Isotope dependence of hydrogen tunneling in niobium

We present ultrasonic experiments on the low-temperature tunneling of H and D interstitials in Nb. Strong isotope effects are observed in the sound velocity and the attenuation. From the temperature dependence of these quantities we determine tunneling matrix elements and relaxation rates for both isotopes. We demonstrate that this isotopic behavior is adequately described by present theories on nonadiabatic interaction of tunneling systems with conduction electrons in the weak-coupling regime