S3): levels F and E with faunal and hominin remains, and an Oldowan lithic industry; and level D with faunal remains and an Early Acheulean industry. 40Ar/39Ar and preliminary magnetostratigraphic analyses had previously dated these levels to shortly before and shortly after 1.7 ± 0.2 million-years ago (Ma) (4, 5), but the age was not well constrained. A new magnetostratigraphic study has recently established the age of the levels (6). In this sequence both the Olduvai/Matuyama boundary (1.925 million years), and the Matuyama/Reunion boundary (2.116 million years) are identified, which provide a maximum and a minimum age for the three archaeological layers. The Olduvai/Matuyama boundary was observed above level D. Using an estimated depositional rate and a model of sediment accumulation, the specific ages have been calculated as: level F = 2.02 (-0.095; +0.096) Ma; level E = 2.00 (-0.0075; +0.1) Ma; level D = 1.95 (-0.025; +0.1) Ma (6) (Fig. 1, C andD). These much older dates for levels D-F have significant implications for the interpretation of the archaeology and the hominin fossil found at the site.

Archaeology

Level E at Garba IV (now dated to ca. 2.00 Ma) was first tested in 1981 and 34 m2 were eventually exposed (fig. S4 and figs. S8 to S10), with an additional exposure of 12 m2 of underlying level F (now dated to ca. 2.02 Ma) (fig. S4 and figs. S8 to S10). Both yielded abundant Oldowan tools (7) and the associated mandible represents one of the only hominin remains directly associated with artefacts in the Early Pleistocene (table S1). It was excavated in level E, a sealed level within a well-established stratigraphic sequence (Fig. 1, C and D; fig. S2; and figs. S9 to S11). The artefacts were mostly produced on obsidian cobbles and pebbles preferentially selected from the rocks available in the local alluvial deposits (Fig. 2A and figs. S12 to S13). Many of these are small flakes, mostly 20-50 mm in length, with ca.30% retouched and transformed into small pointed tools (Fig. 2A, 3 to 5, and fig. S13). In overlying level D, excavated since 1973 to some 75 m2 and redated to ca. 1. [START_REF] Deino | Ar/(39)Ar dating of Bed I, Olduvai Gorge, Tanzania, and the chronology of early Pleistocene climate change[END_REF] Ma, the ca. 7000 lithic tools (table S1) differ from those of the underlying Oldowan assemblage due to the production of large flakes (i.e., >100 mm in maximal length or width). There are true handaxes, cleavers and other types of Large Cutting Tools (LCTs), such as massive scrapers or knives shaped on large flakes (Fig. 2B, 6 to 12, and figs. S15 to S19). The large flakes and LCTs constitute ca. 1% of the analyzed assemblage (large flakes = 41 and LCTs = 23) but are relevant to understanding the technological shift from the underlying Oldowan assemblage (8). The features of the assemblage, including the production of large flakes and LCTs, identify it as Early Acheulean. Thus, Garba IV documents a rapid change between Oldowan and Acheulean lithic production over a period of 50,000-100,000 years in this unique high-altitude paleoenvironment.

