Resonance rotational hyper-Raman scattering intensities of symmetric top molecules

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Abstract
Resonance rotational hyper-Raman scattering cross sections excited by identical linearly polarized incident photons are derived for symmetric tops. In the absence of pure dephasing, the rotational structure of a RHR vibrational transition is described, in general, by five scattering invariants which are derived in an irreducible tensor framework. The pattern of relative rotational RHR intensities is a sensitive function of the two-photon resonant vibronic state dephasing constant. High quality rotational RHR spectra of NH3 are obtained using blue incident radiation at half the X̃→Ã transition energy. Fits to the observed patterns of relative rotational RHR intensities determine lifetimes of 38, 60, and 100 fs for the v′2 =1,2,3 Ã state vibronic bands respectively. Although seven rotational branches (N,O,P,Q,R,S,T) are possible in HR, only three (P,Q,R) are observed in these RHR spectra of ammonia due to the magnitude and relative signs of vibronic matrix elements in the scattering cross section expressions.