Franck-Condon States of Coumarins in Alcoholic Solutions

Several mathematical descriptions of the fluorescence spectra of liquid dye solutions have been proposed [1-6]. Recently we proposed a new one for some liquid coumarin solutions [7], in which we used the concept that the dye molecule, together with the surrounding solvent forms a kind of quasi-molecule. This concept was known for a long time [8]. The total energy of such a quasi-molecule consists of the modified electronic and vibrational energies of the solute molecule and the interaction energy of the solute with solvent molecules in the co-sphere. The forces acting between solute and solvent molecules are much smaller than the forces between the atoms of the molecules. However, in some cases they are strong enough to force librations of the dye molecules about some equilibrium orientations. Typical separations between resulting energy levels are of the order of few c m 1 [9, 10], There exists another picture, where it is postulated that the energy of a solute molecule, modified by the surroundings, is a stochastic variable. The source of stochasticity is the reservoir's noise, which is assumed to be Gaussian. According to this picture a distribution of solute-solvent interaction energies and the existence of a minimum of the interaction energy in each state is assumed. In both approaches, the common and basic assumptions are on one hand the existence of some equilibrium orientations, and on the other the existence of closely spaced energy levels. The latter assumption