Fundamental and Applied Studies of Molecular Plasmas Using Infrared Absorption Techniques

Over the past few years mid infrared absorption spectroscopy (MIR-AS) over the region from 3 to 20 ?m has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of MIR-AS because most of these compounds and their decomposition products are infrared active. MIR-AS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a micro second, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from MIR-AS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of MIR-AS techniques to industrial requirements including the development of new diagnostic equipment. The aim of the present chapter is fourfold: (i) to briefly summarize the basic principles of infrared absorption spectroscopy and related instrumentation, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas using different types of MIR-AS techniques, (iii) to describe examples of industrial process monitoring in the mid infrared and (iv) to discuss the potential of advanced instrumentation based on quantum cascade lasers (QCLs) for plasma diagnostics.