The fusion of free triplet excitons in anthracene crystals at room temperature has been investigated by detection of the changes induced in the delayed fluorescence by a microwave field perturbing a static magnetic field applied to the crystal. This allowed detection of resonances at pair energy levels other than the degenerate one at which the static high-field resonances can be seen. A detailed study in the ab plane gave good agreement with the currently accepted energy-level diagram for the pair states. Possible approaches to account for the observed resonance intensities and linewidths are discussed.

• Received 25 March 1987

DOI:https://doi.org/10.1103/PhysRevB.36.2735