Self-broadening and line mixing in HCN Q branches

Q-branch spectra of the ν1+ν2 (4004 cm−1) and ν2+ν3 (2806 cm−1) combination bands and the ν1−ν2 (2599 cm−1) difference hot band of HCN have been recorded at pressures from 0.13 to 53.3 kPa (1 to 400 Torr) using a tunable difference-frequency laser. The self-broadening coefficients are the same for all three bands involving the ν2 Π bending mode and are within experimental error of those reported previously for other Σ and Π vibrational bands. The J dependence of the self-broadening coefficients exhibits a maximum near the peak of the Boltzmann population distribution, and is well described by semiclassical line-broadening theory incorporating known measured or ab initio dipole and quadrupole moments and polarizabilities, and classical trajectories using an isotropic Lennard-Jones short-range potential. Line mixing is evident in the strongly overlapped, higher pressure Q-branch profiles from the nonadditive Lorentzian superposition of the component transitions. However, line coupling is moderated by the f→e collisional cross relaxation in the l-doubled Π bending vibration. The inelastic rotational collision rates required to fit the spectral line-mixing profiles are poorly represented by empirical energy-gap fitting laws for both R→T and R→R energy transfer. An effective R→T energy-corrected-sudden (ECS) scaling law yields a satisfactory and consistent fit to the Q-branch profiles of all three bands for the pressure range studied.