Theory of spin—lattice relaxation in the triplet state of naphthalene in durene

Abstract

Spin—lattice relaxation in the photoexcited triplet state T 1 of naphthalene in durene can be induced by a two-phonon relaxation process. The intermediate states are the sublevels of another triplet state T 2 which has a spin hamiltonian different from that of T 1. In the present work the relaxation rates in T 1 are calculated within a stochastic and a quantum-mechanical approach. The stochastic treatment describes the thermally activated jump between T 1 and T 2 by a stochasticly time dependent hamiltonian which induces transition probabilities between the sublevels of an averaged triplet state which results from T 1 and T 2 by fast exchange. In the quantum-mechanical treatment Orbach's formulae for the two-phonon relaxation process are adapted to a system in which the intermediate states are represented by the sublevels of the triplet state T 2. The results of both theories are compared with each other and with experimental data.