Christina Herzog

Jean-Marc Pierson

Laurent Lefèvre

Towards Modelling the research in Green IT with Agents

Technology transfer between academia and industry has for long been an important factor for the development of our society. Different organisations mean also different priorities, different points of view and difficulties in working together. Having the same definitions, getting to know the different players, their aims will support the technology and knowledge transfer in order to develop new Green IT technologies. After analyzing the various positions of the different possible participants in proj ects, their different aims but also their common points, this article analyses how to build bridges for a common understanding of innovative projects in the field of Green IT and proposes a start of modelling the relationships with Mul-tiAgent Systems.

Introduction

During the last decades IT is becoming an increasing role in our society. The research has for long been more related to reliability, performance and quality of service than to the ecological impact. Since alarming studies were conducted researchers, govemments, funding organisations and companies started research activities in the direction of Green IT. Based on different existing definitions [19,20], and motivations (social, economie, environmental, etc.) a definition of Green IT is stated, which is for now the basis for this work "Green IT is the environmental and resource saving effort in the IT. The reason for using Green IT may arise from economically or ecologically interests. Actions can affect on the whole lifecycle of information technology-meaningfrom the construction via utilisation through to disposa!. " [START_REF] Herzog | Green IT for Innovation and Innovation for Green IT: The virtuous circle[END_REF].

In [START_REF] Herzog | Green IT for Innovation and Innovation for Green IT: The virtuous circle[END_REF][11], we analyzed the relationships between academia and industry for innovation in the field of Green IT. We focused mainly of the structural and behavioural differences of the two actors, as well as the different possible interactions they can set through research and development projects. Bach agent (companies, researchers, etc.) is having a different access, a different pace and different aims which are making it difficult to have a permanent and an efficient exchange of results.

In this perspective, this article will outline the important findings on the definitions about technology transfers to give common understanding, and will determine the different points of view contextualizing the purpose to the Green IT field. Based on these particular studies, we will elaborate on building multiagent systems to model the inter-relationships.

Technology transfer and mission orientated research

The definition of technology, which is used as the base in this article, covers the knowledge of the appliance of scientific knowledge. Scientific knowledge in the perspective of this work encompasses both empirical and theoretical knowledge; it means all directed processes and methods, including their practical usage and also material artefacts like products, prototypes and software. Technology is the special knowl-edge, know-how, in the sense of instructional knowledge and skills. Occasionally knowledge is divided into know-how, know-why, know-what, and know-who, but simplifying all these definitions are summed up under the definition of know-how [START_REF] Garud | On the Distinction between Know-How, Know-Why, and Know-What[END_REF]. In the economical context knowledge is seen as one factor of production respectively as a resource with an important impact on the technical progress ( e.g. the virtuous circle in the Green IT that we analyzed previously in [START_REF] Herzog | Green IT for Innovation and Innovation for Green IT: The virtuous circle[END_REF]) and the long-term development of companies. The difference to other production factors is that technology can hardly be measured quantitatively compared to raw materials or capital. The evaluation of the technology potential is therefore very difficult. In that sense, it is difficult to mathematically model with equations the potential for technology acceptance by the society, as it was outlined by Kiesling et al. in [START_REF] Kiesling | Agent-based simulation of innovation diffusion: a review[END_REF] in the case of innovation diffusion from the work of Bass et al [START_REF] Bass | A new product growth for model consumer durables[END_REF] and followers. Technology can lose importance in case of exclusive usage and/or when new knowledge is creating new technology, reducing the value of the previous technology. The particularity of Green IT is the pace of development of new ideas and technologies. These are going on very quickly since there are many opportunities where Green IT can be included (Hardware, Software, Cloud Computing, etc.), speeding up the emergence of new technologies and their disappearance. The challenge is to be able to model this pace. Former mathematical models (following the works of [START_REF] Bass | A new product growth for model consumer durables[END_REF]) and the associated Gaussian shaped maturity models may have difficulties to model this pace. Changing the perspective from the generation of technologies towards its usage leads to another useful consideration. The usage of technology suggests linking the term technology to the interaction between scientific/technical perception and the society. This is of parti cul ar importance in the field of Green IT because a general awareness about technical possibilities is missing together with a comprehensive study of the user acceptance level for greener solutions at the priee of potentially degraded quality of services. This section does not lodge claim of a complete classification of interactions of parties in all sorts of technology transfers. This is not feasible as classifications can be done with different parameters (vertical, horizontal, product-oriented, procedure-oriented, infrastructure-oriented transfer, etc.) [START_REF] Brooks | National Science Policy and Technology Transfer[END_REF][START_REF] Auer | Transferunternehmertum. Erfolgreiche Organisation des Technolietransfers. Wiesbaden: Gabier; Deutscher Universitatsverlag[END_REF][START_REF] Corsten | Der nationale Technologietransfer. Formen, Elemente[END_REF]. To examine each interaction in an isolated way has only a limited value for the global point of view. Therefore we decided to focus on two kinds of transfers to reach a profound understanding of the potential interaction of parties. Indeed nonpointed [START_REF] Chakrabarti | Sorne concepts oftechnology transfer. Adoption of innovations in organizational context[END_REF][START_REF] Rogers | Diffusion of innovations[END_REF] and focused transfers are/will be the most profitable ways of technology transfer in Green IT. Reader may refer to [START_REF] Herzog | Link between academia and industry for Green IT[END_REF] for detailed analyses ofthese transfers in Green IT.

