Lise Volkart
email: lise.volkart@unige.ch

Sabrina Girletti

Johanna Gerlach

Jonathan David Mutal

Pierrette Bouillon

Source or Target First? Comparison of Two Post-Editing Strategies with Translation Students

Keywords: post-editing, post-editing strategies, post-editing teaching, machine

We conducted an experiment with translation students to assess the influence of two different postediting (PE) strategies (reading the source segment or the target segment first) on three aspects: PE time, ratio of corrected errors and number of optional modifications per word. Our results showed that the strategy that is adopted has no influence on the PE time or ratio of corrected errors. However, it does have an influence on the number of optional modifications per word. Two other thought-provoking observations emerged from this study: first, the ratio of corrected errors showed that, on average, students correct only half of the Machine Translation (MT) errors, which underlines the need for PE practice. Second, the time logs of the experiment showed that when students are not forced to read the source segment first, they tend to neglect the source segment and almost do monolingual PE. This experiment provides new insight relevant to PE teaching as well as the designing of PE environments.

INTRODUCTION

The importance of post-editing (PE) in the translation industry no longer needs to be demonstrated, as a quick survey of the growing number of PE training options for professional translators is enough to understand the value of this skill. In academia, machine translation (MT) and PE competences are also largely integrated in curricula under the European Master's in Translation (EMT) Competence Framework [EMT Expert Group, 2017]. The content and methods of some PE training sessions and classes are described in academic papers (see for example [START_REF] Koponen | How to teach machine translation post-editing? Experiences from a post-editing course[END_REF] and [START_REF] Doherty | The design and evaluation of a statistical machine translation syllabus for translation students[END_REF]), where standards such as TAUS guidelines [START_REF] Taus | Machine translation post-editing guidelines[END_REF] or the ISO norm on PE1 are often mentioned. However, very few authors give advice on the procedure to be followed when post-editing, i.e. should post-editors read the source or the target first, and does it make any difference? In a survey conducted by [START_REF] Cid | The need for practice in the acquisition of the post-editing skill-set: lessons learned from the industry[END_REF] among PE teachers, half of the respondents confirmed that they do not touch on this issue or give clear-cut advice to their students. A third of the participants recommend reading the source text first, while a fifth recommend starting with the target text. As far as we know, there are no studies that compare the benefits of one strategy or the other. The study presented here aims to provide some preliminary insight into the question. 2 The paper is structured as follows: we start by laying out our motivations and the goals of our study before introducing our experiment and detailing our methodology and results. Finally, we present our conclusions and some prospects for further research.

I MOTIVATIONS AND GOALS

While the source segment (eventually alongside a translation memory suggestion) constitutes the primary source of information for translators, post-editors, in contrast, have the choice to primarily orient their attention toward the source or the MT suggestion (or target text, TT), as underlined by [START_REF] Krings | Repairing Texts: Empirical Investigations of Machine Translation Post-Editing Processes[END_REF]. [START_REF] Nitzke | Problem solving activities in post-editing and translation from scratch: A multi-method study[END_REF] showed that post-editors do not all adopt the same strategy, however several studies indicate that the majority of post-editors tend to look at the target first [START_REF] Carl | The process of post-editing: A pilot study[END_REF][START_REF] Mesa-Lao | Gaze behaviour on source texts: An exploratory study comparing translation and post-editing[END_REF][START_REF] Belam | Buying up to falling down": A deductive approach to teaching post-editing[END_REF]. Furthermore, numerous studies on cognitive processes during translation and/or PE indicate that less attention is paid to the source text (fewer fixations, shorter gaze time) in PE compared to human translation (HT) [START_REF] Carl | The process of post-editing: A pilot study[END_REF][START_REF] Bangalore | The role of syntactic variation in translation and post-editing[END_REF][START_REF] Nitzke | Problem solving activities in post-editing and translation from scratch: A multi-method study[END_REF][START_REF] Mesa-Lao | Gaze behaviour on source texts: An exploratory study comparing translation and post-editing[END_REF][START_REF] Daems | Translationese and post-editese: How comparable is comparable quality?[END_REF]. [START_REF] Carl | The process of post-editing: A pilot study[END_REF], Čulo et al. [2014] and [START_REF] Carl | Why translation is difficult: A corpus-based study of non-literality in post-editing and from-scratch translation[END_REF] have all formulated the hypothesis that this predominance of the MT suggestion/TT might have an influence on the final product through priming or directing effects. Indeed, post-edited texts tend to be more literal, include a high number of typical ST constructions and formal equivalences, and also tend to be closer to the source text than HT [START_REF] Depraetere | What counts as useful advice in a university post-editing training context? Report on a case study[END_REF][START_REF] Čulo | The influence of post-editing on translation strategies[END_REF][START_REF] Martikainen | Ergonomie cognitive de la post-édition de traduction automatique : enjeux pour la qualité des traductions[END_REF]. Furthermore, studies have shown that post-edited texts include more unidiomatic or ungrammatical constructions, especially when post-editors are students [START_REF] Daems | Translationese and post-editese: How comparable is comparable quality?[END_REF][START_REF] Schumacher | Avantages et limites de la post-édition[END_REF]. Student translators tend to be more tolerant towards MT output and are often liable to accept sub-optimal translations [START_REF] Schumacher | Avantages et limites de la post-édition[END_REF][START_REF] Depraetere | What counts as useful advice in a university post-editing training context? Report on a case study[END_REF][START_REF] Carl | Why translation is difficult: A corpus-based study of non-literality in post-editing and from-scratch translation[END_REF][START_REF] Casas | Intégration de la post-édition dans un cours de traduction et revision : étude de cas d'étudiant•e•s à la faculté de traduction et d'interprétation[END_REF]. Finally, a study conducted on bilingual revision (an activity comparable to PE in the sense that the reviser, like the post-editor, has the choice of primarily orienting his/her attention toward the source or target, but attention also seems to be mainly focused on the target text during revision) by [START_REF] Ciobanu | Speech synthesis in the translation revision process: Evidence from error analysis, questionnaire, and eye-tracking[END_REF] showed that revisers produce better quality revisions (especially in terms of correcting accuracy errors) when they listen to the source segment (via speech synthesis) while revising.

