Knowledge Bank

The Q branches of the $v1+v2$ (4004 $cm-1)$ and $v1+v2$ (2806 $cm-1)$ combination bands and the $v1-v2$ (2599 $cm1)$ difference hot band of HCN have been recorded at pressures from 1 to 400 Torr using a tunable difference-frequency laser. The self broadening coefficients are identical for all three bands involving the $v2$ $\Pi$ bending mode and are within experimental error of those reported previously for $ \Sigma - \Sigma $ stretching bands. The J dependence of the self-broadening coefficients exhibits a maximum near the Boltzmann population peak and is well described by semiclassical line broadening theory incorporating known measured or ab initio dipole and quadrupole moments, curve trajectories and an isotropic Lennard-Jones short-range potential. Line-mixing is evident from the non-additive Q-branch collapse when the spectral lines overlap due to pressure broadening, but the line coupling is reduced by the $e-f$ cross-relaxation in the $\ell$-doubled II bending vibration. However, the inelastic rotational collision rates required to fit the spectral line-mixing profiles are poorly represented by empirical exponential energy-gap scaling laws for both R-T and R-R energy transfer.