ORIENTATIONAL ORDERING IN QUENCH-CONDENSED H-2 MEASURED BY AC CALORIMETRY

The heat capacity of quench-condensed normal- and para-hydrogen has be en measured using ac calorimetry. The measurements were made at temper atures from 1.6 to 3.7 K on sapphire and evaporated gold substrates. T he range of exposures studied was 0.24 to 3.3 angstrom-2 yielding esti mated coverages from 0.07 to-2.5 angstrom-2. For reference, monolayer completion of H-2 on graphite occurs at a coverage of 0. 1 angstrom-2. For normal-hydrogen (n-H-2) on a few of our sapphire substrates, the heat capacity as a function of temperature exhibits a peak at 1.8 K, f ollowed by a rapid decrease. For the n-H-2 data on all other substrate s, a negative slope is observed at the lowest temperatures measured, w hich is consistent with apeak below.our temperature range. We attribut e these effects to a bulk-like orientational ordering transition. The coverage dependence of the peak is not consistent with the predictions of a model of finite-size effects. We conclude that the dominant broa dening of the peak is inhomogeneous. The desorption rate is deduced fr om the time dependence of the heat capacity and is found to agree with previously published values. The ortho-to-para conversion rate is com parable to that of bulk hydrogen.