To sum-up the key findings, we can outline the following: in non-pointed research, the diffusion of knowledge is passive, and feedback may or may not happen between the provider of technology and its receivers. As examples, large set of white papers, recommendations, leaflets made by industry, groups of interest or more rarely researchers are freely available online in the Green IT, in particular for datacenter construction and operation: EU Code of Conduct for Datacenter (EU CoC), The Green Grid white papers (53 white papers as oftoday), to name a few.

With the focused transfer it is possible that a precise solution is transferred target-oriented to the recipient. In this situation the partners are in an equal position. Mainly the constructor modifies and develops the technology according to the needs of the user. The developer hasan active role as he supports the user in the implementation and in the gainful use. The interactive relation between the partners is resulting in a lot ofwork but also boosts the success of this technology. Similarly, a focused transfer can deliver results openly in the same way as a nonpointed transfer does, through open source software for instance. A crucial observation level is to know if the technology constructor sends implicit or explicit information to selected users ( e.g. meaning a defined and limited circle of potential users, who he has already a relation with).

Analyzes of links in Green IT

In [START_REF] Herzog | Link between academia and industry for Green IT[END_REF] we proposed a thorough analysis ofkeys to contracted research. We will summarize here the main outputs of this research in line with a modelling in terms of multiagent systems.

For innovation oriented companies the academie research represents a fondamental resource of extemal produced technologies. The more the industry is oriented towards novelties on the market and leadership in technology, the more the cooperation with public research institutes is interesting for it. Academie institutes could be considered as the best partners as they have a wide experience in cutting-edge-research but also because their interest is not only focused on one product. They have no business interests, but an interest in independent and fair research -they are impartial! Despite this high synergetic potential academie researcher and industry operate in unconnected systems. In reality there exist a variety of options for arrangements in the cooperation, like patents, licenses, specifie consulting.

Critics on the model

Before moving on to the contextualization to Green IT, it appears necessary to discuss one hypothesis mentioned hereby: Researchers are impartial, and act for common welfare. This idealistic view is naïve especially in a tierce competition for faculty positions and lack of govemment funding. These ambiguous motivations will have to be included in any model.

Duties and responsibilities

Companies are interested in having a green label on their products or that in their name "Green" is appearing. There are many different aspects making IT to Green IT. It starts with the production over the usage, the energy consumption, the cooling towards the recycling. The individual funding enterprises can get from the State, from individual programs are quite high, therefore they are interested in getting the "green label" [20, 21] even ifit is only "green washing". The Industry is more aware ofthis hype because they see the new market for business and for extending their field of activity than researchers who are more interested in long-term research, in a contribution to the welfare of the society. For instance a solution can be tight to a specifie server (typically by a vendor industry) or general enough to encompass a variety of servers and datacenters at various scale.

Approach

For the industry it is important that a perfect solution (e.g. Datacenter Infrastructure Management-DCIM software ready to use) is presented/proposed. Solutions, which are not costly, working immediately and non high-maintenance products are definitely the ideal outcome. Even if with more investments e.g. measuring the load of servers, or thinking about different methods of cooling the companies could achieve better greener solutions they keep in mind the profit and the management is setting the their priorities.

Conversely the Green IT researchers prefer general approaches as their view is broader-they see also the welfare of the society and the interest to improve research-maybe in also other disciplines.

A very known topic concems Server Virtualization. Server virtualization decreases the number of needed actual servers to handle users' requests to services. This saves a large amount of energy. But the system has to be developed, to be studied and to be proved before large investments are made. Measurements are needed, different solutions challenged against synthetic and real workloads. Only universities had the opportunity to invest time and to create a platform to do so in early ages of server virtualization. With additional exchange with industry about their demands the result were achieved more quickly and more precisely: Indeed using real trace workloads is the key for researchers to prove their results. Unfortunately these workload traces are often kept secret and not shared by company without contracted research and non-disclosure agreement (NDA).

