Electron-resonance spectra

This band was selected rather than the stronger (0,0) band since the Stokes laser is then positioned in the spectral region covered by the dye DCM which has a very broad tuning range necessary to cover the simple, but quite spectrally-separated electronically-resonant CARS spectrum. Electronic resonance enhancement has been observed in OH in flames as seen in Fig. 11. Without enhancement, OH, even at the percent concentrations possible in very high temperature flames, would be submerged within the H2O CARS spectrum whose modulated bandhead is seen near 3652... 

It was noted early by Reed and others that the IETS spectrum could exhibit both absorption and emission peaks - that is, the plots of Fig. 9 could have positive excursions and negative excursions called peaks and dips. The simple analysis suggested in Fig. 9 implies that it should always be absorptive behavior, and therefore that there should always be a peak (a maximum, an enhancement) in the IETS spectrum at the vibrational resonances. It has been observed, however, that dips sometimes occur in these spectra. These have been particularly visible in small molecules in junctions, such as in the work of van Ruitenbeek [92, 109] (Fig. 12). Here, formal analysis indicates that, as the injection gap gets smaller, the existence of an inelastic vibrational channel does not contribute a second independent channel to the transport, but rather opens up an interference [100]. This interference can actually impede transport, resulting in a dip in the spectrum. Qualitatively, this occurs because the system is close to an electronic resonance without the vibrational coupling the conductance is close to g0, and the interference subtracts from the current. 

Instead of one resonance frequency per individual electron, Bethe recovered the spectrum of resonance frequencies for the atom, weighted by dipole oscillator strengths satisfying the sum rule... 

The H NMR spectrum shows resonances for (5.95 ppm), Hg/H g/ exo (4.01 ppm), H /Hg/ endo (3.12 ppm) and Hy (0.30 ppm). The NMR data are in accord with a bridged, puckered bicyclobutonium ion structure that is static on the NMR time scale. The 29Si NMR chemical shift of 43.1 ppm for ion 428 indicates that the silicon is involved in stabilization of the positive charge. The stabilization occurs by shifting electron density from the Cy—Si cr-bond across the bridging bond to the formal carbenium carbon Ca. This y-silyl- type of interaction may be termed silicon homohyperconjugation. 

Fig. 15.8. Schematic one-dimensional illustration of electronic predissociation. The photon is assumed to excite simultaneously both excited states, leading to a structureless absorption spectrum for state 1 and a discrete spectrum for state 2, provided there is no coupling between these states. The resultant is a broad spectrum with sharp superimposed spikes. However, if state 2 is coupled to the dissociative state, the discrete absorption lines turn into resonances with lineshapes that depend on the strength of the coupling between the two excited electronic states. Two examples are schematically drawn on the right-hand side (weak and strong coupling). Due to interference between the non-resonant and the resonant contributions to the spectrum the resonance lineshapes can have a more complicated appearance than shown here (Lefebvre-Brion and Field 1986 ch.6). In the first case, the autocorrelation function S(t) shows a long sequence of recurrences, while in the second case only a single recurrence with small amplitude is developed. The diffuseness of the resonances or vibrational structures is a direct measure of the electronic coupling strength.

Figure 3-21 Comparison of TCNQ - electronic absorption spectrum and resonance Raman excitation profiles, (a) Electronic absorption spectrum from 15,000 to 17,850 cm-1. The extinction coefficient, e, scale is normalized with respect to e at 663.0nm (15,083cm-1) = 3.0 x 103M-Icm-1. (b) Superposition of the v2 (2,192cm-1). v4 (1,389cm-1), v5 (1,195cm-1) and V9 (336 cm-1) excitation profiles. The relative intensity scale has been scaled to 0.00 to lO.Ofor all four spectra. (Reproduced with permission from Ref. 76. Copyright 1976 American Chemical Society.)...

Within the separable harmonic approximation, the < f i(t) > and < i i(t) > overlaps are dependent on the semi-classical force the molecule experiences along this vibrational normal mode coordinate in the excited electronic state, i.e. the slope of the excited electronic state potential energy surface along this vibrational normal mode coordinate. Thus, the resonance Raman and absorption cross-sections depend directly on the excited-state structural dynamics, but in different ways mathematically. It is this complementarity that allows us to extract the structural dynamics from a quantitative measure of the absorption spectrum and resonance Raman cross-sections. 

Figure 3. Resonance Raman and fluorescence spectra of V isolated in solid Ar matrix (15 K) for different exciting laser lines for a matrix with V/Ar 2 y. 10 spectral slits 4 cm laser power at the sample was 20 mW for the 6348-nm line and 5 mW for all others. Key a, ground state resonance Raman spectrum b, excited state resonance Raman spectrum c, electronic resonance...

The three Fe-S clusters have been studied extensively by Epr 85,86 g values of2.05,1.94 and 1.86, Fb has values of 2.07, 1.92 and 1.89, and Fx2.04, 1.88 and 1.78. If both Fa and Fb are reduced at the same time, the clusters interact to give a composite spectrum with resonances at 2.05, 1.94, 1.92 and 1.89. When the electron transfer is intermpted, the recombination is from an earlier acceptor and is almost always faster than when a later acceptor is reduced. 

Since the spectrum of the amide groups shows electronic resonance coupling in the crystal, one could guess that the presence of orderly arrays... 

Mossbauer spectroscopy has proved to be a very useful technique for studying electron transfer and mixed valency in the FeOCl intercalates. The nature of the electron-transfer process which occurs on formation of FeOCl Fe(Cp)2 o.i6 has been studied in detail by Fe Mossbauer spectroscopy. At temperatures in the range 77-100 K, the Mossbauer spectrum exhibits resonance characteristic of the ferrocenium cation and two chemically distinct Fe sites. The two Fe " " sites are different in the number of nearest-neighbor Fe ions. Quantitative measurements confirm that all the ferrocene guest molecules are oxidized and complete electron transfer to the host has occurred. At 300 K, only one... 

Fig. 18. The dashed curves represent the axially-symmetric powder pattern for the dangling bond observed at Si-SiOj interfaces (Caplan et al 1979) for several values of isotropic broadening W. The solid curve is the ESR spectrum in a-Si H. [Reprinted by permission of the publisher from Electron spin resonance studies of amorphous silicon, by D.K. Biegelsen, Proceedings of the Electron Resonance Society Symposium, Vol. 3, pp. 85 - 94. Copyright 1981 by Elsevier Science Publishing Co., Inc.]...

The TDMEPs that are of concern in this review acquire their serious difficulty not only from the fact that the eigenfunctions of the states of real systems, have many-electron structures and are not solvable one-electron models but also from the fact that many discrete states may be involved while the multichannel continuous spectrum, including resonances with multiply excited structures, becomes critically important. 

Fig. 10. Energy level diagram for electronic resonance enhancement in the CARS spectrum of hydroxyl, OH, in which the pump laser is tuned into resonance. Strong enhancement occurs only for allowed downward Stokes transitions leading to a triplet spectrum.

Fig. 11. Portion of electronically-resonant CASS spectrum of OH In a flame for pump laser tuned to the (1-0) Qj(2) transition.

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