Semiclassical approach to fluorescence spectra of polar dye solutions. Anharmonic approximation

On the basis of the quasi-molecule model of electronic spectra of polar dye solutions, a description of structureless fluorescence spectra is accomplished. In the model, parameters of the spectrum have a simple physical interpretation and allow us to specify the dye-solvent interaction potentials in the initial and final states of fluorescence. These parameters for a few coumarin solutions are found by fitting theoretical distributions to the experimentally obtained spectra. A whole fluorescence spectrum can be theoretically reproduced only when the anharmonicity of the motion of molecules in solution is taken into account. However, a main part of the spectrum can also be recovered in harmonic approximation. A criterion for the fitting range in this case is formulated.