Trapped hydrogen vibrational density of states measurement using IINS PdH0.0011 at 5 K and 300 K with incident neutron energy of 250 meV

Hydrogen trapped at dislocations in Pd was studied with the incoherent inelastic neutron scattering (IINS) at two temperatures over the energy transfer range of 250 meV. The primary objective of this study was to study the vibrational density of states (VDOS) of deformed PdH0.0011 at 5 and 300 K over an energy transfer range corresponding to the multi-phonon and second order harmonic regime respectively. A free recoil neutron subtraction method was developed to identify the proton recoil intensity within the impulse approximation. The mass of recoil particle is 1.10 amu and 1.12 amu while the recoil energy is located at 208 and 206 meV for 5 K and 300 K respectively. The 5 K VDOS primary peak energy was observed at 61.4 meV, and is attributed to the β phase palladium hydride. The 300 K VDOS first harmonic peak at 68.2 meV corresponds to the α phase palladium hydride. The comparison of experimental and calculated results indicate the higher energy peak found at 5 K is due to multiphonon events and shows spectral hardening leading to a higher energy; comparing to the 4 K PdH0.63 data. The environment of trapped hydrogen in this research is perturbed and results in different peak positions. The comparison of experimental and calculated results of second harmonic peak at 300 K shows good correlation with the data of PdH0.014 at 295 K, in both the anharmonicity parameters (19 meV) and the peak position. The given value of 137 meV in the published data was not found in the measurement, likely due to a loss of degeneracy and associated broadening. This free recoil subtraction method was supported by the two dimensional plots of energy transfer versus wave vector transfer (E v.s Q) before and after recoil subtraction. The shift of intensity maps at free recoil region and second peak area confirmed the effectiveness of the recoil subtraction method