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RESEARCH PRODUCT

High-resolution non-linear Raman spectroscopy in gases

subject

description

The resolution in the Raman spectra of gases has been greatly improved by the development of the different methods of non-linear Raman scattering. When two laser beams, one of which has a tunable frequency, are focused in a sample, a stimulated Raman process occurs as soon as the frequency difference between the two lasers is equal to a Raman-active rovibrational or rotational transition frequency. The Raman resonance can be detected in different ways: by coherent anti-Stokes Raman scattering (CARS) or the corresponding Stokes process (CSRS), by a gain in one of the beams (stimulated Raman gain spectroscopy, SRGS) or a loss in the other (inverse Raman spectroscopy, IRS), or even by detection of a photoacoustic signal (photoacoustic Raman spectroscopy, PARS). The selective ionization of the excited molecules by a third ultraviolet laser (ionization-detected stimulated Raman scattering, IDSRS) has considerably increased the sensitivity in special cases. The instrumental resolution is determined by the convoluted line widths of the lasers used for excitation. The narrowest line widths can be achieved with stabilized continuous-wave lasers. Their relatively low power has been compensated for either by intracavity excitation of CARS spectra or by injection locking of dye laser amplifiers which are pumped by pulsed lasers or by flashlamps. Examples of investigations of the structure of rovibrational bands and of line-width measurements as a function of pressure are reviewed.