Raman spectroscopy and theoretical modeling of HCl vibrational frequency shifts in high pressure argon

Raman vibrational frequencies of HCl in argon were measured at pressures up to 110 MPa. The mean frequency of the asymmetric Q-branch is shown to accurately measure vibrational shifts through a density region where line shape changes due to motional;narrowing render the peak maximum an inaccurate measure of pressure induced frequency shifts. A semiclassical, analytical expression utilizing Hutson's HCl-Ar pair-potentials is used to determine the derivative of the HCl vibrational frequency with respect to Ar density in the limit of zero density. The predictions are in reasonable agreement with experimental results, although the experimental frequency shifts are about 20% smaller (less redshifted) than theoretical predictions, which may represent the influence of multibody interactions. Experimental HCl Raman Q-branch and S-branch linewidths and peak shifts are compared qualitatively with previous R-branch (IR absorption) results. Separation of the vibrational (Q-branch) and rotational parts of the frequency shift suggest that the rotational contribution is positive (blueshifted) for all J values and approaches zero with increasing J. (C) 1996 American Institute of Physics.