A picosecond CARS study of vibron dynamics in molecular crystals: temperature dependence of homogeneous and inhomogeneous linewidths

An extensive picosecond CARS and spontaneous Raman investigation of temperature dependent dephasing of the totally symmetric vibrons in naphthalene (h8N), perdeuteronaphthalene (d8N), anthracene (h10A), and perdeuteroanthracene (d10A) crystals is presented. The low temperature picosecond decays, ranging from 128 to less than 10 ps, are due to vibrational relaxation by phonon emission. Unlike optical phonons in these crystals, vibron lifetimes of h8N and h10A do not decrease monotonically with frequency, a pattern which is observed in other molecular systems. The low temperature decay rate is proportional to the square of a cubic anharmonic matrix element and a density of difference states function which has a contribution from phonons and lower frequency vibrons. This function is evaluated for each substance and together with measured lifetimes yields matrix elements which are roughly equal for each vibron. We conclude that vibrational relaxation rates are controlled primarily by the density of final states. Relaxation in the deuterated crystals is faster than predicted by this model due to residual isotopic impurities and purely intramolecular vibrational relaxation. Three h8N and one h10A vibrons are studied over an extended temperature range. The homogeneous linewidth increases as Tn with n=2.0–2.4. The observed increase is consistent with the above energy relaxation model up to ∼80 K. From ∼80 K to the melting point, this temperature dependence is consistent with a pure dephasing process involving energy exchange with several thermally populated low frequency vibrons. From ∼250 K to the melting point inhomogeneous broadening is observed. It is suggested that the inhomogeneity is due to slowly (≳10 ps) varying density fluctuations which modulate intermolecular attractive interactions. In this respect, molecular crystals near the melting point resemble some molecular liquids.