Probing the Interaction of Hydrogen Chloride with Low-Temperature Water Ice Surfaces Using Thermal and Electron-Stimulated Desorption

Abstract

The interaction and autoionization of HCl on low-temperature (80-140 K) water ice surfaces has been studied using low-energy (5-250 eV) electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). There is a reduction of H(+) and H(2)(+) and a concomitant increase in H(+)(H(2)O)(n=1-7) ESD yields due to the presence of submonolayer quantities of HCl. These changes are consistent with HCl induced reduction of dangling bonds required for H(+) and H(2)(+) ESD and increased hole localization necessary for H(+)(H(2)O)(n=1-7) ESD. For low coverages, this can involve nonactivated autoionization of HCl, even at temperatures as low as 80 K; well below those typical of polar stratospheric cloud particles. The uptake and autoionization of HCl is supported by TPD studies which show that for HCl doses ≤0.5 ± 0.2 ML (ML = monolayer) at 110 K, desorption of HCl begins at 115 K and peaks at 180 K. The former is associated with adsorption of a small amount of molecular HCl and is strongly dependent on the annealing history of the ice. The latter peak at 180 K is commensurate with desorption of HCl via recombinative desorption of solvated separated ion pairs. The activation energy for second-order desorption of HCl initially in the ionized state is 43 ± 2 kJ/mol. This is close to the zero-order activation energy for ice desorption.

Depletion of chlorine into HCl ice in a protostellar core

“…Upon warmup, HCl will evaporate close to the H 2 O evaporation temperature. This likely involves the relaxation of the water ice matrix, followed by the recombination and evaporation of HCl (Olanrewaju et al 2011). The observed depth of depletion outside of the hot core in FIR 4 is consistent with such a codesorption scenario, although with the present data we can only place an upper limit of 10 −8 on the gas-phase HCl abundance in the ∼ 500 AU size hot core (or 10 −7 within 100 AU).…”

“…The grain surface reactions are described in Hersant et al (2009) and Semenov et al (2010). The binding energy of Cl on H 2 O ice is 1100 K, for HCl we adopted 5174 K from (Olanrewaju et al 2011). The gas-phase network is based on kida.uva.2011, from Wakelam et al (2012), and was updated with data from Neufeld & Wolfire (2009).…”

An Approach to Estimate the Binding Energy of Interstellar Species

“…Ward et al (2012) used TPD experiments coupled with time-of-flight mass spectrometry to determine the yield of OCS and additionally yields value for the computation of desorption energies of O atom and OCS. The interaction and auto-ionization of HCl on low-temperature (80 − 140 K) water ice surfaces has been studied by Olanrewaju et al (2011) by using low-energy (5 − 250 eV) electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). Dulieu et al (2013) also performed TPD experiment to derive the BE of different species on the silicate substrate.…”

Distinct Speciation of Naphthalene Vapor Deposited on Ice Surfaces at 253 or 77 K: Formation of Submicrometer-Sized Crystals or an Amorphous Layer

“…The adsorption of various compounds on (and/or diffusion through) ice was studied in a number of field and laboratory-based experiments utilizing, for example, nitrogen, formaldehyde, , acetaldehyde, , acetic acid, − acetone, − acetonitrile, dichloromethane, HCl, − other hydrogen halides, HNO 3 , − NO 2 , alkanes, alcohols, , Xe, Kr, CF 4 , and other light gases . The environmental aspects of the adsorption of gases on the ice surface were previously excellently reviewed , and remain a subject of active research.…”