Interaction of Hydrogen Chloride with an Ultrathin Ice Film: Observation of Adsorbed and Absorbed States

Abstract

With the objective of gaining insight into how heterogeneous reactions occur in the Antarctic stratosphere, we have initiated a program to study the adsorption and reaction of simple molecules on model polar stratospheric cloud surfaces. In this work, the temperature-programmed desorption of hydrogen chloride from ultrathin (5-20 monolayers thick) water films is described. Two distinct HC1 desorption states, designated a-and 0-HCl, are observed a t 140 and 180 K, respectively. Water sublimation occurs at 180 K and is concurrent with @-HCl evolution. 8-HCl, which is formed exclusively a t low H C l exposures, is derived from the thin film bulk, while a-HC1 is associated with an adsorbed state. P-HCl is assigned to the sublimation of a stoichiometric phase of HC1 and water, probably H C l h H 2 0 , and a-HCl is assigned to the thermal desorption of HCl from the hexahydrate surface. Desorption spectra of HC1 from ice-dz show that H-D exchange between HCl and D 2 0 is much less than would be expected for a dissociatively adsorbed state of HCl. The a state is therefore assigned to molecularly adsorbed HC1. The activation energy for a-HC1 desorption is 33 f 5 kJ-mol-', a value which is highly suggestive of formation of a hydrogen bond between H C l and the hexahydrate surface. Two possible structures of H C l adsorbed on the hexahydrate surface are considered. Implications of these results for heterogeneous polar stratospheric chemistry are discussed.

Hydration of ammonia and hydrogen chloride at cryogenic temperature studied by TP-TOF-SIMS

“…To date, TP-TOF-SIMS has revealed that both HCl and NH 3 molecules adsorbed on pure ASW films are hydrated considerably up to 140 K [22][23][24], which is consistent with the conclusions drawn using different approaches [3][4][5][6]8,9,13,14]. However, very little is known about intermolecular interactions at the surface of acidic and basic water-ice films including hydrated HCl or NH 3 molecules, respectively.…”

“…Molina et al [1] proposed that the reaction rate between HCl and ClNO 2 is enhanced considerably by the existence of ice particles, but detailed reaction pathways for the ionization and solvation of HCl adsorbed on a water-ice surface are not yet fully understood. So far, a number of groups have been extensively tackling these fascinating topics by infrared spectroscopy (IR) [3][4][5][6][7][8][9], reactive Cs + ion scattering (RIS) [10,11], near edge X-ray absorption fine structure (NEXAFS) [12], and temperature-programmed desorption (TPD) [8,9,13,14] analyses, and computer simulations [3,5,[15][16][17][18][19][20][21]. Recently, arguments tend to converge on the conclusion that both the ionization and solvation of HCl molecules occur as proposed by Devlin and coworkers [3][4][5].…”

Investigation of the Heterogeneous Reactivity of HCl, HBr, and HI on Ice Surfaces

“…41,42 It should be pointed out that at the temperatures studied by Graham and Roberts ice micropores are thermodynamically unstable and rapidly collapse. 25 Graham and Roberts reported that the uptake of HCl on amorphous and crystalline ice films at 120 K results in the formation of a crystalline HCl‚6H 2 O hydrate layer.…”

Chemistry at the Surfaces of Ice and Sulfuric Acid: Toward an Understanding of Adsorption at Molecular Solids

“…Their bulk properties, for instance, their sublimation energies and infrared optical constants, are indistinguishable from those of macroscopic samples of the same temperature and composition. 13,[15][16][17] Also, for films of nominal thickness greater than ∼5 ML, the substrate is completely masked from the gas phase, and beyond ∼10 ML the substrate has no discernible influence on the structure or properties of a film. 15,18 There are several reasons for studying the surfaces of ultrathin films rather than those of macroscopic samples.…”