Spectroscopy of Main Group Reactive Intermediates

This proposal sets forth a program of research to study the electronic spectra of main group reactive intermediates, concentrating on species containing silicon, germanium, tin, arsenic, boron or phosphorus. Our approach will be to use a variety oflaser spectroscopic techniques to study the reactive intermediates in the gas phase. These species will be generated by electron impact fragmentation of stable precursor compounds or by laser ablation. In order to simplifY complex spectra and obtain as much information as possible, the reactive species will be prepared in supersonic free-jet expansions so that they can be studied at very low rotational temperatures under collision-free conditions. Laserinduced fluorescence (LIF), wavelength-resolved fluorescence (WRF), stimulated emission pumping (SEP), fluorescence depletion, and resonance enhanced multi photon ionization (REMPI) techniques will be used to characterize the jet-cooled molecules. In our proposed spectroscopic studies ofsilicon-, germanium- and tin-containing molecules, we have targeted a variety of species whose electronic spectra are currently unknown. The siloxy radicals (SiXP; X = H, F, Cl), silicon analogs of the methoxy radicals, will be produced by discharge jet methods. Preliminary mass spectrometry studies have shown that a variety of carbides can be produced by laser ablation and we propose to study them by REMPI and LIF techniques. As an extension of our earlier work on the H2C=Si, H2C=Ge, SiCH and GeCH species, we also propose methods of producing and detecting the various C2H2Si isomers and the HC2Si linear free radical. From a fundamental point of view, the chemistry of silicon, germanium, and tin compounds is very different from that of their carbon analogs, which strongly motivates this work. Of particular interest will be the detection and characterization of molecules involving multiple bonds to silicon and germanium, an area of research that has recently received a lot of experimental and theoretical attention. Our studies of III-V semiconductor growth intermediates will focus on three types of free radicals. The arsino (AsH2) radical will be produced in a discharge jet and the spectroscopy and excited state dynamics will be probed by laser techniques. BCI2 and BF2 will also be synthesized by electron impact and the linear-bent transitions studied by LIF spectroscopy. Finally, the phosphorus-containing free radicals PF2, PCI2, PBr2, HPF and HPBr will be produced from simple halophosphine precursors. Preliminary laboratory studies have established the feasibility of most of the proposed experiments, providing strong evidence that the program of research will be successful and productive. Although the proposed experiments are fundamental in nature, they have a much broader impact. Many of the species to be studied may playa significant role in semiconductor growth processes and it is important to develop sensitive spectroscopic methods for detecting, identifYing, and characterizing them. Molecules and radicals containing silicon are also of current interest in astrophysics, as they are known to exist in interstellar gas clouds and circumstellar atmospheres. Our interest in arsenic, boron and phosphorus species stems from their importance as reactive intermediates in processes used to modify the electronic characteristics of semiconducting materials. Doping with group III and V elements substantially changes the properties of intrinsic semiconductors and our research is aimed at detecting and characterizing free radicals that can be formed in such processes. The spectroscopic information obtained from the work outlined in this proposal will provide necessary data for future studies of these reactive species in industrial processes, chemical reactions, and in the interstellar medium. The data will also be invaluable for laboratory studies of their reaction kinetics. These spectroscopic studies will establish important primary data about the molecular structures, energy levels, and electronic transitions of a series of intermediates for which little precise information is currently available.