Diagnostics of molecular plasmas and trace gas analysis using mid infrared lasers

Mid infrared (MIR) absorption spectroscopy between 3 and 20 µm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas and for trace gas analysis. The increasing interest in molecular processing plasmas has lead to further applications of IRLAS. IRLAS provides a means of determining the absolute concentrations and temperatures of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Since plasmas with molecular feed gases are used in many applications such as thin film deposition and semiconductor processing this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes and for trace gas analysis. The aim of the present contribution is threefold: (i) to report on selected studies of the spectroscopic properties and kinetic behaviour of the methyl radical, (ii) to review recent achievements in our understanding of molecular phenomena in plasmas and the influence of surfaces, and (iii) to describe the current status of advanced instrumentation for quantum cascade laser absorption spectroscopy (QCLAS).