Electron attachment in Ice-HCl clusters: An ab initio study

Experimental work has shown that small amounts of HCl strongly enhance electron capture in ice films. The purpose of the present study was to investigate the effect of adsorbed HCl on the interaction of electrons with small clusters of water. Studies were made with clusters of 6 and 12 water molecules with various geometries both with and without one HCl attached. A number of distinct HCl coordination motifs were examined. All of the neutral structures with HCl exhibited zero thresholds for electron attachment and formed dipole bound anionic states (DBS). The relaxation processes for these initial DBS depended on the number of H2O (n) and on the number and type of H-bonds to the HCl (x). The initial DBS of systems with only O-H center dot center dot center dot Cl H-binding underwent dissociative electron attachment (DEA), forming H atoms. Relaxation for systems with ClH center dot center dot center dot OH2 bonds was more complex. For the two layer n = 12 systems with x = 2 or 3 the HCl proton moved to the nearest oxygen to form H3O+. Then rearrangement of the proton network occurred, and the Cl- became solvated by three HO-H center dot center dot center dot Cl- bonds. The presence of Cl- and H3O+ increases the dipole moment and the electron binding energy (EBE) of the network. Further stabilization is achieved by decay into deeper DBS electron traps and/or by reaction of the excess electron with H3O+ to form H-center dot atoms. The HCl(H2O)(6) clusters with a single Cl-H center dot center dot center dot OH2 bond behaved differently. They increased their stability by becoming more linear. This raised the dipole moment and the EBE therefore increased, reducing the total energy. None of these species showed any signs of increasing the number of H-bonds to Cl. The implication of these observations for the interpretation of the results of the experiments with 0.2 monolayer of HCl on 5 monolayer of H2O at 20 K, and on the possible role of cosmic ray-induced ionization in polar stratospheric clouds in ozone depletion is discussed.