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39504-B6
A Study on the Role of Triplet States in the Photochemistry of Organosilanes

Samuel A. Abrash, University of Richmond

The purpose of the project was to examine the photochemistry of the SiH₂CO system of isomers on triplet electronic hypersurfaces. The planned approach was access to the isomer group by codeposition of Si with formadehyde in solid xenon, followed by wavelength selected photochemistry. Our plan included an initial step of carrying out preliminary experiments in Ar in order to learn the new techniques associated with deposition of highly refractory materials into cryogenic crystals. During the Fall of 2006 and the Spring and Summer of 2007, a team of students, including Diomedes Saldaña-Greco, Tran Doan, Allison Parker and Pradeep Kushwaha, worked to implement the project. Despite a number of serious equipment problems during that period (the FTIR laser, and both our turbopump and turbopump controller failed, and took substantial time to repair), our group made substantial progress in these initial experiments. In addition to completing the integration of the oven for evaporation of the solid Si to our cryogenic apparatus, our students succeeded in setting up the photolytic apparatus, combining a 1000 watt Hg-Xe lamp, a 1/4 meter monochromator, prisms and an optical telescope. Our approach to deposition of the formaldehyde in the matrix was flow pyrolysis of paraformaldehyde. We successfully demonstrated the technique at various pyrolysis temperatures, and were able to clearly control the concentrations of Ar to the extent that we could at will obtain clear spectra of formaldehyde monomers, or at slower Ar deposition rates and higher temperatures, mixtures of monomers, dimers and higher order clusters. In addition, using a UV-Vis spectroscopy as the analytical method, we were able to clearly demonstrate the deposition of Si atoms in the matrix. However, we were unable to succeed in co-deposition of the Si and formaldehyde in a way that dominant reaction product - silaketene formed. After many attempts to modify our procedure we were still unable to succeed in the deposition. In order to aid in diagnosis of the problem and to simplify the problem, we changed to depositon of Si in solid N₂ matrices, since Si spontaneously reacts with nitrogen molecules to form silicon nitride. These experiments failed as well. The challenge is the conductive properties of Si as a function of voltage. Since it is a semiconductor, at certain voltages, the resistance drops drastically, making careful temperature control extremely difficult. However, this problem can be conquered and we continue to work with it.

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