Reaction and diffusion in heterogeneous atmospheric chemistry studied by attenuated total internal reflection IR spectroscopy

Abstract

The adsorption of gaseous HCl onto the surface of water ice films in the range 90–160 K is known to result in the formation of a layer composed of hydrated protons and chloride ions, with the general formula (H ₂ O) _n H ₃ O Cl ^- . Using a novel attenuated total internal reflection (ATR)-based IR spectroscopic probe in which ice films are condensed upon the surface of a variable temperature internal reflection element in vacuo, the diffusion of material from this surface layer into the bulk of the ice film has been studied. The validity of the ATR-IR method for the study of these systems has been verified by monitoring the growth of ice films as a function of exposure time and comparing the resulting absorbance vs. time curves with simple models. For ice films with thicknesses greater than the effective depth of penetration of the probe beam, spectra recorded immediately after exposure to HCl do not show features attributable to ionised HCl hydrates. However, as material from the interfacial layer penetrates into the bulk of the film, spectra recorded as a function of time can be used to measure the rate of diffusion of HCl into the film from the ice/vacuum interface. By comparing the absorbance due to the H ₃ O ⁺ ion vs. time curves with theoretical predictions, the Fick's law diffusion coefficient of (H ₂ O) _n H ₃ O Cl ^- in ice is estimated to be ca. 10 ^-15 m2 s ^-1 at 150 K, in good agreement with values estimated from indirect techniques.