RAMAN-SPECTROSCOPY AND THEORETICAL MODELING OF HCL VIBRATIONAL FREQUENCY-SHIFTS IN HIGH-PRESSURE ARGON

Raman vibrational frequencies of HCl in argon were measured at pressur es up to 110 MPa. The mean frequency of the asymmetric Q-branch is sho wn to accurately measure vibrational shifts through a density region w here line shape changes due to motional;narrowing render the peak maxi mum an inaccurate measure of pressure induced frequency shifts. A semi classical, analytical expression utilizing Hutson's HCl-Ar pair-potent ials is used to determine the derivative of the HCl vibrational freque ncy with respect to Ar density in the limit of zero density. The predi ctions are in reasonable agreement with experimental results, although the experimental frequency shifts are about 20% smaller (less redshif ted) than theoretical predictions, which may represent the influence o f 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-bran ch) and rotational parts of the frequency shift suggest that the rotat ional contribution is positive (blueshifted) for all J values and appr oaches zero with increasing J. (C) 1996 American Institute of Physics.