Considering the results of the above-mentioned studies, the question of the role of reading order in PE and the potential consequences of a mainly target-oriented PE strategy appears to be worth investigating. We therefore decided to compare PE by students in two different conditions: when they start by reading the source segment (source condition) and when they begin with the target segment (target condition). In doing so, we aimed to observe whether or not adopting a strategy closer to the HT process would influence the amount of corrections made to the MT suggestion and the total PE time. Additionally, we surveyed students after the experiment to gauge their satisfaction with regard to one strategy or the other.

II EXPERIMENT

Methodology

Variables

The experiment we designed aimed to measure the influence of the PE strategy (reading the source or target segment first) on three variables:

• Total PE time per source word;

• Ratio of corrected errors (i.e., number of errors post-edited by participants out of the total number of MT errors in the raw output);

• Number of so-called 'optional modifications' per word (i.e., the number of PE actions performed by students on elements that were not indicated as MT errors).

Our hypotheses are that the ratio of corrected errors and the number of optional modifications per word will be higher in source condition as the students will possibly be less primed by the MT output.

Text

We performed our experiment on a news article in English on a general topic from Times Magazine, which consisted of around 540 source words. The text was translated into French and Italian using DeepL in February 2020 and raw MT output for each language was annotated by a translation professor from the Faculty. Professors annotated MT errors (i.e., errors that students should correct in a PE task) using their usual correction system for HT. 52 errors were identified in the French translation and 42 in the Italian.

Participants

We recruited 20 Master translation students from the University of Geneva to take part in our experiment. 12 of them were native French speakers and the other 8 were Italian native speakers, and all had English in their language combination. Before the experiment, they completed a questionnaire on their experience in PE. It revealed that they all had little to no practical experience, however they had some theoretical knowledge, as they all had taken at least one course in the Faculty that included content on MT and PE. Students were not paid for their participation but they received a voucher as compensation.

Design

The experiment was conducted on the tailor-made PE platform COPECO3 [START_REF] Mutal | A collaborative post-editing corpus in pedagogical context[END_REF].

The platform allows two kinds of PE tasks to be created: a 'source condition' task, where the source segment is displayed by default and the user has to click on a button to display the MT suggestion, and a 'target condition' task, where the MT suggestion is displayed by default and the source segment has to be manually displayed. The platform records the time the user spends on the segment displayed by default before clicking on the button to display its counterpart (socalled 'default reading time'). Figures 1 and2 provide a view of the platform in source and target condition, respectively. Before the experiment, participants were assigned a test task to familiarize themselves with the platform. For each language pair, the text was split into two parts (A and B) and the participants were divided into two groups (1 and 2). The experiment was set up as a cross-over design in which group 1 does the PE of part A in source condition and part B in target condition, while group 2 post-edits part A in target condition and part B in source condition. During the experiment, participants had access to all online resources of their choice, with the exception of MT engines. We recorded the screens of participants during all the tasks in order to be able to observe participant behaviour during the experiment.

Instructions to the participants

Participants were given instructions to perform the two PE tasks with the aim of producing a high-quality translation (or one of publishable quality). They were not given precise information on the goal of the experiment and did not receive any instructions on the reading strategy to adopt (reading source or target first). With this design, our aim was to prompt students to adopt one strategy or the other, rather than providing them with a specific guideline that might influence their PE behaviour. Participants were also asked to fill out a questionnaire after the task in order for us to collect their impressions and comments.

