Vibrational states of atomic hydrogen in bulk and nanocrystalline palladium studied by neutron spectroscopy

Abstract

The vibrational states of hydrogen atoms in bulk and nanocrystalline palladium were examined in a wide energy region $0\ensuremath{\le}\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\le}300$ meV using neutron spectroscopy. In bulk ${\mathrm{PdH}}_{0.73}$, the vibrational excitations of H atoms were roughly reproduced by the quantum harmonic oscillator (QHO) model. In ${\mathrm{PdH}}_{0.42}$ nanocrystals with a diameter of 8 nm, however, additional vibrational excitations were found at energies above 80 meV. The energies and intensities of the additional states were not explained by the QHO but reasonably described as vibrations in a highly anharmonic trumpetlike potential. The additional excitations are attributed to the vibrations of H atoms at tetrahedral sites in the subsurface region stabilized by surface effects. Thus, this is an experimental work which clearly detects hydrogen vibration inside metal nanoparticles.