Self-broadening and line mixing in HCN Q branches

Abstract
Q‐branch spectra of the ν12 (4004 cm1) and ν23 (2806 cm1) combination bands and the ν1−ν2 (2599 cm1) 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 fe 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 RT and RR energy transfer. An effective RT 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.