DECOUPLING IN THE LINE MIXING OF ACETYLENE INFRARED Q-BRANCHES

The Q-branch profiles of the ν1 + ν5, ν3 + ν4 and ν2 + 2ν4 + ν5 Πu-Σg combination bands in the 2.5 μm C-H stretch-bend region of acetylene have been recorded with a difference-frequency laser spectrometer at pressures from 1 to 500 Torr (0.13 to 66.7 kPa). The broadening coefficients, obtained from the ν1 + ν5 band at pressures low enough to avoid significant spectral overlap, can be well fit with empirical rotationally inelastic energy-gap scaling laws or satisfactorily modeled with semiclassical line broadening theory using known intermolecular potential parameters. At pressures when lines are overlapped, collisional interference or line mixing is manifest as a deviation of the Q-branch profiles from an additive superposition of individual transition components. However the line coupling given by the state-to-state collisional scaling laws used to fit the broadening coefficients predicts far more collisional narrowing or Q-branch collapse than is observed. We find that only about one-third of the collisions that broaden the individual lines effectively couple the lines within the f sublevel of the l-doubled excited Π vibrational state observed in the Q branch. This decoupling indicates that there is little or no propensity for preserving the vibrational angular momentum sublevel upon collision, and that elastic reorientational and dephasing collisions may also be-significant. Additionally, we find that the collisional parameters and decoupling are independent of the vibrational state despite dramatically different spectral overlaps exhibited by the three bands studied and a close Fermi resonance between the lower two vibrations. This implies that vibrational relaxation and dephasing collision rates are negligible compared with rotationally inelastic and reorientational rates and usually can be ignored for infrared spectral broadening. © 1990 American Institute of Physics.