Inhomogeneous broadening of electronic transitions of chromophores in crystals and glasses: Analysis of hole burning and fluorescence line narrowing experiments

Abstract

A variety of nonphotochemical hole burning experiments are performed on the system pentacene (PC) in benzoic acid (BZA) crystals. These experiments and previously reported fluorescence line narrowing experiments are used to probe the nature of the inhomogeneous broadening of electronic transitions of chromophores in crystals and glasses. A microscopic model is presented which explains detailed results from both types of experiments. Two fundamental assumptions are made in formulating the model. First, there is a large degree of accidental degeneracy in the absolute energy distributions of the chromophores; i.e., molecules in different environments can have the same absolute energy. Second, this degeneracy is lifted when the electronic state is changed. Narrow band excitation of PC causes a well-defined change in the PC’s local environment, producing a narrow spectral hole in the inhomogeneously broadened PC absorption origin and an antihole which is spectrally well-separated from the PC origin. The microscopic model is successful in explaining experimental results on correlations between hole widths and antihole widths, hole positions and antihole positions, and in explaining fluorescence line narrowing experiments on the correlation between the frequencies of resonant and nonresonant fluorescence and the broadening of nonresonant fluorescence. The result is a microscopic picture which demonstrates that narrow band excitation of an inhomogeneously broadened electronic transition involves molecules with a very wide distribution of absolute energies.