SELF-BROADENING AND LINE MIXING IN HCN Q-BRANCHES

Q-branch spectra of the nu(1) + nu(2) (4004 cm-1) and nu(2) + nu(3) (2806 cm-1) combination bands and the nu(1) - nu(2) (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 nu(2) PI-bending mode and are within experimental error of those reported previously for other SIGMA and PI-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 PI-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.