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Reference : UMR8214-ANNLAF-001
Workplace : ORSAY
Date of publication : Tuesday, April 13, 2021
Scientific Responsible name : Anne LAFOSSE
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 September 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
Non-thermal desorption from model molecular ices – towards quantitative astrochemical data
Chemistry in the interstellar medium (ISM) is taking place in the gas phase as well as on the surface of interstellar dust grains, where films of physisorbed molecules build up. These ices are processed by photons and cosmic-rays, and unavoidably by the bunches of secondary low-energy electrons produced by these radiations. This chemistry feeds the medium with new molecular species. Desorption is the key step in this interplay between gas phase, solid phase and radiations, and is central in the interpretation of the most recent observational data obtained by the last generation of telescopes (ALMA, NOEMA, future JWST). It can be thermally (induced by heating) or non-thermally induced by radiations, or it can result from initiated exothermic chemical processes. The proposed studies will be dedicated to non-thermal desorption processes induced by electrons (high energy typical for Auger electrons ~500 eV, low-energy typical for secondary electrons < 20 eV) and photons (soft X-rays) from thin physisorbed films, consisting of molecules embedded in a H2O/CO2 matrix, most often observed in the ISM. How does the electronic excitation of the matrix molecules do participate to the non-thermal desorption? To which extend do the Auger electrons (vs secondary low-energy electrons) resulting from the X-ray irradiation drive the non-thermal desorption? Answers to these physical-chemical questions will ultimately lead to a better understanding of the non-thermal desorption in the planet-forming regions of the ISM, namely the protoplanetary disks. They will allow a proper modeling of X-ray and electron induced processes in these media, so far poorly constrained experimentally, and provide ways for understanding their still unexplained chemical richness.
Model molecular ices (H2O and CO2 matrices doped with observed light organic species like methanol CH3OH, ammonia NH3, formic acid HCOOH…) will be deposited on gold at low temperature (< 25 K). Combined vibrational spectroscopy (IRRAS / HREELS) and Temperature Programmed Desorption (TPD) analysis will provide insights in the morphology and thickness of the deposited ices. Non-thermal desorption will be monitored by Mass Spectrometry in order to identify and quantify the involved species. Desorption under photon irradiation in the soft X-ray range will be studied in the SPICES setup (LERMA, Sorbonne Université), which will be installed at the end of a synchrotron beamline [Basalgète A&A 2021]. Desorption under electron irradiation will be studied in the “electrons-solids” setup (ISMO, Université Paris-Saclay) [Sala PhD 2018]. The ultimate aims are: (i) to disentangle the elementary processes at work at the molecular scale, (ii) to determine quantitative desorption rates of neutral species (in molecules per incident electron/photon) and (ii) to access effective cross sections for selected processes.
The PhD student will be enrolled in the Doctoral School 2MIB, "Chemical Sciences: Molecules, Materials, Instrumentation and Biosystems", of the University of Paris Saclay (https://www.universite-paris-saclay.fr/ecoles-doctorales/sciences-chimiques-molecules-materiaux-instrumentation-et-biosystemes-2mib) and will benefit from a wide and adapted offer of complementary training.
The proposed work will be carried out under the co-direction of:
(i) M. Bertin, from the team "Spin, Photons and Ice" of the laboratory LERMA (Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères), a joint research unit (UMR 8112) shared by CNRS, Observatoire de Paris (OP), Sorbonne University (SU) and Université Cergy Paris (CYU),
(ii) A. Lafosse, from the SIM2D team "Surfaces, Interfaces, Molecules & 2D Materials" of ISMO (Institut des Sciences Moléculaires d'Orsay), a joint research unit (UMR 8214) of CNRS and the University of Paris-Saclay.
The PhD student will participate in experiments at the SOLEIL synchrotron.
All the work will be carried out within the consortium constituted by the 3 partners of the ANR PIXyES (Photodesorption Induced by UV-X-rays and Electrons on ice Surfaces, 2021-2025): LERMA (project leader), ISMO and PhLAM (Laboratoire de Physique des Lasers Atomes et Molécules, joint research unit (UMR 8523) of CNRS and the University of Lille).
The research work of the PhD student will not be subject to confidentiality. It will be published in international peer-reviewed journals. In addition, the doctoral student will participate in at least one national conference during the first year of his/her thesis (if sanitary conditions and his/her level of French allow it) and then in an international conference during the second year of his/her thesis.
Constraints and risks
The risks associated with the experimental context are: high voltages, cryogenics, pressurized gases, ultra-high vacuum, and chemicals (some of the compounds considered present limited risks).
Mandatory "safety - newcomers" training is given at ISMO and before accessing synchrotron radiation.
The most frequent trips to be planned will all take place between Orsay-plateau, Paris and Gif-sur-Yvette (SOLEIL synchrotron).
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