Vibrational cooling in large molecular systems: Pentacene in naphthalene

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Abstract
Ultrafast laser experiments are conducted on low temperature crystals of pentacene in naphthalene (PTC/N) to study the process of vibrational cooling. A vibration of the excited singlet state, denoted Sν1, is excited, and the decay out of this state, as well as the subsequent arrival at the vibrationless ground state S01, are monitored by photon echoes, absorption recovery, and a new technique, pump‐induced coherent Stokes Raman scattering [T.‐C. Chang and D. D. Dlott, Chem. Phys. Lett. 1 4 7, 18 (1988)]. Eight vibrational modes of PTC, ranging from 260 to 1350 cm−1 are studied. The experimental results are interpreted using a previously developed model of vibrational cooling [J. R. Hill and D. D. Dlott, J. Chem. Phys. 8 9, 830 (1988)]. This model predicts the dependence of the vibrational cooling rate on the amount of excess vibrational energy and the temperature. The motion of the vibrational probability distribution toward the ground state is predicted to occur with a temperature independent ‘‘vibrational velocity’’ which describes the rate of vibrational energy dissipation. Using the model, we fit all eight data sets with a single adjustable parameter, the vibrational velocity, and we obtain the value V0=10±2 cm−1/ps. The prediction of a nearly temperature independent V0 is confirmed over the temperature range 1.5 to 35 K. Finally, we discuss the application of these measurements to the problem of heme cooling in optically excited heme proteins.