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 H2 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 μm region of the infrared. A novel heterodyne spectrometer operating near 5 μm 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 (∼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 H2. For H2, there appears to be a decrease of the shift with the rotational state j of the transition. For both He and H2, 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.