High-resolution Raman spectroscopy of gases and the determination of molecularbond lengths

This review highlights the developments that have taken place in the field of high-resolution Raman spectroscopy of gases from the pioneering studies of Stoicheff and Welsh in the early fifties to the present day. This period has seen major changes in the methods that have been employed for investig ating pure rotation and vibration-rotation spectra from these initial studi es with Hg excitation through to the deployment of laser sources for incohe rent Raman scattering at enhanced sensitivity, and the subsequent developme nt of the techniques of nonlinear Raman spectroscopy at resolutions of simi lar to 10(-3) cm(-1). A central theme in this review is the measurement of accurate rotational constants for nonpolar molecules that have then been em ployed for the determination of molecular geometries and bond lengths. The studies by Stoicheff of the pure rotational spectra of a wide range of line ar and symmetric-top molecules provided an extensive data base that served to supplement bond-length determinations from other methods and enabled him to correlate CC and CH bond length variations in noncyclic compounds with changes in their environment. The discovery of laser sources in the sixties provided exciting new opportunities for the examination of pure rotation a nd vibration-rotation spectra at enhanced resolution and sensitivity and br oadened dramatically the scope of the field. Apart from the improvements in the incoherent scattering methods afforded by these new sources, the disco very of a range of new nonlinear Raman phenomena, a field in which Stoichef f made equally important contributions, led to the creation of a range of n ew coherent nonlinear Raman methods that have been widely employed for the study of all rotor classes. Representative examples of the many investigati ons performed with the various spectroscopic methods over this period are g iven, together with the results of the structure determinations achieved fr om the analyses of the rotational spectra.