High-resolution infrared difference frequency laser spectroscopic characterization of methane discharge plasmas

Abstract

Using an original double modulation set-up of an IR difference frequency laser spectrometer and a hollow cathode discharge cell with methane, in addition to a double phase sensitive detection method, it has been possible to distinguish between (nu) <SUB>3</SUB> band spectral lines of the CH<SUB>3</SUB> radical and the much stronger lines belonging to the C-H stretching bands of the precursor and other stable species (C<SUB>2</SUB>H<SUB>4</SUB>, C<SUB>2</SUB>H<SUB>6</SUB>) that appear in the discharge. The method has proved to be very sensitive to detect population variations versus temperature of very weak transitions from highly excited levels of CH<SUB>4</SUB> and other stable species. The temperature characterization of several neutral species involved in the hollow cathode discharge has been tested and remarkably low temperatures have been obtained.