Coherent excitations at the neutral-ionic transition: Femtosecond dynamics on diabatic potential energy surfaces

Upon decreasing temperature (T-c similar to 81 K) tetrathiafulvalene-chloranil (TTF-CA) undergoes a discontinuous phase transition from a neutral regular stack to an ionic dimerized stack. The same system may undergo a phase transition upon photoexcitation. We discuss the early time (<3 ps) dynamics following ultrafast (<20 fs) excitation of TTF-CA crystals either near or far from the neutral-ionic transition (T similar to 90 and 260 K, respectively). Adopting a modified Hubbard model with linear coupling of electrons to a zone-center lattice (dimerization) phonon and to a molecular vibration, both assumed to be harmonic, we are able to quantitatively reproduce coherent oscillations recently reported for this system [H. Uemura and H. Okamoto, Phys. Rev. Lett. 105, 258302 (2010); T. Miyamoto, H. Uemura, and H. Okamoto, J. Phys. Soc. Jpn. 81, 073703 (2012)]. We demonstrate that the microscopic model with parameters optimized for ground state properties of TTF-CA also applies to the coherent dynamics following photoexcitation. The modulation of the frequency of coherent molecular oscillations at the frequency of the lattice phonon emerges naturally as due to strong anharmonic coupling between the two harmonic vibrational degrees of freedom through coupling to delocalized electrons. Detailed comparison with experiment validates the model and sheds light on other aspects of photoinduced dynamics, including the almost instantaneous transfer of several electrons upon absorption of a single photon, as due to a cooperative effect of electronic correlations and molecular vibrations.