The applicability of Raman spectroscopy to the investigation
of gases has been greatly improved by the development of the
different methods of nonlinear Raman scattering. When two laser
beams, one of which has a tunable frequency, are brought to a
common focus in a sample, a stimulated Raman process occurs, as
soon as the frequency difference between the two lasers is equal
to aRaman active rovibrational or rotational transition frequency
of the sarnple, and the corresponding state is popuJated above
equilibrium. The Raman resonance can be detected in different
ways: by coherent anti-Stokes Raman scattering (CARS) or the
corresponding Stokes process (CSRS), by again in one of the
beams (stimulated Raman gain spectroscopy, SRGS) or a loss in
the other one (inverse Raman spectroscopy, IRS), or even by detection of a photoacoustic signal (photoacoustic Raman spectroscopy,PARS). The selective ionisation of the excited molecules by a third ultraviolet laser pulse (ionisation detected stimulated Raman scattering, IDSRS) has considerably increased the sensitivity in special cases. The instrumental resolution of the se techniques is determined by the convoluted linewidths of the lasers used for excitation. This is of special importance for the investigation of high resolution rotation-vibrational spectra of gases
