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A distributed feedback laser with a wavelength of 2.8 μm was used to measure the species produced by water vapor glow discharge.Only the absorption spectra of OH radicals and transient H2O 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 H2O is always opposite to the other peaks.The different spectral orientations of OH and H2O 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 H2O 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.