Abstract: Hydroxyl radical (OH) plays an important role in the atmospheric chemical reaction, combustion and plasma diagnosis. The plasma containing OH was produced through glow discharge with water vapor and its mixture as parent gases, and then the absorption and emission spectral intensities changing with the OH generation parameters were measured simultaneously. The dissociation of water vapor molecules by high-energy electron collision is the main way to produce OH. When the parent molecular pressure is low, both of the emission and the absorption spectral intensities increase with the water vapor concentration, and the variations of OH concentration measured by the two spectral methods are consistent. However, when the parent gas pressure exceeds a certain threshold, the continuous increase of parent gas pressure will reduce the electron energy and weaken the dissociation of water vapor molecules, resulting in the decrease of OH absorption spectral intensity. When pure water vapor is used as the parent gas, the collision caused by increasing pressure will increase the A-state population of OH, resulting in the slow increase of the emission spectral intensity with the water vapor pressure. However, nitrogen oxide molecules in nitrogen-containing parent molecules will collide and react with OH, resulting in the decrease of OH emission spectral intensity as the parent gas pressure increases.

Key words: spectroscopy, OH radicals, emission spectrum, absorption spectrum, wavelength modulation spectroscopy