SYMMETRY SPECIES CONVERSION IN ROTATIONAL TUNNELING SYSTEMS OBSERVED BY HOLE-BURNING - HIGH-RESOLUTION SPECTROSCOPY OF DIMETHYL-S-TETRAZINEIN N-OCTANE

We present a summary of our results on methyl group spectroscopy and r elaxation measurements of the dye molecule dimethyl-s-tetrazine and it s CD3- and CDH2-substituted derivatives in a n-octane host. In the CH3 - and CD3-substituted derivatives, two hole burning mechanisms occur: one is based on nuclear spin-transformation, the other is a structural transformation. The mechanism based on spin transformation leads to s harp antiholes, spaced by 37 and 20 GHz from the burning laser frequen cy for CH3 and CD3, respectively. The structural burning mechanism lea ds to side holes. Surprisingly, the solitting of the side holes is dif ferent from that of the antiholes. This phenomenon is interpreted in t erms of two different dye species, which are distinguished through the ir local environment. Both species have very different yields for the structural and the nuclear spin phototransformation process. From the recovery of the central hole, the relaxation of the rotational tunneli ng states was measured as a function of temperature. The data support a Raman-type phonon scattering process. Deuteration does not slow down the relaxation but, instead, increases it by almost two orders of mag nitude. According to our knowledge these are the first measurements of symmetry species conversion times of isotopic derivatives of methyl g roups. Within the assumption of a Raman-type conversion mechanism we e stimate a rather low hindering potential barrier for the rotors.