Acidity of hydrogen chloride at the surface of low-temperature (40-150 K) water-ice films

We use near-edge x-ray absorption spectroscopy (NEXAFS) at the chlorine 2p levels with two detection modes: total electron yield and total ion yield to obtain direct information on chemical states of HCl molecules at the surface and in the bulk of thin water-ice films at low temperature (40-150 K). NEXAFS spectra show that HCl species are the same at the surface and in the bulk, and that when HCl is adsorbed in excess of a monolayer at T>90 K, transitions from the Cl 2p levels to the antibonding sigma(H-Cl)(*) are totally suppressed as a result of HCl dissociation into an H3O+Cl- ion-pair complex.