Raman spectra resonance with continuum

The very different spectra of iodine obtained under continuum and discrete resonance-Raman conditions are illustrated in Fig. 11 for resonance with the B state, whose dissociation limit is 20,162 cm . In the case illustrated of discrete resonance-Raman scattering, Xl =514.5 nm, and specific re-emission results from an initial transition from the v" = 1 vibrational, J" = 99 rotational level of the X state to the v = 58, J = 100 level of the B state, i.e. the transition is 58 - l" R(99). Owing to the rotational selection rule for dipole radiation, AJ = 1, a pattern of doublets appears in the emission. Clearly, the continuum resonance-Raman spectrum of iodine (Xl = 488.0 nm) is very different from the discrete case spectrum. The structure, which arises from the 0,Q, and S branches of the multitude of vibration-rotation transitions occurring, can be analysed in terms of a Fortrat diagram, as done for gaseous bromine (67). 

Continuum resonance-Raman scattering can be observed under discrete resonance-Raman scattering conditions only if the resonance fluorescence is quenched, either with an inert gas, or (in the case of condensed phase studies) by the solvent or matrix. Thus, on excitation of a liquid, solution, or solid within the contour of an absorption band, the Raman spectrum observed has the characteristics of the continuum rather than the discrete case or, in other terminology, of resonance Raman, rather than resonance fluorescence spectra. Such spectra provide unique information on the spectroscopic properties of radical cations and ions, some of which species are unstable in air. Particularly noteworthy have been the studies by Andrews et al. (65) which have... 

Finally, both anharmonic and vibronic effects (with the inclusion of Duschinsky mixing and Herzberg-Teller contributions) have been considered for the simulation of resonance Raman spectra, along with the environment description by a mbced discrete/continuum solvent model. The resulting RR spectrum up to the... 

Zhang, J.Z.H. and Miller, W.H. (1990). Photodissociation and continuum resonance Raman cross sections and general Franck-Condon intensities from 5-matrix Kohn scattering calculations with application to the photoelectron spectrum of H2F- + hv —> H2 + F, HF + H+e, J. Chem. Phys. 92, 1811-1818. 

Finally, we like to mention that equivalent to the conventional energy frame KHD formulation, the time-dependent theory of Raman scattering is free from any approximations except the usual second order perturbation method used to derive the KHD expression. When applied to resonance and near resonance Raman scattering, the time-dependent formulation has shown advantages over the static KHD formulation. Apparently, the time-dependent formulation lends itselfs to an interpretation where localized wave packets follow classical-like paths. As an example of the numerical calculation of continuum resonance Raman spectra we show in Fig. 6.1-7 the simulation of the A, = 4 transitions (third overtone) of D excited with Aq = 488.0 nm. Both, the KHD (Eqs. 6.1-2 and 6.1-18) as well as the time-dependent approach (Eqs. 6.1-2 and 6.1-19) very nicely simulate the experimental spectrum which consists mainly of Q- and S-branch transitions (Ganz and Kiefer, 1993b). 

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