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 ~nergy 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-I. 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 iv ± 4, 118.3 ± 5, and 147.6 ± 3 K for a sample at temperatures of 34.2, 54.1, and 74.0 K respectively. These data show the same peak structure as did the solid para-hydrogen experiment.U of I OnlyThesi