Vibrationally unrelaxed bands

The case of N = 1 is trivial in a dynamics sense in that it corresponds to no IVR. A fluorescence spectrum belonging to this case consists entirely of vibrationally unrelaxed (u-type) bands. Each of these bands decays in the same manner. In most situations, these flecays are unmodulated, single exponentials, although quantum beats and multiexponential decays arising from couplings other than those associated with IVR are possible. 

Dispersed fluorescence spectra68 (Figs. 24 and 25) obtained upon excitation of various bands in Fig. 23 reveal spectral trends similar to those of anthracene426163 and many other large molecules (e.g., Refs. 10-14, 23, and 24). Excitation of low-energy bands gives rise to spectra that are structured and that contain a significant fraction of vibrationally unrelaxed (a-type) fluores-... 

Anti-Stokes picosecond TR spectra were also obtained with pump-probe time delays over the 0 to 10 ps range and selected spectra are shown in Figure 3.33. The anti-Stokes Raman spectrum at Ops indicates that hot, unrelaxed, species are produced. The approximately 1521 cm ethylenic stretch Raman band vibrational frequency also suggests that most of the Ops anti-Stokes TR spectrum is mostly due to the J intermediate. The 1521 cm Raman band s intensity and its bandwidth decrease with a decay time of about 2.5 ps, and this can be attributed the vibrational cooling and conformational relaxation of the chromophore as the J intermediate relaxes to produce the K intermediate.This very fast relaxation of the initially hot J intermediate is believed to be due to strong coupling between the chromophore the protein bath that can enable better energy transfer compared to typical solute-solvent interactions. ... 

Fig. 6. Schematic diagram showing the possible vibrational transitions seen in emission from an unrelaxed excited state following laser excitation in the 2—1 band.

Estimates of IVR rates have also been inferred from spectral studies. The principal spectral approach, the measurement of the emission spectra of isolated molecules (for a review, see, for example, Ref. 3), relies on the fact that the spectral characteristics of emission from a molecule will depend intimately on the vibrational character of the excited molecular state. If one prepares a gaseous molecule in a well-defined vibrational state and subsequently observes emission bands that would not be expected to arise from this initially prepared state, then some IVR process can be inferred. Moreover, a rate can be calculated for the process by comparing the intensities of expected emission bands (vibrationally unredistributed emission) and unexpected emission bands (vibrationally redistributed emission). Redistributed and unredistributed emission often are loosely termed relaxed and unrelaxed. Since energy is conserved in isolated molecules, there is no real energy relaxation, as occurs in solution or in solids. 

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