Scattering near-resonant

We note that since Q involves the scattering coefficients, the radiation pressure force has resonance or near-resonance behavior. This first was observed and analyzed by Ashkin and Dziedzic (1977) in their study of microparticle levitation by radiation pressure. They made additional measurements (Ashkin and Dziedzic, 1981) of the laser power required to levitate a microdroplet, and Fig. 19 presents their data for a silicone droplet. The morphological resonance spectrum for the 180° backscattered light shows well-defined peaks at wavelengths corresponding to frequencies close to natural frequencies of the sphere. The laser power shows the same resonance structures in reverse, that is, when the scattered intensity is high the laser power required to levitate the droplet is low. 

While near-resonant light exerts both scattering forces and dipole forces on single atoms, similar forces are also exerted on larger dielectric objects. 

NIH NIR NMR NRS NSOM National Institutes of Health Near Infrared Nuclear Magnetic Resonance Nuclear Resonant Scattering Near-field Scanning Optical Microscopy... 

Becker, M., Gaubatz, U., Bergmann, K. and Jones, P.L. (1987). Efficient and selective population of high vibrational levels by stimulated near resonance Raman scattering, J. Chem. Phys., 87, 5064-5076. 

In extreme cases a multiple-scattering, sharp resonant structure can result in which the electron is in a quasi-bound state (155). One example is the white line, which is among the most spectacular features in X-ray absorption and is seen in spectra of covalently bonded materials as sharp ( 2eV wide) peaks in absorption immediately above threshold (i.e., the near continuum). The cause of white lines has qualitatively been understood as being due to a high density of final states or due to exciton effects (56, 203). Their description depends upon the physical approach to the problem for example, the LiUii white lines of the transition metals are interpreted as a density-of-states effect in band-structure calculations but as a matrix-element effect in scattering language. 

The relative contributions of A-and 6-term scattering under resonance conditions is a subject of considerable interest and different conclusions have been found for different systems. For example the A-term predominates for n-electron systems, e. g. polyenes, especially for their main intense absorption band (Warshel 1977). Vibrational wavefunc-tions of non-totally symmetric modes are more nearly orthogonal. Thus, vibrations may only derive their intensities from the 6-teim. A-term and 6-term enhancement can be distinguished experimentally by their excitation profiles. For A-term scattering a peak in the excitation profile is expected at the origin of the resonant electronic transition and subsidiary peaks at successive excited state vibrational levels. The amplitudes of the peaks depend on the successive Frank-Condon factors. For 6-term scattering excitation profile maxima are expected at the 0 0 and 1 0 positions for each of the mixing... 

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). 

J. L. Carlsten and A. Szoke. Spectral resolution of near-resonant Rayleigh scattering and collision induced resonance fluorescence. Phys. Rev. Lett., 56 667-671 (1976). 

J. L. Carlsten, A. Szoke, and M. G. Raymer. Collisional redistribution and saturation of near-resonance scattered light. Phys. Rev. A, 75 1029-1045 (1977). 

The ET spectrum of trans-butadiene shows two well-defined resonances which we attributed (18) to occupation of the two empty it orbitals. The lower resonance lies below that of ethylene and exhibits sharper structure. Figure 2 shows these data on an expanded energy scale for both "high rejection" conditions In which the signal derives from the total scattering cross section, and "low rejection" which reflects the differential elastic scattering near 180° (IS). The symmetric C-C vibrations of the anion are the most pronounced, but there Is evidence for low frequency out-of-plane modes as well. The upper resonance lies above that of ethylene and Is featureless. 

A schematic view of the process that occurs in RRS is given in Figure 1.1.1b. Excitation is made within an absorption band to a virtual state nearly of the same energy as one of the stationary states of the system. The near-resonance electronic interaction enables the molecule to interact much more effectively with the light and provides an enhancement factor of 10 to 10 in scattering probability. 

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