Orientational ordering in quench-condensed H2 measured by ac calorimetry

Abstract

The heat capacity of quench-condensed normal- and para-hydrogen has been measured using ac calorimetry. The measurements were made at temperatures from 1.6 to 3.7 K on sapphire and evaporated gold substrates. The range of exposures studied was 0.24 to 3.3 A−2, yielding estimated coverages from 0.07 to 2.5 A−2. For reference, monolayer completion of H2 on graphite occurs at a coverage of 0.1 A−2. For normal-hydrogen (n-H2) on a few of our sapphire substrates, the heat capacity as a function of temperature exhibits a peak at 1.8 K, followed by a rapid decrease. For then-H2 data on all other substrates, a negative slope is observed at the lowest temperatures measured, which is consistent with a peak below our temperature range. We attribute 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 broadening of the peak is inhomogeneous. The desorption rate is deduced from the time dependence of the heat capacity and is found to agree with previously published values. The ortho-to-para conversion rate is comparable to that of bulk hydrogen.