Laser induced phonons: A probe of intermolecular interactions in molecular solids

A new type of density dependent spectroscopy is developed and applied to the study of acoustics phonons and excited state–phonon interactions in molecular crystals. It is shown that intermolecular separations in solids can be varied in a controlled manner along a selected crystalline direction and that the consequences of the molecular displacements can be observed optically in time-resolved transient grating, absorption, and fluorescence experiments. The technique involves the generation of specific acoustic waves (phonon modes of definite wave vector) by transient grating optical excitation of the sample. The resulting time-dependent density changes cause time-varying spectral shifts, the magnitude of which are dependent on the detailed nature of the anisotropic intermolecular interactions in the crystal, e.g., anisotropic van der Waals interactions in molecular crystals can be directly investigated. First a formalism is presented which permits the determination of the nature of the phonons produced (longitudinal, quasilongitudinal, or quasitransverse) by transient grating excitation in anisotropic crystals. Next, the effect of the transient grating generated phonons on experimental observables (absorption, fluorescence, and transient grating) due to anisotropic spectral shifts is obtained. Model calculations illustrate the results. Finally experimental data for the perylene pure crystal excimer system and the pentacene in p-terphenyl mixed system are presented. In the perylene system, the various types of phonons predicted by theory are observed and phonon dispersions are obtained for several crystal directions. In pentacene in p-terphenyl, the theoretically predicted phonon-induced spectral shifts are clearly observed.