Optical dephasing and vibronic relaxation in molecular mixed crystals:A picosecond photon echo and optical study of pentacene in naphthalene and p-terphenyl

Picosecond photon echo and spectroscopic measurements have been used to establish that optical dephasing in the pure electronic and several vibronic transitions of pentacene in naphthalene is induced by pseudolocal phonon scattering in the ground and excited state. The frequency and low-temperature lifetime of this local phonon are 18 cm–1 and 3.5 ps in the groundstate and 13.8 cm–1 and 11 ps in the excited state (vibron independent). The deuterium isotope effect on the local phonon frequency demonstrates its molecular character and suggests its assignment as an in-plane librational mode. For the system pentacene in p-terphenyl photon echo measurements indicate the existence of a librational mode of ≈30 cm–1 with a ≈1.5 ps lifetime. Line shift measurements on both mixed crystal systems further show that the observed temperature induced shift is due to a difference in quadratic electron–bandphonon coupling of the ground and excited state and crystal anharmonicity (thermal expansion). We further report for a number of vibronic transitions the low-temperature dephasing times. Stimulated picosecond photon echo measurements are used to show that these relaxation times are only determined by population relaxation. No systematic variation is observed between the vibronic relaxation times and excess vibrational energy which indicates that the intramolecular anharmonic coupling determines the initial step in the vibrational energy dissipation process