Photodesorption induced by UV, X-rays and Electrons on Surfaces – PIXyES

The chosen strategy focuses on the collaborative work carried out by the three project partners. Experiments carried out at the synchrotron will make it possible to measure absolute rates of photodesorption for a large number of molecular species, including small organic molecules, pure and in solids rich in H2O and CO, in the vacuum UV ranges (5-14 eV) and “soft” X-rays (300-800 eV). The same systems will also be studied, by a similar methodology, by the irradiation of high (300-500 eV) and low (5-20 eV) energy electrons, thus making it possible to better constrain the role played by the photoelectrons or secondary electrons in photodesorption processes, and to access intrinsic parameters such as the electron inelastic scattering cross section in ices. In parallel to this activity, the microscopic mechanisms of photodesorption will be studied for simple systems, such as CO solids or mixed CO:N2, both experimentally by a laser method combining VUV pulsed source and detection by REMPI laser ionization, allowing access to the kinetic and internal energy distributions of the desorbed species, and theoretically through molecular dynamics calculations. The comparison between the two approaches will make it possible to understand in particular the phenomena of indirect desorption and energy transfer between an excited molecule and a surface molecule, the importance of which is crucial for better constraining photodesorption in complex systems.

During the first part of the project (2020-2022), a certain number of objectives were achieved. The mechanisms of photodesorption in CO ices have been unveiled, thanks to the combined approach of new laser experiments at LERMA – a VUV laser source based on the frequency mixing of two tunable lasers in a noble gas cell, and a probe method based on resonant multiphoton ionization spectroscopy conducted using a third tunable laser and a mass spectrometer – and theoretical simulations at the PhLAM of molecular dynamics based on the AIMD method. The indirect nature of the phenomenon, never shown by theoretical approaches, but also the distribution of kinetic and internal energy of the desorbates, made it possible to establish the dynamics of the process, triggered by the generation of a molecule in a highly vibrational excited state. The photodesorption yields of «simple« molecular species, such as N2, and more complex ones, such as NH3, HCOOCH3, HCOOH or even CH3CN, have been measured and studied in pure ice and in CO-rich and H2O-rich ices, i.e. in systems mimicking the composition of real interstellar ice. These studies have been carried out in both vacuum UV and soft X, two energy ranges particularly relevant in star and planetary formation regions, through measurement campaigns on the SOLEIL synchrotron. The preponderant role played by the photodissociation of these species has in particular been highlighted during the desorption mechanism, but also, in the case of X-ray photodesorption, the crucial importance of the thermalization of Auger electrons has been demonstrated experimentally. These studies were led jointly by teams from LERMA and ISMO. At the same time, comparative studies of induced desorption, carried out on the same systems, but using electrons as primary particles, were carried out at ISMO. The excellent agreements obtained between the desorption efficiencies per photon or per electron open the way to more systematic comparative studies, which will eventually make it possible either to evaluate X photodesorption efficiencies thanks to measurements by electrons, which are simpler to implement, or to access intrinsic quantities hitherto poorly understood, such as the inelastic scattering cross section of low-energy electrons in ices.

During the second phase of the project, the comparative studies carried out by laser experiments and theoretical simulations will continue, and will focus on binary systems (CO:N2) and systems in which photodissociation can play an important role (H2O) . At the same time, electron-induced desorption experiments will be systematically carried out on systems already studied by photon at the synchrotron. The study of factors, poorly constrained on the mechanism of photodesorption, namely the role played by the flux and the fluence of the radiation, will be the subject of studies carried out on synchrotron, and will allow a better extrapolation of the experimental data towards astrochemical models. Finally, the development of a detector making possible to resolve the distribution of the velocity vectors of desorbed molecules (VMI detector, never applied in the context of molecules desorbed from physisorbed ices) will also be implemented.

R. Basalgète, D. Torres-Diaz, A. Lafosse, L. Amiaud, G. Féraud, P. Jeseck, L. Philippe, X. Michaut, J.-H. Fillion & M. Bertin
Indirect X-ray photodesorption of 15N2 and 13CO from mixed and layered ices
Journal of Chemical Physics 2022 – accepted for publication

J.-H. Fillion, R. Dupuy, G. Féraud, C. Romanzin, L. Philippe, T. Putaud, V. Baglin, R. Cimino, P. Marie-Jeanne, P. Jeseck, X. Michaut & M. Bertin
Vacuum-UV photodesorption from compact amorphous solid water: photon energy, isotopic and temperature effects
ACS Earth and Space Chemistry 6 (2022), 100-115

R. Basalgète, A.J. Ocaña, G. Féraud, C. Romanzin, L. Philippe, X. Michaut, J.-H. Fillion & M. Bertin
Photodesorption of Acetonitrile CH3CN in UV-irradiated Regions of the Interstellar Medium: Experimental Evidence
The Astrophysical Journal 922 (2021), 213

The objective of the PIXyES project is to build up an innovative and performant molecular physics, chemical physics and laboratory astrophysics consortium. The goal is to bring new results on the photo-processing of physisorbed molecular solids – referred to later as molecular ices – which are expected to impact deeply the chemistry of the star and planet formation regions of the interstellar medium. The project will be focused on the desorption processes induced by photons – in the Vacuum UV (7-14 eV) and soft X-ray (0,5-1,5 keV) ranges – and by electrons in condensed molecular systems at very low temperatures. These joined studies, with the help of theory, will give new perspectives on the desorption mechanisms. The challenge is to access thorough molecular-scale data and properties of gas-to-ice photoprocesses which are playing a major role on macroscopic scales in the interstellar medium (e.g. gas-to-ice abundance ratios, snowline locations, ...).
Currently, the quantitative studies on non-thermal desorption from molecular ices are limited to the measures of absolute efficiencies from a limited number of systems inspired by astrophysical systems, and the lack of comprehension on the involved microscopic steps and mechanisms prevents to derive general trends and a priori predictions for realistic astrophysical systems. Due to the ever increasing detection of different gaseous molecules in the cold interstellar medium, the need for photodesorption efficiencies for a large number of species, based on laboratory astrophysics data, becomes critical to understand the observations and modelize the chemistry in space. Future space missions such as the James Webb Space Telescope (JWST), whose launch is expected in the coming years, will further accentuate this need. The current laboratory strategies for estimating photodesorption yields therefore need to be renewed. We propose here a chemical physics approach targeting the complex processes at the origin of desorption.
By identifying and understanding the main desorption mechanisms from chosen condensed molecular ices with state-of-the-art surface science techniques and advanced theoretical simulations, we aim at proposing absolute photodesorption efficiencies to the astrochemical community, applicable to any frozen molecules embedded in H2O ices and CO/CO2 ices – by far the most abundant species in the interstellar ices. Based on previous experimental works, our hypothesis is that the incoming radiations on ices will mainly interact with their major constituents, and that the desorption is a consequence of the energy dissipation into the frozen molecular system eventually leading to a desorption event. If this is confirmed, unveiling the energy dissipation mechanisms will make possible to predict the photodesorption yields from realistic interstellar ices for any molecular constituent, including coadsorbed organics, without having to experimentally study one by one all the possible frozen systems, which, in regards of the number of different molecules nowadays detected in the ISM, would be virtually impossible.
For this, complementary and independent experimental and theoretical approaches will be used in synergy, that are quantitative synchrotron-based UV and X-ray studies of the energy-resolved desorption from model molecular ices, the development of a new experimental laser-based approach implying a VUV laser source and a detection method resolved in kinetic, internal and velocity vector of the desorbates, theoretical modelling of the energy dissipation within binary ices, and systematic comparative experiments between photon-induced and electron-induced desorption.