Line mixing and broadening in the Raman Q branch of HD at 304.6 K

The Q-branch lines of pure HD were measured at densities ranging from 1 to 7 Amagat at 304.6 K. Each profile was fitted to the well-known Rosenkranz e xpression to extract the size of the asymmetry due to line mixing as well a s to the linewidth. Line mixing and broadening coefficients were obtained b y fitting the asymmetries and widths to a straight line as a function of ga s density, Apart from a single existing measurement for the Q(0) line, our mixing coefficients are the first direct measurements of the asymmetry due to Line mixing in HD. Our broadening coefficients are consistent with the b est earlier measurements but are an order of magnitude more precise. Agreem ent is found with some existing semiclassical calculations of broadening. W e have fitted our PID broadening coefficients to a variety of empirical ene rgy gap laws. Our conclusions are that none of the exponential gap law (EGL ), the modified exponential gap (MEG) law, and the statistical power gap (S PG) law successfully models our broadening coefficients,. We present a modi fied version of the EGL and the MEG laws, which are successful in reproduci ng the experimental results. Using the fitted parameters of the new gap law , we have calculated the relaxation matrix of HD at room temperature. With this relaxation matrix, we have simulated the e-branch spectrum at a number of densities between 49.1 and 490 Amagat and compared the results with pre vious high-density measurements. At all densities and frequencies, the simu lated spectral intensity was found to agree with the measured strength with in about 5% of the peak of the spectrum. In addition, the comparison provid es evidence of a nonlinear vibrational dephasing Shift in HD. (C) 1998 Acad emic Press.