Line-mixing in the 106—000 overtone transition of HCN

By using cavity ring-down spectroscopy ~CRDS!, we have obtained visible overtone absorption spectra of HCN which display a large collisional line-mixing effect in the proximity of the R branch band heads, for J;18. We consider in detail the 106←000 ~15CN, 05bend, 65CH! parallel transition. The R branch profile was modeled using the modified-exponential-gap ~MEG! and energy-corrected-sudden approximation ~ECS! population transfer rate laws. We used the rates previously determined by Pine and Looney ~PL! by fitting the self broadening coefficients measured for the Q branches of P–S infrared perpendicular stretch–bend combination bands of HCN @J. Chem. Phys. 96, 1704 ~1992!#. Contrary to what is found by these authors, in the present case the MEG law reproduces the R branch line-mixing satisfactorily, while the ECS model fails. This reflects an increasing propensity at higher J for collisional transitions with smaller DJ . Using the MEG law, we found we need to include, as had PL in their fits to the infrared Q branches, an empirical dephasing scale factor F;0.6 for the coherence transfer rates to obtain a satisfactory simulation of the R band head. PL suggested that dephasing in the Q branch spectra are due to cross relaxation across l-type doublet levels of the P state, but no such mechanism would be available in the present case. However, we have found that by using a 50/50 linear combination of the ECS and MEG rate laws, it is possible to fit our data even with F51, which would imply no dephasing of coherence. We take this as a demonstration that the dephasing factor F cannot be reliably extracted from line-mixing studies alone but instead requires some independent source of information on the relative value for state to state inelastic collision rates. © 1996 American Institute of Physics. @S0021-9606~96!02025-9#