Speaker
Description
Photon-stimulated desorption or photodesorption by UV photons is a fundamental process playing a role in interstellar environments and the surface icy satellites in our solar system. Photodesorption has been proposed to provide an efficient non-thermal desorption route of the interstellar ices present in the cold regions of the ISM, contributing to molecular gas phase abundances measured by radio and space telescopes. In the last decade, several photodesorption studies from cold CO condensates have been focused on the experimental determination of UV photodesoption yields using broad band UV sources [1,2]. Complementary studies of these photodesorption yields, using synchrotron radiation, have additionally been able to highlight the first stage of the desorption mechanism, revealing in particular an indirect desorption, induced by an electronic transition (DIET) involving the first electronic excited state of CO A$^1$Π [3] in the 8-10 eV region. We report here more recent pulsed-laser based experiments and ab-initio calculations to fully characterize the desorption mechanism.
This presentation reports on the translational and rovibrational energy of photodesorbed CO molecules from a CO polycrystalline ice (T = 15 K) irradiated at ~8 eV. The electronic excitation was induced by a pulsed Vacuum-UV (VUV) laser, and the rovibrational states of the photodesorbed CO molecule in their electronic ground state were probed using resonance enhanced multiphoton ionization (REMPI). Pump-probe experiments enable to measure, for the first time, time-of-flights and rotationally resolved spectra providing the kinetic and internal energy distributions of the desorbing particles. Vibrationally cold CO molecules were observed, with rotational and vibrational energy peaking well-below 300 meV [4]. This study is supported by Ab Initio Molecular Dynamics (AIMD) simulations which focused on the description of the vibrational energy redistribution within a 50 CO molecules aggregate [5]. Measured and theoretical energy distributions present both correlation between rotational and translational energy. These studies allow to fully validate the so-called indirect-DIET, triggered by a highly vibrationally excited CO molecule (v=40) within the CO cluster.
Références
[1] K. I. Öberg, G. W. Fuchs, Z. Awad, H. J. Fraser, S. Schlemmer, E. F. Van Dishoeck, and H. Linnartz, “Photodesorption of CO ice,” Astrophys. J. 662, L23–L26 (2007).
[2] G. M. Muñoz Caro, A. Jiménez-Escobar, J. Ã. Martín-Gago, C. Rogero, C. Atienza, S. Puertas, J. M. Sobrado, and J. Torres-Redondo, “New results on thermal and photodesorption of CO ice using the novel InterStellar Astrochemistry Chamber (ISAC),” Astron. Astrophys. 522, A108 (2010).
[3] 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,” Astrophys. J. 739, L36 (2011).
[4] A. B. Hacquard, R. Basalgète, S. Del Fré, J. Rakovský, A. Rivero Santamaria, F. Benoit, X. Michaut, G. Féraud, M. Bertin, M. Monnerville and J-H. Fillion, “Photodesorption of CO ices: Rotational and translational energy distributions,” J. Chem. Phys. 161, 184306 (2024).
[5] S. Del Fré, A. R. Santamaría, D. Duflot, R. Basalgète, G. Féraud, M. Bertin, J.-H. Fillion, and M. Monnerville, “Mechanism of ultraviolet-induced CO desorption from CO ice: Role of vibrational relaxation highlighted,” Phys. Rev. Lett. 131, 238001 (2023).
