Molecular dynamics simulations of the ice temperature dependence of water ice photodesorption

Abstract

The ultraviolet (UV) photodissociation of amorphous water ice at different ice temperatures is investigated using Molecular Dynamics (MD) simulations and analytical potentials. Previous MD calculations of UV photodissociation of amorphous and crystalline water ice at 10 K [S. Andersson et al., J. Chem. Phys. 124, 064715 (2006)] revealed -for both types of ice -that H atom, OH, and H 2 O desorption are the most important processes after photoexcitation in the uppermost layers of the ice. Water desorption takes place either by direct desorption of recombined water, or when, after dissociation, an H atom transfers part of its kinetic energy to one of the surrounding water molecules which is thereby kicked out from the ice. We present results of MD simulations of UV photodissociation of amorphous ice at 10, 20, 30, and 90 K in order to analyse the effect of ice temperature on UV photodissociation processes. Desorption and trapping probabilities are calculated for photoexcitation of H 2 O in the top four monolayers and the main conclusions are in agreement with the 10 K results: desorption dominates in the top layers, while trapping occurs deeper in the ice. The hydrogen atom photodesorption probability does not depend on ice temperature, but OH and H 2 O photodesorption probabilities tend to increase slightly (∼ 30 %)with ice temperature. We have compared the total photodesorption probability (OH + H 2 O) with the experimental total photodesorption yield, and in both cases the probabilities rise smoothly with ice temperature. The experimental yield is on average 3.8 times larger than our theoretical results, which can be explained by the different time scales studied and the approximations in our model.

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][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. This binning (method 2) is used in this study and it consists of choosing a molecule and finding the first 23 closest molecules in terms of (x, y) coordinates.…”

“…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.…”

“…Thus, the outcome probabilities strongly depend on the monolayer in which the photoexcited molecule is initially located: the photoexcitation in the top MLs leads mainly to photodesorption, while deeper into the ice, it leads to trapping. [40][41][42][43] The average D atom photodesorption probability and the average H atom photodesorption probability 43 taken over the top four monolayers (e.g., P Hdes = 4 i=1 P i Hdes /4) are plotted in Fig. 1.…”

“…[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.…”