The theoretical basis for photoacoustic Raman spectroscopy (PARS) is developed. An expression for the Raman gain coefficient is derived. This gain coefficient is used to determine the change in the internal translational energy of a gaseous sample that is produced by illumination with two laser beams whose frequency difference corresponds to a Raman frequency shift. The magnitude of the pressure change associated with this nonlinear Raman process was deduced for a simple quasi-equilibrium model appropriate for modulated cw laser excitation and for a kinetic model applicable to pulsed laser excitation. The kinetic model explicitly accounts for the various pumping and relaxation rates associated with the PARS process.
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