Analysis

Because of the small number of participants, results for both languages were analysed together. We collected the different kinds of data recorded by the platform, as well as the screen recordings. The latter were used to verify whether or not participants responded as expected to the incentive of reading the source or target segment first. Only three participants out of 20 did not behave as expected in source condition and one in target condition. They clicked directly on the display button without spending any time reading the segment displayed by default. We performed our statistical analysis to both include and exclude these participants, but as it did not change the overall tendency of our results, and as we only have a small number of participants, we ultimately decided to include them. We manually counted the number of annotated errors that were post-edited and the number of optional modifications for each text part and each participant. Annotated errors and optional modifications were counted following the principle of single logical edits as described in [START_REF] Blain | Qualitative analysis of post-editing for high quality machine translation[END_REF]. This means that one correction/modification can involve several mechanical actions (insertion, deletion, substitution, etc.) that are interdependent. Figure 3 shows an example of a single logical edit. were counted as one logical edit, as adding the word longue implies adding a comma and the adverb pourtant in order to build a correct sentence.

Results

Main variables

Table 1 presents the results obtained for each variable. The association between the total PE time per word and reading condition was investigated using the U-Mann-Whitney test (as normal distribution is not achieved). The total PE time per word was lower in the target condition, but this difference is not statistically significant (difference = -0.173s; Z = -1.722; p = 0.087). The associations between the ratio of corrected errors and the reading condition, and the number of optional modifications per words and the condition, were investigated using a linear regression model with mixed-effects. A random effect was set on the intercept to account for participant variability. The ratio of corrected errors was not significantly higher in the source condition (difference = +1.2%; 95% CI; -4.7-7.3; p = 0.675), but the number of optional modifications was significantly higher in the source condition (difference = +0.009; 95% CI; 0.0013-0.016; p = 0.021). It is interesting to note that the ratio of corrected errors is relatively low, as the participants corrected on average 50% of the MT errors annotated by the professors. This is in line with the results of previous studies, which show that students tend to be tolerant toward MT output.

It is important to mention that statistical analyses revealed an influence of the text part (A or B) on the ratio of corrected errors and the number of optional modifications. Participants made on average more corrections (difference = +7.6%; 95% CI; 1.5-13.7%; p = 0.014) and more optional modifications (difference = +1.7%; 95% CI; 1.05-2.53%; p = 0.014) to the second part (B) of the text. Thanks to the cross-over design, this aspect had a limited effect on our results.

Other variables

Alongside our three variables, we made an interesting observation on the 'default reading time' (i.e., the time participants spend on the side that is displayed by default before clicking to display the other part of the segment). In the source condition (i.e., when the source is displayed by default), the average default reading time by segment is 4.2 s, which corresponds to an average reading speed of 240 to 300 words per minute (comparable to standard English reading speed according to [START_REF] Brysbaert | How many words do we read per minute? A review and meta-analysis of reading rate[END_REF]), while in the target condition (i.e., when the target is displayed by default), the average default reading time is 57.8 s which is far more than what is needed to read the MT output. The screen recordings revealed that most participants start doing research or begin post-editing the MT output before displaying the source. Some participants even postedit the whole segment and only look at the source when they are done.

Questionnaire

In the post-task questionnaire, participants were asked which task they preferred. The answers were almost perfectly equally distributed, with 10 students indicating a preference for source condition, 9 for target condition and 1 indicating 'none of them'. Finally, we asked them if they thought the condition had influenced their PE behaviour. 11 replied 'yes', 7 said 'no' and 2 'I don't know'. It was interesting to note that a majority of them was conscious that the condition could influence their PE approach.

CONCLUSION AND FUTURE PROSPECTS

Our experiment has shown that the ratio of corrected errors was not significantly influenced by the order in which the source and target are displayed, which contradicts our first hypothesis. However, our results tend to confirm our second hypothesis, as the number of optional modifications was slightly higher in the source condition. These results might give rise to the assumption that the more attention students give to the source text, the more likely they are to deviate from the raw MT output. As for the time spent on the PE tasks, no significant difference was found between the two conditions.

