Isotope effects on the photodesorption processes of X2O (X = H,D) and HOD ice

Abstract

To investigate the isotope effects on the photodesorption processes of X 2 O (X = H,D) ice, molecular dynamics calculations have been performed on the ultraviolet photodissociation of an H 2 O or a D 2 O molecule in an H 2 O or a D 2 O amorphous ice surface, and on HOD photodissociation in an H 2 O amorphous ice surface, where the photodissociated molecules were located in the top four or five monolayers at ice temperatures of 10, 20, 30, 60, and 90 K. Three photodesorption processes can occur upon X 2 O photodissociation: X atom photodesorption, OX radical photodesorption, and X 2 O (or HOD) molecule photodesorption. X 2 O (or HOD) photodesorption can occur after recombination of X and OX, or after an energetic X atom photofragment kicks a surrounding X 2 O molecule from the ice surface. Isotope effects are observed for the X atom and the OX radical photodesorption as well as for the kick-out photodesorption. However, no isotope effects were noticeable for the photodesorption of recombined X 2 O molecules. The average D atom photodesorption probabilities are about a factor 0.9 smaller than those for the H atom, regardless of the isotope of the surrounding ice system. Also, the kick-out mechanism is more likely to occur if a D photofragment is created upon dissociation than if an H atom is created. These observations can be explained by more efficient energy transfer from the D atom to water molecules than from the H atom. Reasoning based on the X 2 O phonon frequencies associated with the librational modes and energy transfer efficiencies explain why the OX radical photodesorption probabilities are noticeably larger if the OX radical desorbs from a D 2 O ice system than from an H 2 O ice system. Also, the OX radical photodesorption is more probable upon dissociation of DOX (X = H,D) than upon dissociation of HOX (X = H,D), because the initial kinetic energy of the OX radical is larger if the dissociation products are D + OX than H + OX. The branching ratio of OD OH desorption following photodissociation of an HOD molecule in ice (about 1.0) is much lower than the OD OH branching ratio in gas-phase HOD photodissociation. This may lead to differences in isotope fractionation in OH(g) formation in dense and diffuse clouds in the interstellar medium.

Grain Surface Models and Data for Astrochemistry

“…The first 4-5 monolayers are, however, taking part in the desorption process, which indicates again that the assumption of a single reactive top monolayer cannot be valid. The desorption depth dependence has been observed for a range of excitation energies, ice temperatures and isotope compositions (Andersson et al 2006;Andersson and van Dishoeck 2008;Arasa et al 2010;Koning et al 2013;Arasa et al 2015). The studies further show that radical species which are created through photodissociation remain in the bulk of the ice, can use their excitation energy to move some short distance before they thermalize.…”

“…Most work to date has been conducted on water ice (Andersson et al 2005(Andersson et al , 2006Andersson and van Dishoeck 2008;Arasa et al 2010), including its deuterated analogues, HDO and D 2 O (Koning et al 2013). Andersson et al (2005Andersson et al ( , 2006 studied the photodesorption mechanisms of H 2 O in both crystalline and amorphous water ice at 10 K following photoexcitation of H 2 O into the first excited state with photons with wavelengths between 130-150 nm.…”

“…The probability of the parallel "kick out" mechanism was found to have a more complex behavior with temperature which the authors attribute to the amorphous nature of the ice. Koning et al (2013) studied isotopic effects, and looked at the astrophysically relevant cases of HDO and D 2 O photodissociation in an amorphous H 2 O water ice. D atoms were found to photodesorb with a slightly lower efficiency than H atoms.…”

Methanol along the path from envelope to protoplanetary disc

“…As a first order approximation, the equivalent rates for the gas phase are used. This is likely over-estimating the grain-surface photodissociation rates, since the mechanisms for UV photodissociation and photodesorption of ices are now understood to be related as demonstrated in molecular dynamics studies (Andersson et al 2006;Andersson & van Dishoeck 2008;Arasa et al 2010Arasa et al , 2011Arasa et al , 2013Koning, Kroes & Arasa 2013) and experimental work (Bertin et al 2012;Fayolle et al 2013).…”

Water deuteration and ortho-to-para nuclear spin ratio of H2in molecular clouds formed via the accumulation of H I gas

“…Photodissociation of HDO ice has two branches, (OD + H) and (OH + D) with a branching ratio of 2:1 (Koning et al 2013). If most OH ice photofragments are converted to H 2 O ice, HDO ice photodissociation leads to the removal of deuterium from the water ice chemical network on the timescale of photodissociation.…”