Excited-state vibrational tunnel splitting of hydrogen trapped by nitrogenin niobium

High-resolution inelastic-neutron-scattering spectra of hydrogen trapped by interstitial nitrogen impurities in NbN0.007H0.004 have been measured to s tudy the vibrational lineshape of the broad 105 meV feature previously obse rved for NbN0.004H(o.003). The spectrum measured at 4.2 K exhibits clear ev idence of a doublet that disappears at higher temperatures. We interpret th is result as the first spectroscopic observation of excited-state tunnel sp litting of light-atom (H) vibrations in a solid-state double-well potential . We also show that both the observed excited-state splitting of 4.0 +/- 0. 3 meV and the previously measured ground-state splitting are well represent ed by a hardened-harmonic-well potential.