High-Resolution Raman Spectroscopy of Gases with cw-Laser Excitation*

Abstract

Several techniques of exciting Raman spectra of low-pressure gases and vapors with cw laser radiation are described. Both the He–Ne laser with output at 6328 å and the Ar+ ion laser with output at 4880 and 5145 å have been used. The use of the He–Ne laser with a multiple-pass Raman tube inside the laser cavity yields Raman spectra which can be photographed in exposure times that are equivalent to those required with the classical Hg-arc and large-volume Raman tube as the source of scattered radiation. The Ar+-ion laser allows the photoelectric recording of rotational Raman spectra at atmospheric pressure with a signal/noise ratio of 10:1 when the Raman tube is located outside the laser cavity, and about 100:1 when the Raman tube is inside the laser cavity. A comparison is given for the relative efficiency when the scattering gas is located inside and outside the laser cavity. Pure rotational Raman spectra for N2, O2, CO2, and H3CC≡CH (methylacetylene) are presented. The depolarization ratios of both the Rayleigh and Raman lines of N2, O2, and CO2 have been determined and relative scattering cross section for the pure rotational Raman scattering of oxygen has been obtained.