An integrated system for surface science measurements of adsorbed species on ice surface under UV laser irradiation: application to water vapour deposition, reaction and desorption processes

Abstract

A Knudsen-cell type reactor equipped with a grazing angle reflection absorption infrared (RAIR) spectrometer and a temperature-programmed thermal desorption (TPD) system has been devised for investigating adsorption, desorption and reaction processes on ice surface, sometimes under UV light irradiation. In the Knudsen-cell chamber is settled a temperature-controlled Au surface on which a desired amount of water vapour, and then a desired amount of target gases can be deposited. The surface can be covered or opened with a sliding cover from outside. The pressure in the Knudsen-cell chamber before and during the deposition is monitored, from which the uptake coefficient is estimated. Adsorbed species on ice surface and ice film are monitored during adsorption and desorption by grazing angle RAIR spectroscopy. The reactor was used for investigating the adsorption and desorption processes of water. The reaction under 193 nm UV laser irradiation was also studied. The uptake coefficient of water vapour on the ice film at 120 K was found to be 0.45 ± 0.2, weakly depending on the pressure of water vapour above the surface. The RAIR spectral shape of the ice film changed during the adsorption and desorption processes, slightly differently for the two processes. The UV laser irradiation made a small change in the RAIR spectra of the OH stretching region, although the activation energy for the desorption of the ice film was not different before and after substantial UV irradiation. The principal part of the deposited ice film was thus kept un-reacted during the UV irradiation.