This experiment also produced other interesting results. First, the fact that students spotted just half of the MT mistakes is in line with the results of other studies and emphasises the importance of MT and PE teaching in the translation curriculum. Second, it showed that the display design has a great influence on the post-editor's behaviour: when presented with the MT suggestion first, participants tend to omit the source segment and consult it only after having done a great part of, if not all, the PE. This aspect, even if its implications still need to be further investigated, cannot be ignored when giving PE guidelines to students, preparing specific PE exercises, as well as choosing, setting up and/or designing PE environments. Our results further support the idea already formulated by [START_REF] Moorkens | Assessing user interface needs of post-editors of machine translation[END_REF] that post-editors, who generally work with classical HT interfaces (CAT-tools), would benefit from environments optimized for the specific nature of PE tasks. Those environments could improve PE efficiency by offering, for instance, text-to-speech synthesis of the source text (as investigated by [START_REF] Ciobanu | Benefits of speech technologies for translators and revisers[END_REF]), or different positioning options for the source text and MT suggestions.

Even if our study provided valuable insight into the PE processes of students, we acknowledge that our experiment has some limitations, including the small number of participants and the fact that MT errors were only annotated by one annotator per language.

Figure 1 .

 1 Figure 1. View of the 'source condition'.

Figure 2 .

 2 Figure 2. View of the 'target condition'.

Figure 3 .

 3 Figure 3. Example of an optional modification. Here, three mechanical actions (a word shift and two additions)were counted as one logical edit, as adding the word longue implies adding a comma and the adverb pourtant in order to build a correct sentence.

Table 1 .

 1 Average total time per word, ratio of corrected errors and additional modification per word for each condition. (* indicates that scores are significantly different at p < 0.05).

		Source condition	Target condition
	Total PE time/word	6.596	6.423
	Ratio of corrected errors	0.505	0.500
	Optional modif./words	0.051	*0.043

  This observationconfirms what other studies (see introduction) have shown, which is that the source text tends to be neglected in PE and attention is mainly oriented toward the MT suggestion/target text.

http://jdmdh.episciences.org eISSN 2416-5999, an open-access journal https://doi.org/10.46298/jdmdh.9067

ISO 18587. Translation servicespost-editing of machine translation outputrequirements,

 2 We would like to warmly thank Professor Annarita Felici and Professor Mathilde Fontanet for their participation, as well as the Master students of the faculty who took part in this experiment.

https://copeco.unige.ch


Z P Wang

P M Dinh

P.-G Reinhard

E Suraud

G Bruny

C Montano

S Feil

S Eden

H Abdoul-Carime

B Farizon

M Farizon

S Ouaskit

T D Märk

Microscopic studies of atom-water collisions

Keywords:

The influences of water molecules surrounding biological molecules during irradiation with heavy particles (atoms, ions) are currently a major subject in radiation science on a molecular level. In order to elucidate the underlying complex reaction mechanisms, we have initiated a joint experimental and theoretical investigation with the aim to make direct comparisons between experimental and theoretical results. As a first step, studies of collisions of a water molecule with a neutral projectile (C atom) at high velocities (≥ 0.1 a.u.), and with a charged projectile (proton) at low velocities (≤ 0.1 a.u.) have been studied within the microscopic framework. In particular, time-dependent density functional theory (TDDFT) was applied to the valence electrons and coupled non-adiabatically to molecular dynamics (MD) for ionic cores. Complementary experimental developments have been carried out to study projectile interactions with accelerated (≤ 10 keV) and mass-selected cluster ions. The first size distributions of protonated water cluster ions H + (H2O)n(n = 2 -39) produced using this new apparatus are presented.

I. INTRODUCTION

While the use of ionizing radiation is wellestablished, notably in therapies and analytical techniques, each new development opens a field of investigation around the possible dangers to our health and the environment 1,2 . Concurrently, besides applications and risk evaluations, the effect of ionizing radiation in biomolecular nanosystems is emerging as a major area of research, both on a fundamental level and as a source for experimental and technical innovations. The irradiation of biomolecular nanosystems in the gas phase un- * Dept. Physics and Astronomy, The Open University (OU), Walton hall, Milton Keynes, MK76AA, UK † Université Hassan II-Mohammedia, Facult des Sciences Ben M'Sik (LPMC), B.P.7955 Ben M'Sik, Casablanca, Morocco der single collision conditions represents a significant new subject in radiation science. From an experimental standpoint, it has great potential for the development of new analytical techniques and synthesis techniques. Complementary theoretical progress can provide new descriptions of radiative energy transfer mechanisms on the molecular scale, opening new perspectives for the elucidation of the radiation dose in living systems.

Our principle goal is to investigate microscopic mechanisms responsible for irradiation effects in biological molecules with a particular focus on the influence of water molecules in the environment of these molecules. This is achieved here in a joined experimental and theoretical effort with the aim to make direct comparisons between experimental and theoretical results. The originality of this work lies in the ability to quantify the role of a biomolecule's immediate environment in proton radiation-induced processes through the precise control of the number of associated water Typeset by REVT E X molecules. This is a key aspect from the fundamental physics point of view and it is also crucial to allow realistic and detailed comparisons between experiments and theory. The path towards such an ambitious goal is long and, due to the many elementary processes involved, one needs to validate the various experimental and theoretical aspects. First of all, we consider here the simplest case of a single water molecule interacting with a projectile, which constitutes a prerequisite for any further developments. The next step involving an assemble of water molecules around a biomolecule raises no major theoretical difficulty because the number of molecules can be efficiently controlled. Monitoring this number experimentally constitutes a key issue for further experimental developments and for comparisons with theory.

