Raman scattering discrete resonance

Discrete Resonance Raman Scattering (Resonance Fluorescence 1... 

An interesting consequence of the long lifetimes for discrete resonance is that the molecule may perform numerous rotations before re-emission. This causes depolarisation of the resonance-scattered radiation. For totally-symmetric vibrational modes in the normal Raman effect the scattering is polarised since the lifetime of the intermediate state is very short compared with a rotational period ( 10 s). 

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... 

Fig. 11. A. Spectrum showing discrete resonance Raman scattering from I2 gas. Equation was with a single mode of the 514.5 nm Ar" laser line in resonance with the transition B v =...

Fig. 2. Photon molecule interaction processes. (A) Normal Raman scattering, (B) discrete resonance Raman scattering, (C) continuum resonance Raman scattering. All these processes are amenable to direct scattering experiments generally, only (B) can be easily studied by time-resolved observation.

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