Translational tunneling of protons in benzoic-acid crystals

Using high-resolution optical hole burning, we have determined the tunneling level structure of protons in hydrogen bonds at liquid-helium temperatures: The tunneling matrix element is J=8.4±{}0.1 GHz. The strain deformations of the tunneling potential are reflected in characteristic asymmetric inhomogeneous shapes of the spectral lines and can be explained by correlated variations of J and A, the energy asymmetry of the potential.