Recent progress in infrared laser spectroscopy to characterize low and atmospheric pressure plasmas

Despite the ever-growing applications of low-temperature plasma physics and technology, many plasma processes are far from being completely understood, in particular, the physical and chemical interaction of plasmas with solids and liquids. It is therefore essential to diagnose the fluxes of the generated species, to identify the relevant reaction pathways, to be able to tailor the reaction products for specific applications, and to gain further insight into plasma-induced reactivity in condensed matter systems. This requires high precision measurements of reactive molecular precursors, free radicals and short-lived species. The typical low abundances of the key transient reactive species, nowadays often in combination with small plasma dimensions, make the detection of these species a challenge.

To detect atomic and molecular species, absorption spectroscopy has become the method of choice for characterizing the fluxes of species in plasmas as it has several advantages over other optical diagnostic techniques. Molecular spectroscopy in the near- and mid-infrared regions is highly favourable because of the plethora of molecular fundamental, overtone and combination bands that can be accessed. As a result, selective and very sensitive spectroscopic measurements of a large number of compounds can be performed. I will discuss the recent progress in plasma spectroscopy in the infrared spectral region using various types of lasers.

The sensitivity of laser spectroscopy can be enhanced by combining it with a high finesse optical cavity using cavity-enhanced spectroscopy techniques. I will discuss the application of cavity-enhanced spectroscopy to determine species concentrations in, e.g., atmospheric pressure plasma jets, where we achieved effective absorption path lengths of up to 100 meters in mm sized plasma jets and therewith detection limits of ppb down to ppt levels.