HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study.

The reaction of HCl on water ice provides a simple case for understanding dissociation and proton transfer in this non-optimal, incomplete solvation environment, playing a central role in atmospheric chemistry. This reaction has been repeatedly reported as thermally dependent, whereas the theoretical models predict a spontaneous dissociation. We examine the adsorption of HCl on ice at low temperature (50 K and 90 K) via a combination of near-edge X-ray absorption spectroscopy (NEXAFS) at the chlorine L-edge, photoemission (XPS and UPS), and reflection-adsorption infrared spectroscopy (FT-RAIRS). We show the complete dissociation of HCl into Cl(-) and H(+) through 3 hydrogen bonds, predominantly by direct reaction with water (80%) and by self-solvation (20%), in full agreement with the prediction of a barrierless process.