Paleoecology

The vegetation of the Garba IV site belonged to the Dry evergreen Afromontane forest and grassland complex, which currently develops on the mountains of East Africa above 1800 m asl (9, 10). Around 1.8 Ma or possibly earlier, the palynology indicates bushy woodlands alternating with patches of grassland in the area of Melka Kunture (1). A more open vegetation and a mountain grassland followed, which further expanded during the deposition of level D at Garba IV. This Afromontane vegetation develops under a mountain climate with large diurnal temperature ranges, resulting in plant species that are distinct from those of the lowland savanna or woodland. The mammalian faunal remains of levels E and F are highly fragmented, and species-level identifications are rare. The fossil sample from level D is larger (table S3), but the three assemblages cannot be ecologically distinguished. A rare occurrence in level E is the extinct civet Pseudocivetta ingens, also known from several eastern African sites be-tween c. 2.4 and 1.5 Ma. Theropithecus, a genus quite rare at Melka Kunture, is represented by a single specimen from level D . In this layer, the ungulate fauna is dominated by large hippos and grazing bovids, of which almost 90% are alcelaphins and antilopins, unambiguously pointing to an open environment and consistent with the relatively high frequency of equids (2). Also consistent with an open environment are measurements of 13C/12C isotopic ratios of dental enamel carbonates carried on hippopotamus (n=11), bovid (n=1) and equid (n=1) teeth from layer D. The results show that hippos, bovids and equids consumed C1 plants, suggesting C4 high-elevation grasslands (11) (fig. S20). The bovid material from levels E-F cannot be identified beyond generic rank, while level D alcelaphins (Connoohaetes and Damaliscus) are close to those known from contemporaneous East African lower elevation sites, up to 1500 m asl, but are distinct at least at sub-specific level. Thus, the fauna record testifies to some endemicity (2, 12) of the Ethiopian highlands, a factor to be considered when dealing with hominin evolution and subsistence in this paleoenvironment.

Taxonomy of the Garba IVE mandible

Previous analyses of the external and internal morphology of the MK 81 GAR IVE 0043 (abridged name Garba IVE) mandible (Fig. 3) have identified affinities with various early Homo species such as H. habilis, H. rudolfensis, H. ergaster and H. erectus (13). Although GAR IVE had been published as affected by a rare genetic disorder affecting its enamel (amelogenesis imperfecta) (14), this was recently refuted using synchrotron imaging, and features previously interpreted as pathological were rather shown to result from taphonomic processes (15). Based on medical CT scans, Zanolli et al. (3) expanded on previous assessments to examine the sequence of dental development, tooth crown tissue proportions, tooth size in the unerupted permanent dentition, and corpus cortical bone thickness. Their conclusion was that the Garba IVE specimen shared affinities with H. habilis/rudolfensis and H. erectus sensu lato. We used synchrotron computed tomography to produce high-resolution images of the enamel and dentine surfaces of the unerupted permanent dentition (15) as these have been shown to maximize the taxonomic information in hominin teeth (16-18). We conducted a 3D landmark-based geometric morphometric analyses of the enamel-dentine junction of the third and fourth premolar and first molar. We were also able to compare the unerupted permanent dentition of Garba IVE against an expanded comparative sample that crucially includes specimens of H. habilis from both Tanzania and Kenya. The enamel-dentine junction shape of the permanent third premolar exhibits morphological affinities with H. erectus based on similarities in crown height and the position and relative height of the metaconid (Fig. 4A). The enamel-den-tine junction shape of the permanent first molar is intermediate between Australopithecus and later Homo, and plots closest to KNM-ER 992. This reflects a number of features including a relatively reduced hypoconulid, which separates H. erectus from H. habilis (Fig. 4B). The enamel-dentine junction ridge of the permanent fourth premolar is less diagnostic [since the tooth is not crown complete (15), the cervix could not be used] and the GAR IVE specimen shows similarities with both H. habilis and H. erectus (fig. S21). Both premolars show strong similarities to KNM-WT 15000 at the outer enamel surface (fig. S22). The permanent canine is not crown complete [supplementary figure S1 in (15)], but the preserved enamel morphology is similar to H. erectus specimens such as KNM-ER 820 (fig. S22). Relative tooth crown size of the premolars (i.e., having premolars that are similar in crown size) and first molar also resembles H. erectus (Fig. 4C). It is currently difficult to make comparisons with material definitively attributed to H. rudolfensis (e.g., KNM-ER 1482, 1801 and KNM-ER 60000) due to the limited number of specimens and the preservation/degree of attrition of many of the mandibular teeth, but we consider what is measurable of the dentition of both to be more similar to H. habilis than to H. erectus. Overall, this expanded analysis of the external and internal morphology of the Garba IVE specimen is consistent with an attribution to H. erectus rather than H. habilis.

