Abstract
AbstractThe 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.
Highlights
As long as no lasers with variable frequency were available, only the Raman lines with the highest peak intensity could be observed with the stimulated Rarnan effect[1,2] because these alone were sufficiently amplified
For the first demonstration of coherent anti-Stckes Raman scattering" the second frequency was generated by stimulated Raman seattering in different liquids
We show in Figure 8 the COHERENT ANTI-STOKES RAMAN SPECTROSCOPY (CARS) spectrum of the VI band of methane l2CH4 which can be compared with the inverse Raman spectrum of Figure 3
Summary
As long as no lasers with variable frequency were available, only the Raman lines with the highest peak intensity could be observed with the stimulated Rarnan effect[1,2] because these alone were sufficiently amplified. In this article we restrict our attention to those techniques which have been used to record high-resolution spectra of molecules in the gas phase All these coherent Raman processes are generated when two laser beams, one of which has a tunable frequency, are brought to a common focus in the sample (see Figure 1). The Q-branches of the CH-vibrations of the symmetric top molecules ethane C2H614and cyclopropane C3H633 yielded very complicated structures under high resolution In such cases it is useful to reduce the complexity of the spectra by cooling or by supersonic expansion molecular jets in which the temperature and the number of populated rovibrational levels is.
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