Population Change of OH and H₂O in Water Vapor Glow Discharge Measured Using Concentration Modulation Spectroscopy

A distributed feedback laser with a wavelength of 2.8 μ m was used to measure the species produced by water vapor glow discharge. Only the absorption spectra of OH radicals and transient H _2 O molecules were observed using concentration modulation (CM) spectroscopy. The intensities and orientations of the absorption peaks change with the demodulation phase, but the direction of one absorption peak of H _2 O is always opposite to the other peaks. The different spectral orientations of OH and H _2 O reflect the increase or the decrease of the number of particles in the energy levels. If more transient species can be detected in the discharge process, the dynamics of excitation, ionization, and decomposition of H _2 O can be better studied. This study shows that the demodulation phase relationship of CM spectrum can be used to study the population change of molecular energy levels.