Discussion

At ca. 2 Ma, the GAR IVE mandible is one of the earliest H. erectus fossils so far discovered, and the only specimen whose taxonomic identification is based on teeth, which are known to have a strong taxonomic signal (15-18). Furthermore, it is the first to be directly associated in a sealed deposit with Oldowan stone tools in Africa. The same industry is also found in underlying level Garba IV F, pointing to a fully-in-grained behavior that we attribute to H. erectus activity. The many small, pointed tools and the systematic use of obsidian make it conspicuously different from the 2.4-1.6 Ma Oldowan found elsewhere, and notably at sites where remains of Homo habilis were also discovered (table S2). Six knapped stones classified as Oldowan have been reported at Drimolen (19) and while a ca. 2.04 Ma neurocranium (20) has been classified as H. aff. erectus based on affinities with the Mojokerto cranium (and we note that there is no similarly aged Paranthropus cranium to which it can be compared), the site is dominated by Paranthropus robustus remains and thus the hominin that produced the material cannot be determined with certainty (19, 21). The innominate hip bone KNM-ER 3228 from Koobi Fora is not directly associated with stone tools [START_REF] Leakey | New hominid fossils from the Koobi Fora formation in northern Kenya[END_REF], while at Olduvai PTK, the OH 86 proximal phalanx attributed to Homo cf. H. erectus sensu lato is associated with Oldowan stone tools [START_REF] Domínguez-Rodrigo | Earliest modern human-like hand bone from a new >1.84-million-year-old site at Olduvai in Tanzania[END_REF], but is dated to 1. [START_REF] Braun | Oldowan behavior and raw material transport: Perspectives from the Kanjera Formation[END_REF] Ma. Hominin taxa such as H. habilis, Paranthropus, and Australopithecus lived at the low to middle elevation of eastern and southern Africa, i.e., up to 1500 m asl, and likely coexisted with H. erectus, such as at Drimolen, Olduvai, and Koobi Fora [START_REF] Bobe | Hominin diversity and high environmental variability in the Okote Member, Koobi Fora Formation, Kenya[END_REF]. Based on the current study, however, only H. erectus is known to have inhabited the Ethiopian highland, at or above 2000 m asl.

