Optical measurements of methyl group tunneling in molecular crystals: Temperature dependence of the nuclear spin conversion rate

The tunneling methyl groups in dimethyl-s-tetrazine (DMST) doped single crystals of durene were investigated by high resolution optical spectroscopy using spectral hole burning. The experiments probe the level structure as well as the relaxation dynamics of the tunneling methyl groups in different electronic states of DMST. The tunneling splitting differs by 1.24 GHz in the ground and the first excited singlet states of DMST. In the ground electronic state, relaxation (spin conversion) between the spin 3/2 (A) and 1/2 (E) tunneling levels was measured between 1.5 and 12 K. The spin conversion time is larger than 100 h at 1.5 K and decreases with Arrhenius-type behavior above 3.5 K. The activation energy of 20 cm−1 also is observed as a phonon sideband in emission, and is, in agreement with theoretical predictions, tentatively assigned to a librational mode of the methyl group.