Self-, N2- and Ar-broadening and line mixing in HCN and C2H2

Self-, N2- and Ar-broadening coefficients were measured for the stretch-bend infrared combination bands nu-1 + nu-1/2 (4004/cm) of HCN and nu-1 + nu-1/5 (4091/cm) of C2H2, using a tunable difference-frequency laser. At atmospheric pressures, the Q branches of these bands exhibit significant rotational narrowing or line mixing. The broadening coefficients are fit with empirical rotationally inelastic collision rate laws, which are then used to model the line mixing in the overlapped Q-branch profiles. Simple energy gap fitting laws appear to be suitable for the shorter-range intermolecular quadrupole-quadrupole and induction forces, whereas an energy-corrected-sudden scaling law works better for the longer-range dipole-dipole and dipole-quadrupole collision partners. In all cases, the line-coupling coefficients are substantially reduced from the rotationally inelastic rates fit to the broadening coefficients, indicating that 35-70 percent of the broadening may be due to other collisional mechanisms such as cross-relaxation to the degenerate H state vibrational level.