High-resolution cw-diode laser cavity ring-down spectroscopy in hydrogen plasma at room temperature

Reported is a description of an infrared cavity ring-down (CRD) spectrometer employing a cw-diode laser, that is applied for absorption measurements in a microwave discharge. The apparatus was recently built for study of recombination of H-3(+)(v = 0) with electrons. In cavity ring-down spectroscopy (CRDS) the monochromatic light of the laser is coupled into an optical cavity composed of high-reflectivity mirrors. Interrupting the laser beam the light intensity within the cavity decays due to losses on the mirrors and also due to absorbtion by gas-phase molecules (and/or molecular ions). The characteristic time constant of the exponential decay is coupled with the absorption coefficient, which is given by the corresponding absorption cross section and by the density of absorbing species. Using the CRDS method the absolute density of H-3(+) ions in hydrogen containing plasma is determined. The sensibility of the new CRDS system (< 2 x 10(-8) cm(-1)) was found to be sufficient for this kind of measurements.