Anharmonic excited state surface

The failure to achieve selectivity in this model system can be traced to the dynamics on the anharmonic excited-state surface, and, in particular, the wavepacket bifurcation. This observation motivated us to explore more systematically the features of the excited-state potential energy surface and excited-state wavepacket dynamics that are compatible with the proposed selectivity scheme. 

Figure 28. Anharmonic excited-state potential energy surface. The classical trajectory that originates from rest from the ground-state equilibrium geometry is shown superposed. 

We have applied these ideas to the case described in Figs. 24 and 25, where wavepacket spreading on the broad, anharmonic, excited-state potential energy surface destroys the selectivity. A square pulse was used for... 

Apart from the heat bath mode, the harmonic potential surface model has been used for the molecular vibrations. It is possible to include the generalized harmonic potential surfaces, i.e., displaced-distorted-rotated surfaces. In this case, the mode coupling can be treated within this model. Beyond the generalized harmonic potential surface model, there is no systematic approach in constructing the generalized (multi-mode coupled) master equation that can be numerically solved. The first step to attack this problem would start with anharmonicity corrections to the harmonic potential surface model. Since anharmonicity has been recognized as an important mechanism in the vibrational dynamics in the electronically excited states, urgent realization of this work is needed. 

In both cases, because of restrictions imposed on the excitation process (e.g. optical selection rules), the initially excited state is not an exact eigenstate of the molecular Hamiltonian (see below). At the same time, if the molecule is large enough, this initially prepared zero-order excited state is embedded in a bath of a very large number of other states. Interaction between these zero-order states results from residual molecular interactions such as corrections to the Bom Oppenheimer approximation in the first example and anharmonic corrections to nuclear potential surfaces in the second. These exist even in the absence of interactions with other molecules, giving rise to relaxation even in isolated (large) molecules. The quasi-continuous manifolds of states are sometimes referred to as molecular heat baths. The fact that these states are initially not populated implies that these baths are at zero temperature. 

In systems in which there is very strong electronic coupling, the ground and excited state (or reactant and product) PE surfaces can be very anharmonic. This can have large effects on Xr, AGda, and /iJ/max- Some aspects of the effects on Xr and AGda were discussed in Sections 7.11.15 and 7.11.2.6. A shift in the electronic absorption, with respect to a reference value, when the D/A... 

Earlier theoretical work on the excited-state dynamics of pyrazine focused on the vibronically induced anharmonic couplings in the Si state, on the interactions between gerade and ungerade rm states or on selected PE functions and electron-vibrational coupling parameters. In a series of papers the Munich group has characterized the S2 and 51-state PE surfaces in the conformational subspace relevant for the short-time photophysics following the S2 — So and 5i — So transitions.In the course of this work, the treatment has steadily improved, as regards the accuracy of the electronic-structure calculation as well as the number and description of the vibrational modes considered. The most accurate calculations are based on the CASSCE/MRCI method with a basis set of DZP quality. 

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