Raman Diagnostics of Heterogeneous Chemical Processes

Abstract

A great many technical processes are based on heterogeneous chemical reactions at gas-solid interfaces. Among these are petrol refining or the production of electronic devices by means of chemical vapor deposition (CVD). Hydrodynamic transport mechanisms in the gas phase strongly influence the kinetics of such processes; furthermore, very little is known about the nature and role of intermediate species. A clarification of these questions requires precise knowledge of the fundamental parameters, namely the flow, temperature and concentration fields in the gas phase. In most cases the systems are too complex for satisfactory theoretical approaches. Conventional experimental methods such as thermocouple measurements or sampling techniques have many limitations resulting from perturbation of the system under study by the solid probe or from perturbation of the probe by the system. Optical techniques, in particular laser spectroscopy, however, besides being measurements of a non-intrusive character, meet the further requirements of in situ observation and high spatial resolution. While laser-induced fluorescence is suitable for qualitative detection of gas phase species with high sensitivity, laser Raman spectroscopy provides the possibility of simultaneous determination of local values of the temperature and the molecular concentrations.