Collisional effects on the rovibrational transitions of the HD fundamental band

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Abstract
The effect of collisions with self and buffer gases, Ar, Kr, and Xe, on the linewidth, line shape, and frequency of the rovibrational transitions of HD in its ground electronic state are studied with a high‐resolution tunable laser difference‐frequency spectrometer. Collisional narrowing, manifested in sub‐Doppler width of the transitions, is observed (for the first time in HD) in both P and R branches of the infrared fundamental band. The measured line profiles and the pressure dependence of their widths are analyzed in the limits of strong and weak velocity‐changing collisions. It is shown that both models fit the data equally well in the low pressure range (below about 300 Torr). The strong collision model, however, is found to be the best fit for HD/HD collisions in the entire 10–760 Torr pressure range. In the case of HD/buffer gas collisions, a model incorporating correlations between the velocity and state‐changing collisions appears to be more appropriate. The dependence of collisional narrowing and broadening parameters on the rotational quantum number is reported and discussed.