Criteria of efficiency

Green IT must give more value, in terms of money to the company ( e.g. stock options must go up, more possibilities for getting new contracts ). Green IT is only interesting if there are tax advantages, if it helps to save money immediately and directly-companies often forget the impact of electricity consumption and the costs for electricity; as costs for energy are starting to make a point of discussion (not like costs for stuff) Green IT has to provide clear numbers how much costs can be saved in using Green IT.

For researchers the only criteria of efficiency is the scientific reputation, i.e. the number of published papers (and not research results). There are few conferences dedicated not only to technical Green ICT including other overlapping research topics like management or business process, where researchers can confront their ideas with industry players. Conversely industry forums are not regularly inviting researchers talks. It is mandatory to publish paper for researchers in well established conference or journals, and to reach at the same time a large community interested in energy saving. This criteria in the field of efficiency shows totally divergent points of view, and may result in difficulties in the trans fer of technology and building up a long lasting cooperation.

Freedom of Action

The research institutes are opened conceming new approaches, new ideas. The researchers are free in their decision which research to bring forward and where to put the focus but they have limits through resources like stuff, funding. Many open public calls, where Green IT is present, are limited to iterative research even if Green IT would allow breakthrough research. This effect may lead to difficulties between researchers and the management as the company solda certain result to one of the clients and is not able to change the contract, even if the solution proposed by researchers having better results in saving energy.

Organizational differences

What is clearly an advantage of the researchers is that they have existing cooperation with other research units in their university, while industry partner often have only one major experience or one major activity. Let's take the example of cooling in server rooms. It may appear that a company is able to build energy efficient servers but doesn't have the experience in modelling the airflow in a data center, which is another expertise. It belongs to another industry player for small companies, another department in large groups, can be simply taken from recommendations or best practices developed in the past, or develop with the help of academie partners. Researchers do have workshops on a large field of research where they exchange with each other, where they leam from other as well. Academies are linked to other fields of research and they are not in concurrence if they are in different research fields.

Towards multiagent systems

Following the current trend in modelling the distributed interactions with multiagent systems, in particular in the field of innovation diffusion [START_REF] Kiesling | Agent-based simulation of innovation diffusion: a review[END_REF], we propose in this section a first step for modelling the technological transfer between researchers and industry and its impact for society.

In this initial modelling, we consider 4 kinds of agents: Green IT researcher, IT researcher, general researcher, society.

The technology developed and used by Green IT researcher has to till the gap to IT researchers, researchers and the society. The valuation and the development of the links between these agents has to be different to mo del the difference in Green IT ac cep tance and usage. For instance, an IT researcher might be more influenced by Green IT researchers on their usage of IT equipments than a researcher in another field. However IT researchers have a stronger influence in general on the society (since they represent older links with society) than Green IT researchers (who are less numerous and less seen). This means that the indirect influence of Green IT researchers towards society benefit from their direct influence towards IT researchers. The model will help to figure out how to increase the direct link.

For non-pointed research, given the passive diffusion of information, each agent influences regularly the adoption of Green IT technologies, knowingly or unknowingly. This means that the valuation of the interactions must be dynamic and reactive to possible feedback. For focused research, the Green IT researchers will influence directly and more strongly (higher valuation) only researchers (other Green IT, IT and general) through contracts. In that scheme the impact on society will only be indirect. It can be noted that the direct link between the IT researcher and the society is directly influenced by its link with the Green IT researchers. Also, focused research may produce freely available results as for instance open source software, that may influence directly the society, creating a loose link between Green IT researchers and the society. This link strengthens indirectly the link between IT researchers and Green IT researchers (through Word ofMouth dissemination or facing the fact that such solutions exist).

Another important aspect is the network topology between the agents. As stated before, the research may or may not be spread out towards a selected number of earl y adopters. The impact of the topology of the links between the agents has been shown [START_REF] Bass | A new product growth for model consumer durables[END_REF] in a number ofprevious works as very influential on the final spread of the product. Small world and scale-free topologies are the most common ways of interactions used by the existing studies.

