PHOSPHORESCENCE MICROWAVE DOUBLE RESONANCE IN EXCITON STATES OF MOLECULAR CRYSTALS AND COHERENT STATES OF TRIPLET SPIN ENSEMBLES

Abstract

Saturation of the zero-field electron spin transitions of a phosphorescent triplet state by a microwave field causes changes in the intensity and/or polarization of the emission and thus forms the basis for phosphorescence microwave double resonance (PMDR)† in excited triplet states. Because of the sensitivity of photon detection, the technique is capable of detecting as few as 104 molecules in a sample depending upon the details of the radiative channels being monitored. If phosphorescence is monitored from an exciton band, PMDR can be used to experimentally differentiate between diffusion limited exciton migration and migration describable by the group velocity of the wave packet of k states, i.e. coherent exciton migration. In the following paper the relationships between PMDR spectroscopy, coherent triplet exciton migration and density of states functions in molecular crystals will be illustrated for crystals which can be considered as models for one-dimensional excitons. In particular, a resonance theory will be outlined that incorporates explicitly exciton–phonon scattering into the Bloch equations and allows one to extract both the lifetime of a k state of the band and the coherence length of the exciton. PMDR experiments in ‘one-dimensional’ molecular crystals are presented which illustrate the salient features of the theory. The experimental results are interpreted using a statistical theory which explicitly includes the exciton band dispersion, the density of state function of band and trap states, and the group velocity of the exciton wave packets. From the above experiments, a coherence length between 300 and 104 å and a coherence lifetime of 10-7 s have been found for k states in the centre of the band for certain substituted benzene crystals at 4 K.