Around 2 Ma (i.e., before the Olduvai/Matuyama boundary at 1.925 Ma), there is a shift toward the production of Acheulean tools, such as handaxes and cleavers produced on large flakes. Previously, only statistical models had suggested such an early date [START_REF] Key | Statistical inference of earlier origins for the first flaked stone technologies[END_REF], while published databased on field-work tentatively pointed to the initial development of this technology at ca.1.8 Ma [START_REF] Duke | Before the Acheulean: The emergence of bifacial shaping at Kokiselei 6 (1.8 Ma), West Turkana, Kenya[END_REF][START_REF] Diez-Martín | The Origin of The Acheulean: The 1.7 Million-Year-Old Site of FLK West, Olduvai Gorge (Tanzania)[END_REF][START_REF] Lepre | An earlier origin for the Acheulian[END_REF][START_REF] Beyene | The characteristics and chronology of the earliest Acheulean at Konso, Ethiopia[END_REF]. The Garba IV D data suggest that the Acheulean actually emerged 200,000 years earlier and that there is a substantial chronological gap between the emergence of the Early Acheulean at Garba IV and in the Rift. Compared to small flake production in the Oldowan, this major step in technological and cultural development implies both the accurate search and management of larger blanks of stone, and the strength to produce significantly larger flakes. Some large flakes were then partially retouched, and others were shaped into more complex tools. Outside Melka Kunture, fully knapped handaxes, with a true bilaterally and bifacial balanced shape, are documented 1.7 Ma at FLK West [START_REF] Diez-Martín | The Origin of The Acheulean: The 1.7 Million-Year-Old Site of FLK West, Olduvai Gorge (Tanzania)[END_REF]. At Melka Kunture, the Early Acheulean was also discovered at Gombore IB, later in age at ca.1.66 Ma (6,30), where a very robust Homo humerus was found in association (30, 31). As at Garba IV D, there were fully-shaped handaxes, which implies that in the earliest phases of the Acheulean the mental templates and skills of bifacial shaping were already developed. The aridification of Africa, which started after 2.8 Ma (32, 33), could have driven hominin groups beyond previously occupied environments. A highland environment, such as that at Garba IV, at an altitude of 2000 m asl and more, exposes individuals to less oxygen, higher exposure to UV rays, more rain and cooler temperatures. Will et al. ( 34) have recently examined the Homo fossil record up to 1 Ma, and show that Bergmann's rule applies, which predicts a larger body size in colder environments. The larger-bodied and larger brained H. erectus was possibly better adapted to the highlands than smaller-bodied hominins such as Australopithecus and H. habilis. Additionally, the vegetation was different from that of the savanna, while some animal species testify of a degree of endemism. The hominins would have needed to acquire knowledge of these novel resources. Information on plant use is not preserved, but the nearby forests would have offered resources such as fruit, nuts, tubers, and roots. Butchery marks have been detected on hippopotamus and bovid bones from Garba IV D (35). The fossil and archaeological evidence from Garba IV indicates that H. erectus could cope with demanding local conditions; eventually changing their way of producing stone tools, and configuring the pieces that define the emergence of the Acheulean technocomplex, as also recorded at Gombore IB. The ca. 2 Ma core and flake assemblages made by Homo erectus at Melka Kunture allow us to put into a new perspective the North African and Asiatic assemblages which, later on, still include only small flakes, without any handaxes or other large tools (36-39). They could be the outcome of an earlier expansion or record an expansion from areas where H. erectus and the Acheulean appeared later than in the Ethiopian highlands. In Sub-Saharan Africa, the early Acheulean of Garba IVD means that Oldowan and Acheulean industries co-existed over at least 300,000 years, which revives the de-bates about who were the makers of core and flake assemblages at lower elevations. At Olduvai (Bed II) and in the Middle Awash the coexistence of different hominins producing different lithic industries has been suggested [START_REF] Diez-Martín | The Origin of The Acheulean: The 1.7 Million-Year-Old Site of FLK West, Olduvai Gorge (Tanzania)[END_REF][START_REF] Schick | Acheulean Industries of the Early and Middle Pleistocene, Middle Awash, Ethiopia[END_REF][START_REF] Leakey | Excavations in Beds I and II, 1960-1963[END_REF]. The hominin fossil record is dominated by the discoveries made at low to middle elevations in the Rift Valley and in the cave deposits of South Africa. The fossil and archaeological remains from Melka Kunture demonstrate that the highlands of East Africa, with a different paleoenvironment, is pivotal to understanding H. erectus behavior. Around 2 Ma, there is evidence at Garba IV of H. erectus retaining behavioral characteristics close to those of H. habilis. It was still producing core and flake assemblages but with different features than typical Oldowan technocomplexes. Then around 1. [START_REF] Deino | Ar/(39)Ar dating of Bed I, Olduvai Gorge, Tanzania, and the chronology of early Pleistocene climate change[END_REF] Ma, the Early Acheulean emerged with archetypal bifacial tools. Between 2 and 1.9 Ma, the Melka Kunture site provides the earliest evidence of H. erectus, who quickly adjusted to a high-altitude environment first producing Oldowan technology and then developing Acheulean technology.

REFERENCES AND NOTES

1 R. Bonnefille, R. T. Melis, M. Mussi, "Variability in the Mountain Environment at Melka Kunture Archaeological Site, Ethiopia, During the Early Pleistocene (~1.7 Ma) and the Mid-Pleistocene Transition (0.9-0.6 Ma)" in The Emergence of the Acheulean in East Africa and Beyond. Contributions in honor of Jean Chavaillon., R. Gallotti

Fig. 2 .

 2 Fig. 2. Garba IV artifacts. (A) Levels E and F (2.00 Ma to 2.06 Ma), Oldowan. Images show obsidian cores (1 and 2) and obsidian pointed tools (3 to 5). (B) Level D (1.95 Ma), Early Acheulean. Images show large retouched flakes shaped on basalt or obsidian (6 to 8) and handaxes shaped on lava, basalt, or obsidian flakes (9 to 12).

