DEL FRE Samuel : Theoretical studies of photodesorption of molecular interstellar ices

Résumé de thèse :

In the coldest regions (~ 10 K) of the interstellar medium (ISM), most molecular species apart from H2 accrete on dust grains to form ice mantles, acting as particularly rich molecular reservoirs. The UV-induced desorption of molecules at the ice surface, which may explain the high abundance of gas phase molecules in the coldest environments of the ISM, has been extensively studied for CO, the second most abundant species in the ISM. Experimental investigations have revealed that in pure CO ices, UV photodesorption may predominantly follow an indirect "Desorption Induced by Electronic Transition" (DIET) mechanism1-2. However, understanding the underlying molecular mechanisms, especially the nature of the energy transfers involved in indirect desorption pathways, has long been a subject of scientific debate as the only detailed theoretical study on the CO ice photodesorption failed to reproduce some of the main experimental findings.3 In this astrochemical context, we introduce an innovative study utilizing ab initio molecular dynamics (AIMD) simulations based on Density Functional Theory (DFT) to investigate an indirect desorption mechanism in CO ice.4 Here, a highly vibrationally excited CO molecule (v=40) within 50 CO molecules aggregates initially created, optimized, and then thermalized at 15K, triggers the indirect desorption of surface molecules. This scenario represents the last part of the DIET mechanism, wherein the electronic energy from the excited molecule, redistributed into a high vibrational state of its electronic ground state, is transferred to neighboring molecules, potentially leading to desorption. Our study meticulously analyzes the redistribution of vibrational energy into translational, rotational, and other vibrational modes within the aggregate post-excitation, as well as the desorption mechanism. Our theoretical insights reveal that the desorption process starts with a mutual attraction between the vibrationally excited molecule and one or two neighbors, which is activated by the stretching of the CO bond. It is followed by a sequence of energy transfers initiated by a collision, culminating in the desorption of vibrationally cold CO molecules. In addition, the theoretical energy distributions—vibrational and kinetic—of the desorbed molecules, predominantly found in their ground vibrational state, are in perfect agreement with the experimental ones which supports the pivotal role of vibrational relaxation in the desorption process.

[1] M. Bertin, E. C. Fayolle, C. Romanzin, K. I. Öberg, X. Michaut, A. Moudens, L. Philippe, P. Jeseck, H. Linnartz, and J.-H. Fillion, UV Photodesorption of Interstellar CO Ice Analogues: From Subsurface Excitation to Surface Desorption, Phys. Chem. Chem. Phys. 14, 9929 (2012).

[2] E. C. Fayolle, M. Bertin, C. Romanzin, X. Michaut, K. I. Öberg, H. Linnartz, and J.-H. Fillion, CO Ice Photodesorption: A Wavelength- Dependent Study, ApJ 739, L36 (2011).

[3] M. C. van Hemert, J. Takahashi, and E. F. van Dishoeck, Molecular Dynamics Study of the Photodesorption of CO Ice, J. Phys. Chem. A 119, 6354 (2015).

[4] S. Del Fré, A. R. Santamaría, D. Duflot, R. Basalgete, G. Feraud, M. Bertin, J.-H. Fillion, and M. Monnerville, Mechanism of Ultraviolet- Induced CO Desorption from CO Ice: Role of Vibrational Relaxation Highlighted, Physical Review Letters (2023).

Doctorant : DEL FRE Samuel

Directeur de thèse : MONNERVILLE Maurice, RIVERO SANTAMARIA Alejandro, DUFLOT Denis