In previous experiments at the Institut de Physique Nucléaire de Lyon, the ionization of gasphase water molecules by protons 3 and neutral hydrogen 5 impact has been studied in the velocity range of 0.9 -2.5 a.u.(units of the Bohr velocity), coinciding with the Bragg peak maximum for energy deposition by an ion beam in an absorbing medium. Mass-analyzed H 2 O + and fragment ions of the water molecules were detected in coincidence with the projectile following ionization in single collision conditions. The determination of the charge state of the projectile after the collision enabled ionization processes to be separated on the basis of charge transfer between the target molecule and the projectile. Thus it was possible to measure branching ratios and absolute cross sections for water ionization with and without the capture of an electron by the incident proton.

The present experimental challenge is to study interactions between water molecules in a cluster following 0.9-2.5 a.u. proton impact induced excitation/ionization of water prior to dissociation. Accordingly, a new experimental system has been developed to investigate the proton irradiation of mass-selected protonated water clusters. Many different techniques have been used successfully to generate ensembles of water cluster ions including electrospray ionization 6,7 , electron impact ionization 8 , corona discharge ionization 8 , chemical ionization 9 and electrospray droplet impact 10,11 . The novelty of the present experimental system lies in the acceleration (up to 10 kV) and massselection of molecular and cluster ions of water prior to 0.9-2.5 a.u. collisions with protons.

The microscopic description of radiation induced processes requires an explicit dynamical account of electronic degrees of freedom which respond first in the present collisions. Moreover, it is necessary to treat electrons in a non-adiabatic way and to allow for ionization and/or electron transport. This invalidates most calculations based on the Born-Oppenheimer (BO) approximation except in some specific situations. Indeed, depending on the characteristics of the ionizing projectile (charge, velocity), one can treat the problem in a simplified manner by decoupling electronic and ionic dynamics. A typical example is the case of high velocity charged projectiles in which ions can be safely considered as frozen and the dynamics reduced to the electronic response, at least for short times. Another example is the case of low velocity neutral projectiles for which a ground state (BO) treatment is acceptable. To the best of our knowledge, low velocity charged and high velocity neutral projectiles can nevertheless not be treated by the usually available calculations [START_REF] Kohanoff | AIP Conf Proc[END_REF] , and certainly not in the framework of a unique theoretical approach [START_REF] Gaigeot | [END_REF] . Fortunately, in the case of cluster dynamics, several approaches [14][15][16][START_REF] Domps | [END_REF]23 have been developed which consider this question of coupled electronic and ionic dynamics in relation to irradiation by intense laser fields. A coupling of the optically active spot to a large environment can be added in a hierarchical approach 24,25 . In the present work, we have adapted the non-adiabatic approach of Calvayrac et al. 16 , originally developed for metal clusters, to the case of an organic molecule and applied it to realistic irradiation scenarios. The method of Calvayrac et al. 16 contains as limiting cases pure electron dynamics and BO dynamics (Car-Parinello dynamics) and can thus describe collisions with high-velocity charged or low-velocity neutral projectiles. Moreover, at variance with currently available approaches, it enables complementing cases to be described such as low-velocity charged and high-velocity neutral projectiles. Indeed such a non-adiabatic approach places no restriction on the velocity or charge state of the projectile and thus offers an unified picture of many possible irradiation scenarios.

II. EXPERIMENTAL SET-UP

We briefly present the experimental system in this section. The experimental set up will be thor-oughly described elsewhere 4 . All measurements are taken with a two-sector-field mass spectrometer of EB geometry combined with an electronimpact ion source. Neutral clusters are produced by expanding water vapor from a stagnation chamber at a temperature of around 100 • C and a pressure of approximately 1 bar through a pin-hole nozzle (20 µm diameter) into vacuum. In this isentropic expansion, the temperature drops rapidly with increasing distance from the nozzle leading to super-saturation of the water vapor and subsequent clustering. The clusters are ionized by electrons of 50 eV (experimentally available range from about 0 -50 eV) and the pressure in this region is maintained at 2 × 10 -4 mbar by a 1200 L/s oil diffusion pump. The resulting positive ions and cluster ions (charge state q) are immediately extracted from the center of the ion source by a weak penetrating electric field. The extracted ions pass through a skimmer and are accelerated to (6 × q) keV. An Einzel lens and four pairs of deflection plates are used to direct the beam to the focal point of a hemispherical electric sector field with maximum transmission. The electric sector field serves to select ions according to their kinetic energy, thus increasing the energy resolution of the beam. The kinetic-energy resolved ion beam then passes through a magnetic sector field for ion selection according to momentum. Finally, the massselected water cluster ions H + (H 2 O) n are focused into the collision chamber and detected through a 1 mm collimator by a channeltron electron multiplier operated in a counting mode. Further experimental developments are currently being carried out in order to intersect the cluster ion beam with an intense beam of 0.9 -2.5 a.u. protons or a jet of atomic or molecular gas (He, N 2 , etc.).

