RESONANCE HYPER-RAMAN SCATTERING POLARIZATION - A MEASURE OF METHYL-IODIDE B-STATE SUBPICOSECOND LIFETIMES

The resonance hyper-Raman (RHR) scattering of CH3I vapor is observed as blue incident radiation is tuned through two-photon resonance with the V2 vibronic absorption band of the predissociative B <-- X Rydberg transition in the UV. In analogy to linear resonance Raman spectroscopy, the RHR band shapes and polarization are found to be a sensitive function of the two-photon resonant vibronic state lifetime and detuning. The dependence of these scattering characteristics on the two-photon excited-state dephasing constant provides a technique for determining subpicosecond predissociation rates via nonlinear spontaneous Raman scattering. Theoretical fits to the observed resonant rovibrational hyper-Raman depolarization ratio dispersion curves are obtained for a vibronic B-state lifetime of 0.5 +/- 0.2 ps. This lifetime value is in agreement with previous results obtained from linear resonance Raman scattering studies. The polarization analysis of RHR scattering, in particular, provides a technique for measuring excited-state lifetimes of molecular transitions in the VUV.