Raman scattering in hydrogen halide gases

We have measured the Q branch vibrational Raman cross sections of the hydrogen halide gases HCl, HBr, and HI relative to that of N2. We also measured the variation of the HI cross section as a function of the incident laser frequency. The absence of preresonance behavior in HI suggests that the virtual states primarily responsible for Raman scattering lie near or above the ionization potential. This in turn implies that the variation of the Raman cross section among the hydrogen halide molecules results primarily from a variation in the magnitude of the dipole matrix elements rather than from a variation in the energy of the virtual states involved.