It is obvious that the interests of industry and the rest of the society are different. One could argue to separate the "society" agent between industry and others related agents. For industry, as it was noted in 3.2, the interests in Green IT is also influenced by the society itself (the Green labelling). Conversely the society is increasing its green interests after actions of the industry. Taking into account the different interests between industry and researchers this may lead to a loose link between them, as for example in their approach (see 3.3). This involves that links between agents are not only based on contracted research but also on looser links, for instance social links. Nevertheless it might be the case that the needs in certain areas may strengthen the cooperation, for instance when the law has to be enforced or incentives through funding is available to the benefits ofboth agents. As such the criteria of efficiency (see 3.4) is creating generally loose links between agents. This criteria of efficiency is also advocating for the need to consider Green IT researchers and industry players as different selfish agents that will evolve independently while trying to push other agents in their direction directly ( see for instance part 3.1 ). The criteria of freedom of action (see 3.5) is showing the volatility of the research by Green IT researchers, i.e. they could easily move in and out one research topic without notice. In terms of multiagent systems, the related agents can disappear (or appear) at a faster pace than industry play ers.

From 3.6, we outline that the granularity at which we model the different agents is important. For instance an university partner is a collaboration can be split between administrative and research agents. For an industry agent, the different IT groups can be split also to different agents who influence each other.

It must be understood that this work represents only the first ideas towards a more complete modelling of interactions in the field of Green IT.

Conclusion

Green IT is a rather young research area and therefore there are still a lot oftopics to explore. Energy saving, protection of resources, recycling, limited possibility in the utilization of technology are catchwords which are accompanying our society. Green IT has been recently embraced in different manners by academie researchers and industrialleaders. This should happen in a strong exchange with researchers of different fields but also in including companies, even if they are not at the first sight in the IT field. It is nec-essary to enforce the cooperation between academia and industry to ad vance in the Green IT for our society. Pointing out sorne different objectives, we are convinced that there will be a better understanding between the partners, but also that there will be a change in the cooperation and in the transfer ofknowledge due to the better understanding between industry and researchers but also there will be a change in the different funding systems.

The multiagent systems modelling appears to give the appropriate framework in order to better understand the links. Beyond what is presented in this paper, we still have to analyse a number of links, e.g. the importance of TTO (Technology Transfer Office) and the funding schemes, probably adding new kind of agents in the system. Also, the explicit objectives of this modelling system has to be described, and the actuai influence rules (in terms of deterministic mathematics, probability, etc, ... ) has to be developed. Multiagent systems will help the decision makers about strategies about their involvement in Green IT. This will constitute our main future work.

IRIT, University Paul Sabatier Toulouse 3, CNRS, France email: herzog@irit.fr, pierson@irit.fr

INRIA, Ecole Normale Supérieure de Lyon, Université de Lyon, France email: laurent.lefevre@inria.fr


Jean Jouve
email: jean.jouve@inria.fr

Victor Romero

Rahul Narain

Theodore Kim

Florence Bertails-Descoubes

Characterisation of the Fracture and Healing of a Feather

Keywords: Fracture Healing, Ductile Fracture, Elasticity

Feathers are composed of a central shaft, the rachis, from which hundreds of fibers called barbs branch out. The distinctive surface of a feather is composed of these barbs attached to each other by a myriad of micro-metric spring-like fibres known as barbules. This multi-scale architecture grants feathers peculiar properties essential to the flight of birds. Under loading, the feather surface follows an elastic response until barbules start to disengage, yielding tearing of the surface. In contrast to fracture in solids, a crack in a feather propagates along a predetermined path aligned with the barbs, and can be repaired by carefully pushing the barbs back together [START_REF] Zhang | Repairable cascaded slide-lock system endows bird feathers with tear-resistance and superdurability[END_REF]. Modelling such an interesting reversible mechanical behaviour is of great importance not only to gain a better understanding of feathers, but also to design new meta-materials with healing properties.

In our work, we first conduct tensile and shear tests on rectangular patches cut from real feathers in order to characterise the behaviour of a feather, as shown in Figure 1. We do not take into account the rachis as we consider that the local mechanical response is governed by the interplay between barbs and barbules. From this geometrical and mechanical characterisation, we then develop a simple numerical model at the scale of the feather surface, by taking inspiration from thin sheet simulators possessing fracture capabilities, as in [START_REF] Pfaff | Adaptive Tearing and Cracking of Thin Sheets[END_REF]. To model the healing process, we add a repair mechanism to the simulator. We furthermore account for a real barb distribution projected onto the surface to enforce realistic anisotropy in the elastic, fracturing, and healing behaviours of the surface. We finally validate our numerical model against various experiments.

Fig. 1 :

 1 Fig. 1: Mechanical tests on real feathers. Left, experimental uni-axial traction loading curve. Right, experimental shear loading curve.


