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 $T>\mathrm{}90\mathrm{}\mathrm{K},$ transitions from the Cl $2p$ levels to the antibonding ${\sigma }_{\mathrm{H}-\mathrm{C}\mathrm{l}}^{*}$ are totally suppressed as a result of HCl dissociation into an ${\mathrm{H}}_3{\mathrm{O}}^{+}{\mathrm{Cl}}^{-}$ ion-pair complex.