UV photodesorption of methanol in pure and CO-rich ices: desorption rates of the intact molecule and of the photofragments

TL;DR

This study measures wavelength-dependent UV photodesorption rates of methanol and its fragments from pure and CO-rich ices, revealing lower-than-expected rates that impact astrochemical models and suggest new pathways for complex molecule formation.

Contribution

It provides new quantitative data on photodesorption rates of methanol and its fragments, highlighting the dominance of fragment desorption and its implications for astrochemical processes.

Findings

Photodesorption rate of intact methanol is about 10^{-5} molecules/photon.

Rates decrease to below 10^{-6} molecules/photon in CO-rich ices.

Photodesorption mainly involves desorption of methanol fragments like CO, CH3, OH.

Abstract

Wavelength dependent photodesorption rates have been determined using synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -- 14 eV range. The VUV photodesorption of intact methanol molecules from pure methanol ices is found to be of the order of 10$^{-5}$ molecules/photon, that is two orders of magnitude below what is generally used in astrochemical models. This rate gets even lower ($<$ 10$^{-6}$ molecules/photon) when the methanol is mixed with CO molecules in the ices. This is consistent with a picture in which photodissociation and recombination processes are at the origin of intact methanol desorption from pure CH$_3$OH ices. Such low rates are explained by the fact that the overall photodesorption process is dominated by the desorption of the photofragments CO, CH$_3$, OH, H$_2$CO and CH$_3$O/CH$_2$OH, whose photodesorption rates are given in this study. Our results suggest that the role of the photodesorption as a mechanism to explain the observed gas phase abundances of methanol in cold media is probably overestimated. Nevertheless, the photodesorption of radicals from methanol-rich ices may stand at the origin of the gas phase presence of radicals such as CH$_3$O, therefore opening new gas phase chemical routes for the formation of complex molecules.