A Mukherjee

A Momeni

A R Allouche

E M Staicu Casagrande

T Minea

H Khemliche
email: hocine.khemliche@universite-paris-saclay.fr

Grazing Incidence Fast Atom Diffraction in high-pressure conditions

Keywords: Surface structure analysis, Grazing Incidence Fast Atom Diffraction, Thin film growth, Atom-surface interactions, Decoherence

Grazing Incidence Fast Atom Diffraction (GIFAD) is a recent technique for characterizing surface structures and real-time monitoring of thin film growth. Up to now, GIFAD has only been used in Ultra-High-Vacuum conditions, typically in the range of 10 -10 to 10 -8 mbar, and has therefore only been considered for high vacuum deposition methods like Molecular Beam Epitaxy or very lowpressure Chemical Vapor Deposition (CVD). At pressures exceeding 10 -6 mbar, gas phase collisions along the atom beam trajectory not only reduce the mean free path but also degrade the beam coherence length and thus potentially suppress the diffraction signal. In addition, pressures lower than 10 -5 mbar are required to maintain a low noise level on the scattered particle detector. In a new configuration, we demonstrate that GIFAD can operate at pressure as high as 10 -2 mbar of argon with well-contrasted diffraction patterns. This opens wide avenues for the study of surface reactivity, thin film growth in Magnetron Sputtering Deposition, where electron diffraction is inevitably perturbed by the electromagnetic fields. This High-Pressure version of GIFAD could also be extended to Reactive Pulsed Laser Deposition and many CVD variants.

Introduction

Atom beam diffraction at thermal energies has been used in many branches of surface science for decades to study atom surface interactions, surface structure, nucleation and growth of thin films [START_REF] Farías | Atomic beam diffraction from solid surfaces[END_REF].

Despite its many advantages, this diffraction regime has seldom been used for real-time analysis due to its slowness since the detection of the scattered atoms is performed point by point in space [START_REF] Cardillo | Molecular-beam apparatus for the study of gas-surface interactions[END_REF].

Grazing Incidence Fast Atom Diffraction (GIFAD), the high-energy counterpart, has developed as an effective and complementary method for both surface analysis and thin film growth monitoring. Due to the grazing trajectory of energetic atoms, the parallel and perpendicular motions can be decoupled when considering a flat surface, and the diffraction process can be understood by considering only the slow motion, normal to the surface, which is then similar to the incidence of thermal energy atoms.

For both thermal and fast atoms, the diffraction patterns essentially provide information about the Potential Energy Surface (PES) [START_REF] Winter | Fast atom diffraction at surfaces[END_REF][START_REF] Bocan | Anomalous KCl(001) Surface Corrugation from Fast He Diffraction at Very Grazing Incidence[END_REF][START_REF] Bocan | GIFAD for He/KCl(001). Structure in the pattern for <110> incidence as a measure of the projectile-cation interaction[END_REF]. However, the grazing incidence geometry in GIFAD strongly reduces decoherence due to thermal vibrations of the surface atoms [START_REF] Rousseau | Surface analysis with grazing incidence fast atom diffraction (GIFAD)[END_REF], therefore much higher normal energies can be achieved. In most situations, the normal energy is much larger than the potential well, which is typically 5-15 meV for He atoms [START_REF] Bocan | Dynamical effects as a window into van der Waals interactions in grazing-incidence fast He-atom diffraction from KCl(001)[END_REF][START_REF] Debiossac | Combined experimental and theoretical study of fast atom diffraction on the β2(2×4) reconstructed GaAs(001) surface[END_REF], the weak attractive contribution to the PES can then be neglected, so the PES can be considered equivalent to the intrinsic electron density profile of the surface [START_REF] Esbjerg | Dependence of the He-Scattering Potential at Surfaces on the Surface-Electron-Density Profile[END_REF]. The de Broglie wavelength of an atom with mass m and a velocity v is dB = h / mv. For He atoms with 1 keV energy dB = 0.45 pm. To observe diffraction, the transverse coherence width (i.e. width of the associated wave packet) Lc  dB /  [START_REF] Frankl | Comments on ''Coherence legth and/or Transfer width?[END_REF] where θ is the beam divergence angle, needs to be larger than twice the projected crystal period (d) as depicted in Fig. 1. This condition imposes a stringent limit on the beam divergence, with a typical value below 1 mrad.

