Measurement and calculation of the Q-branch spectrum of nitrogen using inverse Raman spectroscopy and cw Raman-induced polarization spectroscopy

The techniques of inverse Raman spectroscopy, Raman-induced polarization spectroscopy (RIPS), and optical heterodyne RIPS (OHD-RIPS) are compared by probing the Q-branch of the nitrogen molecule. The signal is measured employing either a photomultiplier tube (low background level-RIPS) or a photodetector (high background level-IRS and OHD-RIPS). The measurements are performed using atmospheric mixtures of N-2-Ar with concentrations varying from 0 to 79% N-2. This strategy permits estimation of detection limits using the different techniques. Pump and probe energy levels are varied independently to study signal dependence on laser irradiance. A theoretical treatment is presented on the basis of the Raman susceptibility equations, which permits the calculation of spectra for all three techniques. Calculated Q-branch spectra are compared with the measured spectra for the interactions of a linearly polarized probe beam with a linearly or circularly polarized pump beam. The polarizer angle in the detection path for OHD-RIPS has a dramatic effect on the shape of the spectrum. The calculated and experimental OHD-RIPS spectra are in good agreement over the entire range of investigated polarizer angles. Detection limits using these techniques are analyzed to suggest their applicability for measuring other species of importance in combustion and plasma systems. Copyright (c) 2007 John Wiley & Sons, Ltd.