Striking Isotope Effect on the Metallization Phase Lines of Liquid Hydrogen and Deuterium*

The insulator-metal transition in dense fluid hydrogen is of interest for both fundamental and planetary sciences. Almost all previous studies have treated hydrogen and deuterium without distinction. However, a study of their behavior at similar conditions can reveal the importance of ion dynamics. We have measured the metallization transition line of statically-compressed deuterium in the 1.2-1.7 Mbar range up to measured temperatures of ~3000 K. Compared to previous results on hydrogen, collected using the same experimental technique, deuterium is observed to metallize at substantially higher temperatures than hydrogen at the same pressure. This reveals a prominent isotopic shift in the location of the phase lines. The hydrogen molecule has larger zero-point energy than deuterium and is expected to dissociate at a lower temperature or pressure. Interestingly, the energetic difference in the free-molecule dissociation energies is comparable to the temperature (energy) difference of the phase transition lines. These results highlight the importance of nuclear quantum effects in the hydrogen isotopes at these conditions.