For comparison, the angle between two consecutive diffraction orders (Bragg angle) for 1 keV He atoms falls in the range of 1 to 2 mrad for most inorganic materials. GIFAD was first observed on insulator surfaces [START_REF] Rousseau | Quantum Scattering of Fast Atoms and Molecules on Surfaces[END_REF][START_REF] Schüller | Diffraction of Fast Atomic Projectiles during Grazing Scattering from a LiF(001) Surface[END_REF] and subsequently on metals [START_REF] Bundaleski | Grazing incidence diffraction of keV helium atoms on a Ag(110) surface[END_REF][START_REF] Busch | Fast atom diffraction at metal surface[END_REF] and semiconductors [START_REF] Debiossac | Combined experimental and theoretical study of fast atom diffraction on the β2(2×4) reconstructed GaAs(001) surface[END_REF][START_REF] Khemliche | Grazing incidence fast atom diffraction: An innovative approach to surface structure analysis[END_REF]. With He atoms as a probe, GIFAD has also shown an effectiveness in providing detailed real-time information on the growth and organization dynamics of semiconductors [START_REF] Atkinson | Dynamic grazing incidence fast atom diffraction during molecular beam epitaxial growth of GaAs[END_REF] and ultrathin organic films [START_REF] Momeni | Ultrafast Crystallization Dynamics at an Organic-Inorganic Interface Revealed in Real Time by Grazing Incidence Fast Atom Diffraction[END_REF] thanks to its non-destructive character.

All previous studies were performed in Ultra High Vacuum (UHV) conditions. However, most methods for growing thin films operate at pressures well beyond 10 -6 mbar. Among these, Magnetron Sputter Deposition (MSD) typically operates between 10 -3 and 10 -2 mbar [START_REF] Gudmundsson | Physics and technology of magnetron sputtering discharges[END_REF]; reactive Pulsed Laser deposition operates up to 0.1 mbar [START_REF] Bílková | Reactive pulsed laser deposition of zinc oxide thin films[END_REF], and CVD pressures span from UHV to the atmosphere [START_REF] Sun | Photocatalytic TiO2 films prepared by chemical vapor deposition at atmosphere pressure[END_REF][START_REF] Mueller | Growing graphene on polycrystalline copper foils by ultra-high vacuum chemical vapor deposition[END_REF].

Possible operation of GIFAD at higher pressures raises the central question regarding the subsequent coherence length. Elastic collisions eventually produce broadening of the beam that, as described above, could be detrimental to observation of diffraction. This paper demonstrates that the beam coherence can be preserved by the careful design of a double differential pumping system. Well-resolved diffraction patterns from LiF (001) can be observed at argon pressures up to 10 -2 mbar.

Methods

A schematic of the scattering chamber is shown in Fig. 2. The production, collimation and detection of the neutral He beam is similar to that described in previous publications, e.g. [START_REF] Bocan | Anomalous KCl(001) Surface Corrugation from Fast He Diffraction at Very Grazing Incidence[END_REF]. The He 0 beam was produced by neutralizing a He + beam in a gas cell and further collimated by two apertures, the first one with size 400×60 µm and the second one with a diameter of 150 µm. The scattered atoms are collected on a detector composed of microchannel plates and a phosphor screen; the diffraction images are further captured by a CCD camera located on the air side.

The major evolution is the differential pumping configuration. Here the second beam collimation aperture (150 µm) also serves to reduce the gas leakage between the atom gun and the scattering chamber. To reduce the noise level on the detector, the pressure in the detection volume must remain in the 10 -6 mbar range, despite the 10 -2 mbar in the scattering chamber. Therefore, the challenge was to find an effective compromise that considerably reduces the conductance between the scattering and detection volumes while collecting both the direct and scattered beams. We shall recall that collection of the direct beam is essential for an accurate determination of the normal energy and subsequently for the corresponding equipotential line of the PES. The solution consists in a series of four apertures, with dimensions from 4 to 10 mm, set close to the sample. In this configuration, the effective length of the high pressure region is reduced to l2 = 65 mm ≅ 2l1 (Fig. 2). At an argon gas pressure P = 10 -2 mbar, the mean free path lMFP of thermal He atoms is in the range of a cm. For fast He atoms, the cross section σ for elastic scattering on Ar has been measured from 40-850 eV [START_REF] Ruzic | Total scattering cross sections and interatomic potentials for neutral hydrogen and helium on some noble gases[END_REF], so lMFP can be expressed as:

lMFP = kT / Pσ (1)
From a rough extrapolation on the measured cross-sections to 1 keV, we set σ  4×10 -16 cm 2 , which yields a mean free path of 10 cm at 10 -2 mbar. Increasing l2 would result in a strong beam attenuation; the chosen value is therefore a reasonable compromise that lets sufficient space for the sample holder.

