Single Particle Kinetic Energies of Para-Hydrogen Molecules in Para-Hydrogen and Argon Solids.

Single particle kinetic energies of para-hydrogen molecules in para-hydrogen and argon solids have been determined by deep inelastic neutron scattering. The Low Resolution Medium Energy Chopper Spectrometer at the Argonne National Laboratory's Intense Pulsed Neutron Source was used to make the measurements at momentum transfers up to 12 A^{-1}. Peaks corresponding to transitions from the molecule's rotational ground state J = 0 to the excited states J = 1, 3, and 5 are observed. The peaks are broadened by the translational center of mass motion of the molecule. A new technique for analyzing data taken with a chopper spectrometer at a pulsed source is implemented. This method employs a numerically determined resolution function. Single particle kinetic energies for solid para -hydrogen are found to be 92.6 +/- 1.1, 108.9 +/- 4, 129.9 +/- 5, and 145.8 +/- 4 K for molar volumes of 23.2, 21.26, 19.49, and 18.16 cc/mole respectively. These values are shown to be those of the ground state. This measurement is the first of its kind to span such a large range of molar volumes. The scattering from dilute para-hydrogen molecules embedded in solid argon has also been measured. The single particle kinetic energies are found to be 84.4 +/- 4, 118.3 +/- 5, and 147.6 +/- 3 K for a sample at temperatures of 34.2, 54.1, and 74.9 K respectively. These data show the same peak structure as did the solid para-hydrogen experiment.