Zero‐Phonon Lines in the Luminescence Spectra of Impurity Organic Crystals and Electron–Phonon Interaction

The temperature dependences of the shapes, widths, positions, and intensities of the zero‐phonon lines in the fluorescence spectra of 1.12‐benzperylene and coronene in a normal hexane matrix are investigated experimentally. The results obtained are discussed in terms of the theory of the electron‐phonon interaction in the impurity crystals. Thestrength of the impurity‐matrix coupling in the systems studied is shown to he very small. At high temperatures the zero‐phonon linewidth is a result of the Raman seattering of the lattice phonons by the impurity molecule. The estimation of the change in normal mode frequencies upon the phototransition in the impurity molecule and the mixing of the initial state normal modes of the crystal show that those values are small. The anharmonicity of the crystal vibrations in the “soft” organic crystals in contrast to the “hard” inorganic crystals is shown to play an important role and to determine considerably the zero‐phonon line characteristics.