Photodesorption of ices I: CO, N2 and CO2

TL;DR

This study measures UV photodesorption yields of CO, N2, and CO2 ices under astrochemical conditions, revealing molecule-specific mechanisms and the influence of ice layering on desorption efficiency, which impacts understanding of gas presence in cold space regions.

Contribution

It provides the first experimental quantification of photodesorption yields for CO, N2, and CO2 ices, including effects of mixing and layering, advancing astrochemical models of molecule retention in space.

Findings

CO desorbs without dissociation from the surface.

N2 has a much lower photodesorption yield, increased by co-desorption.

CO2 desorbs via dissociation and recombination, with yields depending on ice thickness.

Abstract

A longstanding problem in astrochemistry is how molecules can be maintained in the gas phase in dense inter- and circumstellar regions. Photodesorption is a non-thermal desorption mechanism, which may explain the small amounts of observed cold gas in cloud cores and disk mid-planes. This paper aims to determine the UV photodesorption yields and to constrain the photodesorption mechanisms of three astrochemically relevant ices: CO, N2 and CO2. In addition, the possibility of co-desorption in mixed and layered CO:N2 ices is explored. The ice photodesorption is studied experimentally under ultra high vacuum conditions and at 15-60 K using a hydrogen discharge lamp (7-10.5 eV). The ice desorption during irradiation is monitored by reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed molecules. Both the UV photodesorption yields per incident photon and the photodesorption mechanisms are molecule specific. CO photodesorbs without dissociation from the surface layer of the ice. N2, which lacks an electronic transition in this wavelength range, has a photodesorption yield that is more than an order of magnitude lower. This yield increases significantly due to co-desorption when N2 is mixed in with or layered on top of CO ice. CO2 photodesorbs through dissociation and subsequent recombination from the top 10 layers of the ice. At low temperatures (15-18 K) the derived photodesorption yields are 2.7x10^-3 and <2x10^-4 molecules photon-1 for pure CO and N2, respectively. The CO2 photodesorption yield is 1.2x10^-3x(1-e^(-X/2.9)) + 1.1x10^-3x(1-e^(-X/4.6)) molecules photon-1, where X is the ice thickness in monolayers and the two parts of the expression represent a CO2 and CO photodesorption pathway.