The Electronic Stark Effect

Fluctuating interactions with the solvent thus broaden the vibronic absorption lines of the chromophore and shift them to higher energies relative to Eg. As discussed above, however, the mean energy of interaction can shift Eg either upward or downward depending on the chromophore and the solvent. We will discuss generalized solvent coordinates further in Chaps. 5 and 10. 

Fluctuating electrostatic interactions can be treated microscopically by incorporating Eqs. (4.50-4.52) into molecular d3mamics simulations (Box 6.1). The results can be used to construct potential energy surfaces similar to those of Eqs. (4.55a) and (4.55b), or can used in quantum calculations of the eigenvalues of the chromophore in the electric field from the solvent. Mercer et al. [86] were able to reproduce the width of the long-wavelength absorption band of bacteriochloro-phyll in methanol well by this approach. 

In the simplest situation for a molecular chromophore, the magnitude and direction of the shift depends oti the dot product of the local electric field vector (Eext =fEapp where E pp is the applied field and/is the local-field correction factor) with the vector difference between the chromophore s permanent dipole moments in the excited and ground states (Ap)  

The difference between the dipole moments in the two states can be related to the chromophore s molecular orbitals by the expression 

The effect on the overall absorption spectrum of an isotropic sample can be evaluated by expanding the absorption spectrum as a Taylor s series in powers of zlv  

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