Low-temperature proton-magnetic-resonance line-shape measurements in the palladium-hydrogen system

Proton NMR line shapes at 30 MHz in $\mathrm{Pd}{\mathrm{H}}_x$ are reported for polycrystalline samples with hydrogen concentrations of 0.82 and 0.65. Below 130 K these spectra exhibit a fine structure consisting of a narrow and almost temperature-independent central component, superposed on a broad background. The central component is essentially temperature independent. A comprehensive experimental study of this spectral feature is reported for the Pd${\mathrm{H}}_{0.82}$ sample. One possibility is that the central component originates from correlated tunneling motions among protons. Two viewpoints are presented in answer to the obvious question as to "which protons" are involved.