3 year PhD position on laboratory astrophysics and chemical modeling

Context: Water is the third most abundant molecule in the interstellar medium (after H2 and CO). It has been observed in various objects, but the very rich spectrum of rotational, vibrational and ro-vibrational transitions of water, and sometimes in surprisingly excited energy states is observed only in the extended atmosphere of the envelope of O- rich evolved stars. Water is a key molecule for understanding the energy exchanges and physico-chemical processes that can take place in these media. Very recently, with the ALMA network, we discovered several very high energy transitions of water, well above the energy of the transitions detected until now. To understand the excitation of water in these environments, it is necessary to model the excitation of water not only by the radiation from the central star and by radiative coupling with the background radiation, but also by collisions with the most abundant partners (H2, He, H, e-). This implies knowing the collisional rates for pure rotational transitions of water molecules within its first four excited vibrational states (ν1=1, ν2=1, 2 and ν3=1) as well as the ro-vibrational transitions between the ground state and these excited vibrational states. The first objective is therefore to obtain these collision rates. The second is to include these rates in an advanced radiative transfer code to reproduce the ALMA observations (ATOMIUM consortium, (https://fys.kuleuven.be/ster/research-projects/aerosol/atomium/atomium) and interpret them.

Experimental work (ISM): The crossed molecular beam experiment in Bordeaux allows the study of inelastic collisions between atoms, molecules or radicals with other atoms or molecules, down to very low collision energies (equivalent to a few Kelvin). Studies on rotational excitation to the first excited levels of H2O but also D2O by collisions with H2 have highlighted quantum effects at these low energies, but have mainly allowed to validate the theoretical calculations made in order to provide reliable data to astrophysicists. The first objective of the thesis will be to extend these experimental studies to vibrationally excited water molecules (to date, no experimental data is published on inelastic collisions of vibrationally excited molecules at low temperatures).

Method: Two molecular beams are generated (in a molecular beam, all molecules go in the same direction and at the same speed): a single collision between the molecules takes place at the intersection of the 2 beams. The angle between the two beams is variable, which allows the collision energy between the molecules to vary. The molecules are then probed by a time-of-flight mass spectrometer (to select the species by their mass) after resonant multiphoton ionization (the wavelength of the laser allows the choice of the quantum state to be ionized). For this study, an IR laser will have to be coupled to the set-up to vibrationally excite the water molecules before the collision center. See Bergeat et al. (2020, Phys. Rev. Lett., 125, 143402-5, et 2020, Phys. Chem. A, 124, 259-264).

Collaborators: Sébastien Morales (engineer in charge of lasers), Christian Naulin and Astrid Bergeat, Licence/ Master internships.