Passage towards metallic hydrogen via electron excitation

Hydrogen is known as a nonmetal, unlike the other elements in the first period of the periodic table. If hydrogen could be made metallic, it could serve as an extremely efficient energy source because much higher energy density can be obtained from metallic hydrogen than from current solid fuels. Hydrogen is nonmetallic under ambient conditions because even solid hydrogen forms an H₂ molecular crystal, so there are no free electrons. If the H₂ bonds are dissociated, hydrogen is expected to become metallic because the electrons would no longer be localized between H₂ molecules and could move freely throughout the crystal. Much research therefore focuses on dissociating H₂ bonds by applying extremely high pressure. However, clear evidence of metallization in solid hydrogen has not yet been found. Here we show signals of bond dissociation in solid hydrogen induced by electronic excitation. Starting from the phase III hydrogen crystal (~75 K, ~180 GPa), we excited electrons from bonding to antibonding states and followed the time evolution using time-dependent density functional theory (TDDFT) calculations. We found that the distinct peak in the radial distribution function at about 0.7–0.8 Å — corresponding to the H–H bond length — became blurred after excitation as the electrons reached a new equilibrium. These results indicate weakening of the H₂ bonds.

We anticipate that electronic excitation could be a pathway toward metallization of solid hydrogen.