Specific heat of quench-condensed hydrogen films

The specific heat C(X,T) of quench-condensed films of H-2 has been measured as a function of ortho concentration X with 0.28 < X < 0.75 for coverages between 24.3 and 92.3 Angstrom(-2) at temperatures between 0.4 and 3.0 K. The films were condensed on evaporated gold substrates held at several temperatures T-cond between 1.0 and 3.5 K. The observed specific heat is attributed to or orientational ordering of the ortho-H-2 molecules. For the films with X = 0.74 condensed at T-cond > 2.5 K, there is a peak which indicates a bulk-like ordering transition. At temperatures below the peak, there is a large contribution to C, which is not present in bulk H-2, and which we attribute to short-range ordering size effects. As T-cond is decreased below 2.5 K, the shape of the specific heat curve changes, and the peak at 1.5 K is replaced by a gradual rise with a sharp drop above 2.6 K. Despite this strong dependence of C on T-cond, the entropy per molecule at 3 K is only weakly dependent on T-cond and comparable to that for bulk H-2. Film annealing at 3.4 K produces a change in the specific heat curve, and a study of this effect is presented. The ortho-para conversion rate of the films condensed at the various temperatures is found to be same as in bulk, well-annealed H-2. As in bulk H-2, the transition temperature inferred from the location of the specific heat peak or anomaly decreases with X. Unlike in bulk H-2, there is no temperature hysteresis in C for any of the quench-condensed films. This implies that the ordering transitions are not accompanied by a martensitic transformation.