III. THEORETICAL METHODS

In order to perform microscopic simulations of dynamical processes, we employ time-dependent density functional theory (TDDFT) 17 for the electrons combined with classical molecular dynamics (MD) for the ionic cores. More precisely, the degrees of freedom of the model are the wave functions of valence electrons of the system, {ϕ α (r), α = 1 . . . N el }, and the coordinates of the system's ionic cores, {R I , I = 1 . . . N ion }. In this letter, the various ions can be H + , C 4+ , and O 6+ . One starts from the total energy of the system E tot :

E tot = E kin ({ϕ α }) + E C + E xc +E el,ion (ρ, {R I }) + E ext (ρ, {R I }, t) . (1)
The electronic density is defined as

ρ = α |ϕ α | 2 .
The various contributions to the E tot in Eq. ( 1) are successively the electron kinetic energy, the direct Coulomb energy, the exchange-correlation energy, the coupling energy between electrons and ions, and the energy coming from the interaction of the system with an external time-dependent field (that of a by-passing atom or ion here). The functional E xc used for the exchange-correlation energy is from Perdew and Wang 18 .

Equations of motion are then derived in a standard manner by variation of E tot . On one hand, variation with respect to ϕ * α leads to the (timedependent) Kohn-Sham equations for the singleparticle wave functions,

i ∂ ∂t ϕ α = ĥKS ϕ α , (2a) ĥKS
= p2 2m e + V C + V xc + V PsP + V el ext . (2b)
The coupling between electrons and ionic cores is treated via a pseudopotential V PSP and is detailed below.

On the other hand, Hamiltonian equations of motion for ions, thus treated by classical MD, are derived by variation with respect to R I and P I and read

∂ ∂t R I = P I M I , (3a)
∂ ∂t P I = -∇ RI   I =J Z I Z J e 2 |R I -R J | + N el α=1 ϕ α |V PsP (r -R I )|ϕ α + V ion ext . (3b)
In the last equation, Z I and M I stand for the charge and the mass of ionic core I, and V ion ext the coupling potential of ions with the external field.

Since Eqs. ( 2) and ( 3) are computed and solved simultaneously, both electrons and ions are fully propagated in time. One can recover two limiting cases from this approach : i) pure electronic motion with a frozen ionic geometry [TDDFT only, Eqs. (2)] 12 , and ii) BO dynamics where ions are propagated in time, Eqs. (3), while electrons are in their ground-state (obtained by standard static DFT methods) [START_REF] Kohanoff | AIP Conf Proc[END_REF]19 . The power of our method, coupling TDDFT and MD, is to provide the nonadiabatic dynamics of the system, which can reduce to an adiabatic description if the ionic motion is slow enough to allow electrons to relax in their ground-state.

TDDFT is used at the level of the local-density approximation (LDA) 20,[START_REF] Dreizler | Density Functional Theory: An Approach to the Quantum Many-Body Problem[END_REF] together with an averagedensity self-interaction correction (ADSIC) [START_REF] Legrand | [END_REF] for the Coulomb and the exchange-correlation potentials in Eq. (3b). This allows us to achieve the correct ionization potentials, a feature which is crucial to describe electron emission correctly, especially close to threshold. Time-dependent fields and wave functions are represented on a 3D coordinatespace grid of dimensions 72 × 72 × 64. Electronic wave functions are propagated in time by the timesplitting method 29 . The Poisson equation is evaluated by a fast Fourier technique 30 . Ionic equations of motion are solved using the Verlet algorithm. Absorbing boundary conditions are used to remove outgoing electrons. Thus the total number of electrons N = N (t) decreases time. The number of escaped electrons N esc = N (t = 0) -N (t) is a measure of average ionization. The details of this method are presented elsewhere (e.g., 16,[START_REF] Reinhard | Introduction to Cluster Dynamics[END_REF] ).