The experiment is performed with a LiF(001) sample that is cleaved in air and immediately transferred to the UHV chamber on a five-axis manipulator. A clean surface is achieved by annealing around 300°C for a few hours. He atoms at 1 keV energy where scattered at an angle between 0.76° and 0.78° from the sample maintained at room temperature. Argon gas with 99.999% purity is introduced into the chamber using a leak valve; at the highest pressure used in the experiment (10 -2 mbar), the mean signal/noise ratio on the detector remained at the acceptable value of ⁓25:1.

Results and Discussions

The experimental diffraction images measured at different pressures along the <100> and <110> directions are shown in Fig. 3a-d diffraction signal measured at 3.6×10 -3 mbar shows an intensity loss by a factor of 3.5 with respect to the signal measured at the base pressure.

It is worth comparing the pressure dependence of the attenuation factor for both the direct beam, where the sample is removed from the incident beam path, and for the scattered beam; these are shown respectively in Fig. 4a and4b. The intensity loss of the direct beam can only be related to the gas-phase collisions between He and Ar atoms. As expected, the direct beam follows a single exponential decay according to the Beer-Lambert [START_REF] Dehmer | Absolute total scattering cross sections for 7 Li on He, Ne, Kr, and Xe[END_REF] principle

I = I0 exp (-nσl2) ( 2
)
where I0 is the intensity measured without gas, n is the number density of the argon gas and l2 is the path length traversed by the beam. We used a modified equation to fit the data as follows,

I = I0 exp (-p/ps) (3)
Where ps = kT/σl2 ; k and T are the Boltzmann constant and absolute temperature respectively. The parameter ps can be obtained from fitting the experimental data, and one can then derive the scattering cross section. We find ps =3.9×10 -3 mbar, leading to σ =1.5×10 -15 cm 2 assuming a fast pressure drop at the entrance and exit tubes apertures.

Contrary to the direct beam intensity, the decay of the scattered beam can no longer be fitted by a single exponential function as shown in Fig. 4b, but fits rather well with a bi-exponential curve (see the semi-log graph in the inset of Fig. 4b), indicating two regimes of scattering. A plausible explanation points to an additional scattering contribution from species (H2O, CO2, etc.) adsorbed on the sample surface due to contamination of the injected gas. This explanation is confirmed by the fact that the scattered beam intensity does not fully recover when the pressure is brought back to its initial value. The decay parameters obtained from the bi-exponential fit yield ps1 = 4.2×10 -3 mbar and ps2 =7.16×10 -5 mbar. One can note that, ps1  ps demonstrating that both direct and scattered beam experience similar scattering process from the surrounding argon gas.

At the highest pressure investigated, 10 -2 mbar, a clear diffraction pattern is still observed, indicating that the collision-induced broadening of the direct beam, along the path preceding the surface interaction (l1), does not significantly reduce the transverse coherence length. The divergence angle of the direct beam along the full beam path l2, shown in Fig. 4c, remains nearly constant up to ⁓5x10 -4 mbar and then increases quasi-exponentially. The divergence angle is derived from the beam diameter at an intensity of 1/e 2 of the peak amplitude. At 10 -2 mbar pressure and based on the quasi-exponential behavior, the actual beam divergence at the sample position is that measured at 5×10 -3 mbar, so only ⁓0.37 mrad as compared to the value of 0.32 mrad measured without the Ar gas. This weak degradation of the beam divergence explains the persistence of a well-contrasted diffraction pattern.

Conclusions and perspectives

We have demonstrated that grazing incidence diffraction of fast He atoms from a clean crystalline surface can be observed at surrounding argon gas pressures as high as 10 -2 mbar. Notwithstanding the fact that, the direct beam intensity has lost 90% of its intensity at this pressure, and that the available differential cross-section for He scattering on Ar [START_REF] Gao | Absolute differential cross sections for small-angle elastic scattering in helium -rare-gas collisions at kev energies[END_REF] drops by less than a factor of 3 in amplitude between zero and 0.05° scattering angles, the coherence width remains sufficient for preserving diffraction. Interestingly, calculations performed by Karlovets et al. [START_REF] Karlovets | Scattering of wave packets on atoms in the Born approximation[END_REF] on the elastic scattering of an electron wavepacket on a hydrogen atom show a clear effect of the wave packet transverse size (i.e.

transverse coherence width) of the differential cross-section; large angle contributions to the later decreases with increasing transverse coherence width. These results have been further confirmed by Sarkadi et al. [START_REF] Sarkadi | Loss of wave-packet coherence in ion-atom collisions[END_REF] on the scattering of 75 keV protons on atomic hydrogen. Although a detailed analysis on the effect of beam coherence on the final beam divergence is well beyond the scope of this work, high beam collimation conditions seem to represent a protection against decoherence.

