Spectroscopic studies of atoms and small molecules isolated in rare gas solids : photodissociation and thermal reactions

Abstract

The spectroscopy and the chemical dynamics of atoms and small molecules in solid rare gas matrices are studied in this thesis. The many-body nature of the surrounding solid environment causes effects on the spectroscopy and chemistry of the species embedded in the solid environment. These effects may lead to very different behaviour than expected according to the gas phase results. For example, the reaction between two thermally mobilized hydrogen atoms in solid xenon is hindered by an energy barrier, and reaction products other than molecular hydrogen are favored. Not only the reactions between atoms or molecules, but also their photochemistry is greatly affected by the solid surrounding. The first half of the present thesis summarizes the studies of the novel rare gas compounds and the second half is dedicated to the studies of formyl fluoride and its dimers. The electronic absorption spectra of the thermal reaction product molecules HXeY (Y = Cl, Br, I, CN, SH, OH, or H) were measured in solid xenon. A broad and structureless UV absorption is characteristic of these molecules. Quantum chemical calculations were carried out in order to support the spectral assignments. A kinetic model was used to simulate the reactions of thermally mobilized hydrogen atoms. The experimental evidence and computational predictions of the HXeSH • • • H2S complex are also discussed. The studies of formyl fluoride were focused on its photodissociation and dimerization. Four stable dimer structures were found computationally, and an experimentally observed blue shifted C-H stretching vibration was assigned to a formyl fluoride dimer containing a double -CH-O- hydrogen bond. The photodissociation of formyl fluoride is affected by the cage effect of the surrounding solid. This effect prevents the separation of the photoproducts and thus in-cage dynamics dominates. In-cage reactions yielded mainly molecular complexes between CO and HF. Computational predictions were made to rationalize the observed photodissociation of the complexes between CO and HF in solid krypton and xenon.