The coupling between electrons and ionic cores is achieved by Goedecker-type 28 pseudopotentials consisting of a local part and a non-local one, that is, for a given wave function ϕ α :

V PsP (r) ϕ α (r) = V loc (r) ϕ α (r) + dr ′ V nloc (r, r ′ ) ϕ α (r ′ ), (4a) V loc (r) = - Z I r erf x √ 2 +e -x 2 /2 C 1 + C 2 x 2 , (4b)
V nloc (r, r ′ ) = m Y 0m (r)p(r)h 0 p(r ′ )Y * 0m ( r′ ). (4c)
In the local term, x = r/r loc and erf is the error function; in the non-local part, Y 0,m denotes the spherical harmonic and p(r) the following radial projector

p(r) = √ 2 r 3/2 nloc Γ(3/2) exp - r 2 2r 2 nloc . ( 4d
)
The original parameterizations employ different Gaussian widths r loc and r nloc for each material and contribution. This hampers numerical precision in coordinate space grids. We have refitted new pseudopotential parameters (that is C 1 , C 2 and h 0 ) for the smallest width of 0.412 a 0 in all terms (local and non-local) and for all elements involved in the study. Further details will be given in a forthcoming publication.

IV. FIRST RESULTS

A. Experimental production of size resolved protonated water clusters

Figure 1 shows a typical mass spectrum obtained for the water cluster beam by scanning the magnetic sector field. This spectrum was recorded for an acceleration voltage of 6 kV and for a water vapor pressure and temperature of 1.1 bar and 100 • C in the stagnation chamber, respectively. The water monomer ion peak is not shown in the spectrum as its intensity is too high leading to saturation of the channeltron detector. The observed series of peaks spaced by 18 amu / unit charge is attributed to protonated water clusters comprising n molecules (n = 2 -39). Water cluster ion intensities decrease exponentially (red dashed line in figure 1) as the number of water molecules increases. The other peaks correspond to components arising from the residual gas in the cluster source. The insert in the top panel of figure 1 shows the higher-mass peaks (n = 18 -39) in greater detail. These peaks correspond to H + (H 2 O) n , n = 18 -39, cluster ions. The present size distribution of cluster ions is rather similar to that observed by Castleman and co-workers 9 . The present results are also plotted in the bottom panel of figure 1 on a logarithmic scale. In this case, a clear drop in intensity can be seen for ion production above cluster size n = 21. This is consistent with previous results 9 supporting a magic number (that is, a particularly stable configuration) at this cluster size.

Importantly for our future experiments, the intensity of the mass-analysed cluster ion beams achieved with the present set-up is rather high. For instance, the intensity of the protonated water trimer beam measured through the 1 mm-collimated channeltron is 5000 counts/s. In a next step, these mass selected cluster ion beams will be crossed by an intense proton beam and the resulting collision events will be analyzed using a molecular imaging detection system.

B. Theoretical studies of the irradiation of water molecules

As discussed in section I, the ultimate aim of our combined theoretical and experimental project is a systematic investigation of ) n for n = 1 -3 will be presented elsewhere. The first models developed here will describe the collision of an atom or ion with an H 2 O molecule. As observables, we study the ionization and the motion of the various atoms in the system. In particular, we have explored the two limiting cases of low-velocity neutral and highvelocity charged projectiles for which one can reduce the dynamics to Born-Oppenheimer or pure electronic motion respectively. We have checked that our non-adiabatic method enables us to recover these limiting cases, as expected. In the present studies, we find that non-adiabatic effects do indeed remain negligible which a posteriori validates previous investigations. We thus focus on the cases of low-velocity charged and high-velocity neutral projectiles for which an accurate coupling between ionic and electronic dynamics becomes compulsory. We illustrate the capabilities of our approach on such cases. the impact parameter b = 2.21 a 0 . There is again a strong impact on the H atom closest to the projectile. As for the smaller impact parameter, there is still a significant ionization of about 0.2 charge units. However, the transferred momentum is sufficiently low that the atom remains bound in the molecule. The H atom rotates around the O atom and pushes the other H atom into the same rotation such that the structure of molecule is essentially conserved. Thus, in this case, atom impact has triggered strong rotational motion only. Finally, the lower left panel shows ionization for medium impact parameter b = 2.21 a 0 and two different velocities (0.1 a.u. and 0.2 a.u.). Ionization increases with velocity although the change is surprisingly small. We have here two counteracting influences. The faster atom exposes the molecule ′ s higher frequency components so it can enhance ionization. On the other hand, the time of interaction becomes much shorter leading to a reduced reaction yield. The combination of these effects leads to a rather small velocity dependence in this range. large dipole moment of the water molecule. As a consequence, the projectile trajectories experience bending depending on the projectile velocity. The upper right panel shows the lowest velocity case and it is clear that the bending angle is very high. The water molecule as a whole is accelerated to a translational motion combined with some rotation of the two H atoms relative to the O atom (compare the initial and final configurations shown in the inserts). The lower right panel represents a collision at 0.05 a.u.. The bending of the projectile trajectory and the effect on the motion of the H 2 O molecule are both markedly smaller than observed for v = 0.025 a.u.. Again, the molecule is accelerated to translational motion with some small rotation, but this time in the other direction. The fastest collision is shown in the lower left panel. The much shorter interaction time reduces bend-ing and impact on the molecule. The left upper panel shows ionization for the various impact velocities. The slowest collision, although having the largest effect on molecular motion, produces the least electron emission. Ionization increases with increasing velocity and seems to level off near the largest velocity. Thus, the enhancement by higher frequency components dominates over reduced interaction time in that (low) velocity range.

