Chromophore state

The absorption spectrum of folded AvGFP in the visible region displays two well-separated peaks at 395 nm and 475 nm, whose relative intensity depends on pH (Fig. 4b). The chromophore states associated to these two peaks have been termed, respectively, A and B [81]. Although this has been initially debated [78, 82, 83], accumulated spectroscopic and theoretical evidences indicate that these two absorption bands, respectively, arise from the neutral (A) and anionic (B) forms of the HBI chromophore [10, 11, 69-71, 84-87]. 

Luin S, Voliani V, Lanza G, Bizzarri R, Nifosi R, Amat P, Tozzini V, Serresi M, Beltram F (2009) Raman study of chromophore states in photochromic fluorescent proteins. J Am ChemSoc 131 96-103... 

With this system of clusters, we are not only able to expand the studies of IVR/VP but we can identify how the final chromophore state is achieved and whether or not a vibrational equilibrium is established in the cluster prior to VP (Hineman et al. 1993a). Clearly, these determinations depend on the relative rates of IVR and VP. In this discussion, the above serial IVR/VP model will serve as a framework within which to interpret the final chromophore product distributions following cluster IVR/VP. 

Intermolecular vibrational energy redistribution may populate several different chromophore states with sufficient energy in the van der Waals modes that VP can occur. If VP, which is predicted to be very fast in the Ar cluster, competes with subsequent IVR, then these chromophore states will be populated in the bare molecule. This would give a more crowded spectrum for 4EA(Ar)x with greater intensity away from the 0 transition, as is observed. These predictions are in qualitative agreement with all cluster data however, quantitative comparison for the Ar cluster is rendered impossible by crowding of the spectra (see Figure 5-11). 

The most important conclusions of these dynamical studies is that van der Waals clusters behave in a statistical manner and that IVR/VP kinetics are given by standard vibrational relaxation theories (Beswick and Jortner 1981 Jortner et al. 1988 Lin 1980 Mukamel and Jortner 1977) and unimolecular dissociation theories (Forst 1973 Gilbert and Smith 1990 Kelley and Bernstein 1986 Levine and Bernstein 1987 Pritchard 1984 Robinson and Holbrook 1972 Steinfeld et al. 1989). One can even arrive at a prediction for final chromophore product state distributions based on low energy chromophore modes. If rIVR tvp [4EA(Ar)i], a statistical distribution of cluster states is not achieved and vibrational population of the cluster does not reflect an internal equilibrium distribution of vibrational energy between vdW and chromophore states. If tvp rIVR, and internal vibrational equilibrium between the vibrational modes is established, and the relative intensities of the Ar = 0 torsional sequence bands of the bare chromophore following IVR/VP can be accurately calculated. A statisticsl sequential IVR/VP model readily explains the data set (i.e., rates, intensities, final product state distributions) for these clusters. 

If the energy gaps between the bridge and chromophore states are large (i.e., several eV), as is the case for bridges comprising saturated hydrocarbon moieties,... 

A change in the population of chromophore states is usually regarded as requiring a quadrature form of the interaction with the fields, as in active processes. Such a transition is quadratic in the incident field amplitudes since the rate of transition is proportional to the intensities of the incident fields. However, Eq. (4.8) indicates that an appropriate nonquadrature form of interaction can also change the population. The signal field in the medium sees only the status of the polarization induced in the medium the details of how the polarization has been created are not important, other than assigning to it an absolute phase. 

These chromophore states will be denoted nu n5)°. We will now focus on the polyad associated with an integer value for N. This polyad contains N + 1 states. For instance, Fermi resonance for the fifth CH overtone results in mixing among the following seven states 0, 12)°, l, 10)°, 2, 8>°, 3, 6)°, 4, 4)°, 5, 2>°, 6, 0)°. 

This results in an overall merged basis set of 207,000 states. The energy thresholds (26,500, 7000, 3500 cm ) used in Eq. (118) were specially selected for the study of the CH chromophore states. 

Eigenvalues (cm ) Projection on the zero-order chromophore states 6 chromophore eigenvector — m, 2m)" of the Chromophore character Assignment Eigenvalues obtained by neglecting the bath modes (cm )... 

Photoinduced unimolecular decomposition reactions are among the simplest reactions which can be studied experimentally and theoretically. One such reaction which has received considerable attention is the vibrational predissociation of small isolated van der Waals (vdW) clusters for which one molecule is a chromophore and the other is a small "solvent" molecule. Two dynamical events may transpire in such a system following the initial photoexcitation to Si vibronic levels vibrational energy may be redistributed to modes other than the optically accessed zero order chromophore states and at sufficient energies the cluster may dissociate. The fundamental theoretical understanding of these two kinetic processes should be accessible in terms of Fermi s golden rulel and unimolecular reaction rate2 concepts. 

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