HETERODYNE SPECTROSCOPY OF CARBON-MONOXIDE LINES PERTURBED BY HYDROGEN AND HELIUM

The pressure-induced broadening and shift of CO spectral lines has been measured at room and liquid-nitrogen temperatures, using He and H-2 as perturber gases. The R(2), P(8), and R(14) rovibrational transitions for the fundamental vibrational band were studied, which absorb in the 5-mu-m region of the infrared. A novel heterodyne spectrometer operating near 5-mu-m wavelengths has been used for these measurements. The local oscillator (LO) for the spectrometer was a frequency-doubled CO2 laser, with CdGeAs2 as the nonlinear crystal. This LO was step tunable and could be frequency stabilized to better than 0.5 MHz absolute accuracy. A spectral resolution of 20 MHz was achieved. The CO-He line-broadening measurements were accurate enough (approximately 2%-3% absolute accuracy) to distinguish between two proposed intermolecular potential models for this system. Also, a measured increase in the broadening coefficient as a function of rotational quantum number j was observed for CO-He at temperatures of 80 K. This is not easy to explain in the context of so-called "sudden approximations" which have been used to interpret pressure-broadening data for this system. The shift measurements are the first reported for the fundamental vibrational band of CO with He or H-2. For H-2, there appears to be a decrease of the shift with the rotational state j of the transition. For both He and H-2, there is a pronounced temperature dependence of the shift for the P(8) transition, the shift becoming "redder" (toward lower frequency) as the temperature decreases from 298 to 80 K.