Spectrally-resolved UV photodesorption of CH₄in pure and layered ices

Abstract

Context. Methane is among the main components of the ice mantles of insterstellar dust grains, where it is at the start of a rich solid-phase chemical network. Quantification of the photon-induced desorption yield of these frozen molecules and understanding of the underlying processes is necessary to accurately model the observations and the chemical evolution of various regions of the interstellar medium.Aims. This study aims at experimentally determining absolute photodesorption yields for the CH 4 molecule as a function of photon energy. The influence of the ice composition is also investigated. By studying the methane desorption from layered CH 4 :CO ice, indirect desorption processes triggered by the excitation of the CO molecules is monitored and quantified. Methods. Tunable monochromatic VUV light from the DESIRS beamline of the SOLEIL synchrotron is used in the 7 -13.6 eV (177 -91 nm) range to irradiate pure CH 4 or layers of CH 4 deposited on top of CO ice samples. The release of species in the gas phase is monitored by quadrupole mass spectrometry and absolute photodesorption yields of intact CH 4 are deduced. Results. CH 4 photodesorbs for photon energies higher than~9.1 eV (~136 nm). The photodesorption spectrum follows the absorption spectrum of CH 4 , which confirms a desorption mechanism mediated by electronic transitions in the ice. When it is deposited on top of CO, CH 4 desorbs between 8 and 9 eV with a pattern characteristic of CO absorption, indicating desorption induced by energy transfer from CO molecules. Conclusions. The photodesorption of CH 4 from the pure ice in various interstellar environments is around 2.0 ± 1.0 × 10 −3 molecules per incident photon. Results on CO-induced indirect desorption of CH 4 provide useful insights for the generalization of this process to other molecules co-existing with CO in ice mantles.

Photon-induced desorption of larger species in UV-irradiated methane ice

“…Photodesorption mechanisms were studied for CH 4 and its daughter molecules. Dupuy et al (2017) found CH 4 photodesorption for monochromatic energies above 9.1 eV. However, in agreement with Cruz-Diaz et al (2014) and Martín-Doménech et al (2016), no evidence of a significant CH 4 photon-induced desorption was found in our work using the MDHL.…”

“…The direct desorption of a molecule from the ice surface (van Hemert et al 2015;Dupuy et al 2017), after absorption of a photon, is of low efficiency compared to the indirect photodesorption, by which the photon is absorbed by another molecule and the photon energy is distributed to the surrounding molecules. In this case, a UV photon can be absorbed by a molecule in the subsurface layers of the ice, leading to electronic excitation.…”

Vacuum Ultraviolet Photodesorption and Photofragmentation of Formaldehyde-Containing Ices

“…CO photodesorption spectrum from H 2 CO:CO was also recorded. It looks like that from pure CO, except it is less intense but this is consistent with the fact that there is less CO at the surface of the mixed ice than in the thick pure ice of Dupuy et al 76 . As for the layered ice, these results illustrate how photodesorption depends on the ice composition and the thickness.…”

UV photoprocessing of NH3 ice: photon-induced desorption mechanisms

“…These studies have been reported for pure H2O (Cruz-Díaz et al (2017), and references therein), CO (Öberg et al (2007CO (Öberg et al ( , 2009b (2016)), CH4 (Dupuy et al (2017)), and NH3 (Nishi et al (1984); Loeffler & Baragiola (2010)) ices. In this work, we present a series of experiments simulating the UV photoprocessing of pure NH3 ices focused on the subsequent photodesorption of ice molecules.…”

“…This mechanism has been proposed by van Hemert et al (2015) for the photodesorption of CO molecules from a pure CO ice based on molecular dynamics simulations. However, in previous works this process had been proven to be inefficient during experimental simulations with CO and N2 ices when compared to indirect photodesorption mechanisms (see, e.g., Muñoz Caro et al (2010); Bertin et al (2012Bertin et al ( , 2013), although it may have a contribution in other cases (Dupuy et al (2017)). Alternatively, if the absorbing surface molecule photodissociates, the resulting photofragment can desorb provided that it is formed with enough kinetic energy, or it may recombine with another fragment, leading to the formation of a photoproduct that could also photodesorb thanks to the excess energy of the parent photofragments and/or the exothermicity released during the recombination reaction (see, e.g., Andersson & van Dishoeck (2008); Fayolle et al (2013);Fillion et al (2014); Bertin et al (2016)).…”