The theoretical test cases have demonstrated the feasibility of detailed dynamical simulations of water systems excited by various projectiles. We have seen a significant difference in the dynamics triggered by neutral (C atom) versus charged (H + ion) projectiles. There is sizeable impact ionization. The effects on the molecular motion as a whole are large and depend sensitively on the impact condi-tions.

V. CONCLUSION AND OUTLOOK

In this paper, we have demonstrated the ability to produce intense beams of accelerated and size-selected water cluster ions in our newly constructed cluster/atom collision apparatus. The accompanying theoretical studies have demonstrated our ability to explore the dynamics of water systems irradiated by various projectiles. The next steps of the experimental investigation will focus on the effects of irradiation of size-selected water cluster ions. The parallel theoretical investigations are currently being extended to consider irradiation of small singly charged water clusters. In par-ticular, comparisons will be drawn between our experimental and theoretic studies of size-selected water cluster ion collisions with high-velocity protons. Attachment of water molecules to a molecule of biological interest will constitute a further step both from the experimental and theoretical sides.

The experimental capability to control the number of attached water molecules is essential for a direct comparison with theoretical calculations. This opportunity will be exploited in the studies to come.

21 FIG. 1 :

 211 FIG.1: Top: Spectrum of accelerated (6keV) and mass-selected water cluster ions obtained by scanning the magnetic sector field. Bottom: Same in a logarithmic scale.

1 .FIG. 2 :

 12 Figure 2 shows results for a collision of a fast (v = 0.1 -0.2 a.u.) neutral C atom with an H 2 O molecule, for impact parameters b varying between 1.87 and 3.15 a 0 (these values are calculated with respect to the center of mass of the water molecule). The water molecule lies in the scattering plane. The upper left panel shows the time evolution of ionization for impact velocity v=0.1 a.u.=20 a 0 /fs and different b. The sudden jump to N esc ≈ 4 is caused by the C atom leaving the numerical box and carrying its four active electrons away. The net ionization of the water molecule is the difference N esc -4. There is negligible ionization (and excitation) of the water molecule for the largest b (3.15 a 0 ). For b = 2.21 a 0 , however, we see a loss of about 0.2 charge units, whereas for the smallest b (1.87 a 0 ) one full electron is lost. The reason for this becomes clear when we consider the right upper panel showing the trajectories in the scattering plane for the smallest impact parameter. The C atom travels from right to left through the box with negligible deviations from a straight line, first triggering ionization by about 0.2 charge units as shown in the left-hand panel. At the point of closest impact, the C atom hits one of the H atoms and transfers a sufficiently large momentum that the H atom dissociates from the molecule and leaves the box taking the attached 0.8 electrons away. The lower right panel shows the trajectories for

2 .FIG. 3 :

 23 FIG. 3: Modeled collision between a water molecule and a H + ion at low velocities (v = 0.015 -0.1 a.u.) for a fixed impact parameter b = 0.81 a0. Top left panel : number of escaped electrons as a function of time. Other panels : ionic trajectories in the xz plane. The arrows indicate the time evolution; only in the top right panel, the arrows on the water molecule are drawn at same instant. The insert in the bottom left panel shows the initial configuration of H2O for all the calculations, while the other inserts present the final configuration for v = 0.05 a.u. (bottom right) and v = 0.025 a.u. (top right).

  collision induced processes in H + (H 2 O) n cluster cations. To achieve this ambitious goal, it is necessary to validate the employed methods at intermediate steps. As the key building block of H + (H 2 O) n clusters, the neutral H 2 O molecule provides the starting point for our theoretical investigation. Systematic studies of small H + (H 2 O

G. Lauritsch and P.-G. Reinhard, Int. J. Mod. Phys. C 5 (1994) 65

Acknowledgments.

This work was supported by the Deutsche Forschungsgemeinschaft (RE 322/10-1), the Humboldt foundation, a Gay-Lussac prize, Institut Universitaire de France, Agence Nationale de la Recherche (ANR-06-BLAN-0319-02), and the French computational facilities CalMip (Calcul en Midi-Pyrénées), IDRIS and CINES.