Homogeneous Broadening of Optical Transitions in Organic Mixed Crystals

We have used the phenomenon of laser-induced molecular photodissociation to determine the homogeneous linewidth at 2 K of the origin (zero-phonon line) and a vibronic transition in the mixed-crystal absorption spectrum of dimethyl s-tetrazine in durene. From the measured 55-MHz (upper limit) homogeneous width of the origin we conclude that in the vibrationless excited state coherence persists at least during the 6-nsec lifetime. The 29-GHz homogeneous vibronic linewidth is ascribed to vibrational relaxation.