Isotope effect on the formation of hydrogen cluster beams

Experiments with cluster beams formed by the hydrogen isotopes H2 and D2 show considerable differences in cluster size and intensity. It is found that, in spite of the marked differences in the thermodynamic properties of both isotopes, under identical stagnation conditions, the cluster size is nearly equal for both isotopes at 300K nozzle temperature. At 770K nozzle temperature, the H2 clusters are about twice, and at 1100K about 2.5 times as large as that of D2; however, the vapor pressure of D2 is smaller than that of H2, and therefore larger D2 clusters would be expected. On the basis of the well-known marked differences in the thermodynamic properties of the two isotopes that control the clustering process, the differences in cluster size and intensity are discussed. It is shown that the pronounced isotope effect in vapor pressure will be nearly compensated for by the effect in molar volume and condensation energy. The differences resulting from the variation of the nozzle temperature are explained by the differences caused by quantum mechanics in the number of excited rotational energy terms of the two isotopes. The number of these terms determines the specific heat, and thus the curve for the adiabatic expansion of both gases. Accordingly, it is possible to discuss cluster beam formation in the case of the ortho- and paramodification of hydrogen, just as in the case of tritium