We studied by neutron spectroscopy H tunnelling states in Nb below the superconducting transition temperature in both the superconducting and normal-conducting electronic state. Our results prove the direct (nonadiabatic) interaction between tunnelling states and conduction electrons. The electronic coupling parameter is derived independently of the damping (relaxation) of the tunnelling states and of the renormalization of the tunnelling matrix elements.

The local jump diffusion of trapped H in two Nb(OH), samples was investigated between 10 and 160 K by neutron spectroscopy. The jump rates vary by less than a factor of 7.Below -70 K, they exhibit Kondo's Tm-' temperature power law for a diffusion process that is controlled by nonadiabatic coupling to conduction electrons. Above -70 K, the diffusion is dominated by the interaction with phonons.

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The jump rate of a particle in quantum diffusion is theoretically expected to be proportional to the square of the tunnel splitting (or tunnel matrix element). This basic relationship is exemplified in neutron spectroscopic experiments on the tunnelling of trapped-H interstitials in Nb.