We report a novel approach to measure rotational tunneling in condensed phases. High-resolution optical spectroscopic methods are used to unravel the tunneling level structure and to measure the rate of the nuclear-spin conversion. The method is demonstrated in experiments on the rotational motion of methyl groups in a crystal of durene doped with di-methyl-s-tetrazine but it can readily be transposed to other tunneling systems.

• Received 18 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.3277