Photodesorption of water ice

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 H 2 O 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 H 2 O 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 H 2 O removal mechanism in the top two monolayers is through separate desorption of H and OH fragments. Removal of H 2 O molecules from the ice, either as H 2 O 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 H 2 O per incident photon is estimated to be 3.7 × 10 −4 , with the probability for photodesorption of intact H 2 O 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.

Molecular dynamics simulations of D2O ice photodesorption

“…[40][41][42][43] Starting from the normal hexagonal ice (I h ) crystalline ice configuration (containing 8 bilayers (BLs) (16 MLs) with 60 (30) molecules in each ML), the amorphous ice surface was set up at 10, 20, 60, or 90 K using the "fast quenching" method" [46][47][48] Further details can be found in our previous studies. [41][42][43] Since the resulting amorphous ice surface has a more irregular bonding structure than the crystalline ice surface, 41,47 assigning molecules to MLs is not straightforward. 43 In our most recent study 43 a new definition of ML (binning method 2) was tested and shown to be a more realistic way to assign molecules to MLs.…”

“…41 To simulate the dynamics of a photodissociation event, Newton's equations of motion are integrated in time with a time step of 0.02 fs and a maximum time of 20 ps. The stop criterion and the six final outcomes after UV photodissociation of D 2 O are analogous to those described previously for H 2 O photodissociation: [40][41][42][43] (1) desorption of D while OD is trapped inside or on the ice, (2) desorption of OD while D is trapped inside or on the ice, (3) desorption of both D and OD, (4) D and OD are both trapped inside or on the ice, (5) D and OD recombine and form a D 2 O molecule which either desorbs, or (6) is trapped inside or on the ice. Besides these six outcomes, an additional channel is possible where D 2 O desorbs through the so-called "kick-out" mechanism.…”

“…Besides these six outcomes, an additional channel is possible where D 2 O desorbs through the so-called "kick-out" mechanism. 41,42 This occurs when a molecule desorbs from the ice by momentum transfer from an energetic D atom resulting from photodissociation of a neighbouring photoexcited molecule.…”

“…[40][41][42][43] The most important photodesorption mechanism after photodissociation of water in the top three MLs of the ice surface is H atom photodesorption, followed by OH radical photodesorption, and H 2 O molecule photodesorption. The calculated H 2 O photodesorption probability is due to two mechanisms.…”

Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

“…However, in these high temperature regimes thermal desorption should be quite efficient. We still consider photo-desorption processes but rather than use the experimentally determined yields for icy mantles from previous work I'll be using a constant value, Y pd = 10 −4 molecules photon , in accordance with the work of Van Dischoeck et al 45 Photo-desorption rates are treated in the following manner: is how the rate for the photo-desorption from secondary UV photons induced by cosmic rays is calculated. In these equation…”