Spectroscopy between 5 and 6 μm with a tunable spin-flip Raman source

We describe the application of a tunable, infrared, spin-flip Raman source to the measurement of the absorption spectra of gases in the spectral region 5–6 μm. Continuous tuning was obtained by using a low finesse Raman crystal cavity to eliminate mode pulling and hopping effects. The effects of atmospheric absorption, and pulse-to-pulse and magnetic field dependent intensity variations were cancelled by using a double beam arrangement. As an illustration of the operation of such a Raman spectrometer we obtain the P and Q branch absorption spectra of nitric oxide at various gas pressures in the spectral region 1890-1835 cm^-1. Studies at low gas pressure and with the tunable source operating close to the threshold for stimulated emission show a resolution of ≈ 0.08 cm^-1, determined by the linewidth of the Raman scattered radiation. This linewidth is shown to be in agreement with a mechanism of stimulated superradiant narrowing of the spontaneously scattered lineshape.