Fig. 3 .

 3 Fig. 3. The GAR IVE mandible. Views showing the preserved deciduous molars (dm1 and dm2) and the germs of the permanent teeth (I2, lateral incisor; C, canine; P3 and P4, third and fourth premolar; M1, first molar). The image was modified from (15).

Fig. 4 .

 4 Fig. 4. Analysis of the internal tooth shape of the permanent postcanine dentition. (A) Plot of first and second principal components (PC 1 and PC 2) of mandibular third premolar enamel-dentine junction shape. (B) First and second principal components of mandibular first molar enamel-dentine junction shape. The variation explained by each principal component is shown in parentheses. In both principal components analysis (PCA) plots, GAR IVE is most closely associated with H. erectus. (C) Tooth-size variation (centroid size of enamel-dentine junction ridge landmarks) between the third premolar, fourth premolar, and first molar in GAR IVE and the comparative sample. (D) Surface models of the third premolar, fourth premolar, and first molar in (left) lingual and (right) occlusal view (enamel surface in light gray and enamel-dentine junction surface in dark gray).

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Acknowledgments

We are grateful to the Authority for Research and Conservation of the Cultural Heritage (currently the Ethiopian Heritage Authority) and to the Ministry of Culture and Tourism of the Federal Republic of Ethiopia for the loan and temporary export permit of MK 81 GAR IVE 0043 (27 November 2015 to 26 January 2016) awarded to M. Mussi. For access to comparative fossil specimens (and associated CT scans), we thank the following individuals and institutions: B. Zipfel and S. Jira (Evolutionary Studies Institute, University of the Witwatersrand); E. Mbua and J. Kibii (National Museums of Kenya); A.

Funding: This research has been funded by Università di Roma Sapienza grant C26S133Z4B (M.M.); Università di Roma Sapienza grant SA11715C7C936C01 (M.M.); Ministero degli Affari Esteri e della Cooperazione Internazionale grant ARC-000799 (M.M.); Ministero degli Affari Esteri e della Cooperazione Internazionale grant ARC-001666 (M.M.); European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program grant agreement 819960 (M.M.S.); Fundación Palarq (Spain) grants 2019 (J.P. and M.M.), 2020 (J.P., M.M., E.M.-Q., and S.R.-J.), and 2021 (J.P., M.M., E.M.-Q., and S.R.-J.); Leakey Foundation grant 42137 (E.M.-Q.). Deutscher Akademischer Austauschdienst (DAAD) grant 57552336 (G.B.); Università di Roma Sapienza grant 0044296 (G.B.); CNRS (A.L.C.); and postdoc Xunta de Galicia grant ED481D-2022/023 (E.M.-Q.).

Author contributions: Conceptualization: MM. Methodology: LDB. reopened the excavated area and reidentified the archaeological levels for sampling. GB. collected the faunal teeth enamel samples (sampling and pretreatment). Investigation: RTM, stratigraphy and geology; MMS, TWD, AG, ALC and JJH, analysis of the dental samples; DG paleontology; HB and GB stable isotopes; EMQ, JP and SRJ archaeological analysis; RB paleobotany; ALC analysis of the synchrotron micro-CT scans. Funding acquisition: MM, MMS, JP, EMQ, SRJ, GB and ALC. Project administration: MM, JP, EMQ. Supervision: MM. Writingoriginal draft: MM, with contributions from all authors. Writingreview and editing: MM and EMQ, with contributions from all authors.

Competing interests: Authors declare that they have no competing interests

Data and materials availability: All data supporting the findings of this study are available within the paper and its supplementary materials. Landmark data and R code used to conduct the geometric morphometric analyses used in this study are available on the Publications page of Human Fossil Record.org (https://human-fossil-record.org/index.php?/category/595). The hominin mandible #MK 81 GAR IVE 0043 as well as the palaeontological and archaeological collections from the site are kept at the National Museum in Addis Ababa and available for scientific research. License information: Copyright © 2023 the authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original US government works. https://www.science.org/about/science-licenses-journal-article-reuse