REACTION AND DIFFUSION IN HETEROGENEOUS ATMOSPHERIC CHEMISTRY STUDIEDBY ATTENUATED TOTAL INTERNAL-REFLECTION IR SPECTROSCOPY

The adsorption of gaseous HCl onto the surface of water ice films in t he range 90-160 K is known to result in the formation of a layer compo sed of hydrated protons and chloride ions, with the general formula (H 2O)(n)H3O+Cl-. Using a novel attenuated total internal reflection (ATR )-based IR spectroscopic probe in which ice films are condensed upon t he surface of a variable temperature internal reflection element in va cuo, the diffusion of material from this surface layer into the bulk o f 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 resulti ng absorbance vs. time curves with simple models. For ice films with t hicknesses greater than the effective depth of penetration of the prob e beam, spectra recorded immediately after exposure to HCl do not show features attributable to ionised HCl hydrates. However, as material f rom the interfacial layer penetrates into the bulk of the him, spectra recorded as a function of time can be used to measure the rate of dif fusion of HCl into the film from the ice/vacuum interface. By comparin g the absorbance due to the H3O+ ion vs. time curves with theoretical predictions, the Fick's law diffusion coefficient of (H2O)(n)H3O+Cl- i n ice is estimated to be ca. 10(-15) m(2) s(-1) at 150 K, in good agre ement with values estimated from indirect techniques.