COLLECTIVE EXCITATIONS IN LIQUID PARA-H-2 OBSERVED BY NEUTRON INELASTIC-SCATTERING

Inelastic-neutron-scattering studies of liquid H-2 enriched in para-H- 2 molecules show the existence of a damped mode, interpreted as a phon onlike collective excitation whose wave-vector dependence is establish ed for 0.6 less than or equal to Q less than or equal to 2.0 Angstrom( -1). Our result, in marked disagreement with previous observations in this system, is in qualitative agreement with theoretical variational calculations. A comparison with liquid normal D-2 is made. The crossov er from collective to single molecule response in both the solid and t he liquid phases has also been determined. [S0163-1829(97)04242-2].