Transverse Acoustic Modes of Coherent Molecular Motions in Tetracene Crystals

Molecules in organic crystals, akin to atoms in inorganic lattices, can act as rigid units whose vibrations mediate thermal transport. While collective molecular motions are expected to generate coherent vibrational modes, including longitudinal acoustic (LA) and transverse acoustic (TA) modes, the coherent excitation of TA modes in molecular crystals remains largely unexplored. Here, we investigate the lattice dynamics of tetracene single crystals using polarization-dependent transient absorption spectroscopy. We observe long-lived oscillations featuring two well-defined gigahertz-frequency modes. Thickness-dependent scaling and interface-controlled damping confirm their origin as standing transverse acoustic phonons, arising from in-plane shear displacements confined by acoustic reflection at the crystal-air interface. These results provide direct spectroscopic evidence that ultrafast photoexcitation can coherently drive transverse collective motion in molecular crystals, opening new opportunities for phonon-engineered control of exciton dynamics, charge transport, and optomechanical coupling in molecular materials.