Nonlinear Raman Spectroscopy of Gases

Abstract

The field of Raman spectroscopy has enjoyed two periods of rejuvenation in the last 25 years. The first came with the discovery of the laser in 1960, a source that provided a dramatic increase in photon density at a scattering site and hence greatly improved spontaneous Raman spectra of liquids, solids, and gases. It was realized early that further gains could be obtained by nonlinear sample mixing of several optical fields, and the observation of coherent anti-Stokes Raman scattering (CARS) was first reported by Maker & Terhune in 1965 (1). Application to gas phase studies followed shortly thereafter (2, 3), but the real impetus for the second rebirth came in 1973 with the pioneering experiments of Taran and co-workers on combustion systems (4) and with the subsequent commercial development of pulsed Nd: Y AG and tunable dye lasers of high power and narrow bandwidth. The last decade has seen a remarkable growth in the number of applications of such sources to the study of molecular and physical properties. More than 2000 papers related to coherent Raman techniques have been published in the last 10 years and, of necessity, this review is restricted to applications to gas phase systems. At present, coherent Raman spectra have been obtained at gas pressures less than a few microbar, at temperatures ranging from a few degrees Kelvin to 3600 K, and at a resolution better than 0.001 cm -1. In addition to CARS, a number of other forms of coherent Raman probing have evolved. These include stimulated Raman gain (SRG) and loss (SRL) spectroscopy, photoacoustic Raman spectroscopy (PARS), Raman-induced Kerr effect spectroscopy (RIKES), and polarization and other variants of these. Recently, studies have been extended from the