EXPERIMENTAL STUDIES OF HYDROGEN ON BORON-NITRIDE .2. NMR-STUDIES OF ORIENTATIONAL ORDERING OF H-2

We report the results of NMR studies of thin films of hydrogen adsorbe d on hexagonal boron nitride. Orientational ordering is observed below 1 K but the ordering is not complete, and a clear two-component order ing is observed. Molecules are either (i) almost completely ordered wi th local order parameters sigma = [1 - 3/2Jz(2)] clustered close to a maximum value of sigma congruent to 0.94 (comparable to the values for long range ordering in bulk samples at high ortho concentrations), an d (ii) a large fraction of the molecules that remain nearly disordered with sigma less than or equal to 0.25. The degree of orientational or dering depends on the number of hydrogen layers and on the ortho-hydro gen concentration, and these studies indicate that ordering occurs pri ncipally in the first four layers closest to the substrate, with weake r orientational ordering in the outer layers near the free surface eve n at temperatures as low as 210 mK.