Photodesorption of water ice

A molecular dynamics study

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: andersson@strw.leidenuniv.nl
² Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
³ Department of Chemistry, Physical Chemistry, Universtity of Gothenburg, 41296 Gothenburg, Sweden

Received: 11 June 2008
Accepted: 24 September 2008

Abstract

Context. Absorption of ultraviolet radiation by water ice coating interstellar grains can lead to dissociation and desorption of the ice molecules. These processes are thought to be important in the gas-grain chemistry in molecular clouds and protoplanetary disks, but very few quantitative studies exist.

Aims. We compute the photodesorption efficiencies of amorphous water ice and elucidate the mechanisms by which desorption occurs.

Methods. Classical molecular dynamics calculations were performed for a compact amorphous ice surface at 10 K thought to be representative of interstellar ice. Dissociation and desorption of H₂O molecules in the top six monolayers are considered following absorption into the first excited electronic state with photons in the 1300-1500 Å range. The trajectories of the H and OH photofragments are followed until they escape or become trapped in the ice.

Results. The probability for H₂O desorption per absorbed UV photon is 0.5-1% in the top three monolayers, then decreases to 0.03% in the next two monolayers, and is negligible deeper into the ice. The main H₂O removal mechanism in the top two monolayers is through separate desorption of H and OH fragments. Removal of H₂O molecules from the ice, either as H₂O itself or its products, has a total probability of 2-3% per absorbed UV photon in the top two monolayers. In the third monolayer the probability is about 1% and deeper into the ice the probability of photodesorption falling to insignificant numbers. The probability of any removal of H₂O per incident photon is estimated to be 3.7 10^-4, with the probability for photodesorption of intact H₂O molecules being 1.4 10^-4 per incident photon. When no desorption occurs, the H and OH products can travel up to 70 and 60 Å inside or on top of the surface, respectively, during which they can react with other species, such as CO, before they become trapped.