This new high-pressure GIFAD configuration, which we name HP-GIFAD, offers many opportunities for the study of surface reactivity and should also represents an alternative diffraction tool for monitoring thin film growth in high-pressure conditions. For surface structure characterization, the He probe has been selected for its inertness; this key property is also favorable in limiting gas phase interactions with the surroundings species.

Real-time monitoring during thin film growth has always been a challenge. Reflection High Energy Electron Diffraction (RHEED) showed powerful in monitoring film growth in Molecular Beam

Epitaxy chambers [START_REF] Mccray | MBE deserves a place in the history books[END_REF][START_REF] Braun | Applied RHEED: Reflection High-Energy Electron Diffraction during crystal growth, 1 st Edition[END_REF]. In its high pressure version, RHEED has seen its use extended to other deposition modes, such as Pulsed Laser Deposition [START_REF] Blank | In-situ monitoring by reflective high energy electron diffraction during pulsed laser deposition[END_REF] or Atomic Layer Deposition [START_REF] Bankras | In situ reflective high-energy electron diffraction analysis during the initial stage of a trimethylaluminum/water ALD process[END_REF]. In a layerby-layer growth mode, RHEED provides a direct information of film thickness through oscillations of the reflected intensity [START_REF] Lehmpfuhl | Interpretation of RHEED oscillations during MBE growth[END_REF]. Despite some complex attempts [START_REF] Podkaminer | Real-time and in situ monitoring of sputter deposition with RHEED for atomic layer controlled growth[END_REF], application of RHEED in MSD suffers severe limitations because of the stray electromagnetic fields. HP-GIFAD may thus offer an alternative solution for accessing film growth properties (growth mode, crystalline structure, defect density). Due to the extreme GIFAD sensitivity to the surface defects, it should provide valuable insight into the influence of plasma parameters on the growing film. However, substrate contamination plays a pivotal role in thin film depositions, and the quality of the film is also affected.

So, the UHV approach (leak-free system, baking, selection of materials, etc.) is the best solution to keep the substrate as well as the growing layer free from contamination, this scheme naturally implies the use of a high-purity process gas. The benefit might be decisive for the more challenging case of HiPIMS (High Power Impulse Magnetron Sputtering), a recent variant of MSD [START_REF] Gudmundsson | High power impulse magnetron sputtering discharge[END_REF]. HiPIMS produces better films with greater density and crystallinity but its development is hindered the complexity related to the large number of adjustable parameters (pulse power, pulse duration, duty

Fig. 1 .

 1 Fig. 1. Schematic of the diffraction condition of a wave packet of width L c interacting with a surface.

  . The lowest pressures, 10 -8 mbar and 5.6×10 -9 mbar respectively, correspond to the base vacuum level. The diffraction spectra for the <100> direction, obtained by projecting the intensity contained in between the two Laue circles of polar radius 0.78°±0.05° (which corresponds to a normal energy of 185 meV±23 meV), are shown in Fig.3e. At first glance, the

Fig. 2 .

 2 Fig. 2. Schematic of high-pressure GIFAD setup. T in and T out are the entrance and exit tubes having diameters 150 µm and 4 mm respectively. The separation between T in and T out and T in and sample are l 2 and l 1 respectively. LiF(001) sample is shown in blue color. l2 = 2l1 = 65 mm.

Fig. 3 .

 3 Fig. 3. He atom diffraction along the <100> direction from LiF(001) surface at (a) 10 -8 mbar and (b) 3.6×10 -3

Fig. 4 .

 4 Fig. 4. Intensity decay of (a) the direct beam and (b) the scattered beam along the <100> direction; the semi-log plots are shown in the insets. (c) pressure dependence of the direct beam divergence angle, the dashed line is to guide the eye.

cycle, argon gas pressure, oxygen pressure for reactive sputtering, substrate temperature, ion acceleration). We suggest that HP-GIFAD could be very effective in precisely controlling the growth process in HiPIMS.

Acknowledgments

This work has been partially funded by 'Investissements d´Avenir', LabEx PALM (ANR-10-LABX-0039-PALM).

We are grateful to Hynd Remita from the Institut de Chimie Physique (LCP, UMR 8000) for the irradiation of the LiF samples.

We also acknowledge Sylvain Lupone for his technical assistance.

Data availability

No data was used for the research described in this article.

Declaration of competing interest

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

CRediT authorship contribution statement