Collisional effects on the rotational and rotation--vibration Raman spectra of HD compressed by argon

Rotation and rotation–vibration Raman spectra of HD compressed by argon were recorded at room temperature (72 to 695 amagat) and 175 K (95 to 364 amagat). At low densities, an important motional narowing in the Q branch conceals the vibrational broadening, while this last mechanism becomes the main contribution to the bandwidth when pressure is sufficiently increased. Discussion of the results in the impact limit is given for the low density range where a linear density dependence of the rotational linewidths is observed. The validity of second order calculations in a semiclassical approach is discussed. The case of the HD–Ar system is compared to that of pure HD for which new data complete previous experimental studies.