Abstract
The results of a photon-echo relaxation study on the lowest 1B 2u ← 1A 1g transition of tetracene and pentacene in a p-terphenyl host crystal are reported. The photon-echo decay times were measured at 1.37 K and found to be 8.2 ± ns for tetracene and 13.8 ± 0.5 ns for pentacene. These relaxation times correspond to homogeneous linewidths of 19.4 and 11.5 MHz respectively and are over a factor of 10 3 smaller than the observed inhomogeneous linewidths in the optical absorption spectrum. Temperature dependent photon-echo intensity measurements further show that, below 3.5 K, optical relaxation is due to an orbach process with a presumably pseudolocalized phonon state. The resonant phonon frequency was measured to be 8.1 ± 1 cm −1 for tetracene and 21.2 ± 0.5 cm −1 for pentacene.
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