OBSERVATION OF PAIR INTERACTION BETWEEN ORTHO MOLECULES IN SOLID H2

A study of the electric quadrupole-quadrupole (EQQ) interaction between ortho-H2 molecules in solid H2 is presented. This work consists in recording and interpreting the pair spectrum of proton nuclear magnetic resonance for low ortho-H2 concentrations. In the theoretical part of this paper, a calculation of the absorption spectrum for an isolated pair of ortho molecules is presented, and the temperature dependence of this spectrum is derived as a function of the rotational states of the isolated ortho pair. Furthermore, the results of similar calculations on the absorption spectrum of isolated triangles of ortho-H2 molecules in solid H2 are briefly discussed. This spectrum is expected to be less pronounced than the pair spectrum and to be mostly responsible for a broad absorption background. The experimental work, carried out at temperatures between 0.45 and 14°K on several low ortho-H2 concentrations shows the maxima expected from theory. From the analysis of the results, and assuming the interactions between molecules to be purely EQQ, the coupling constant kB=6e2Q225R5kB is found to be 0.82°K. This value is smaller than that expected for the solid rigid lattice, where kB=1.00°K. One also obtains from the analysis a dipolar coupling constant d=54.5 kHz which is smaller than that measured for the free H2 molecule, where d=57.7 kHz. It is suggested that the departures both in and in d from expectations are caused by the interactions between lattice vibrations and rotations, since solid H2 will behave more like a quantum crystal with large zero-point motion than like a classical rigid crystal. © 1968 The American Physical Society.