CHARGE-TRANSFER STATES IN DENSE HYDROGEN

THE electronic properties of hydrogen, from the gas phase to the solid state, are fundamental to our understanding of the chemical bond(1). The strong covalent bond of diatomic hydrogen persists in low-density condensed phases, where the molecules interact very weakly through van der Waals forces(2). At very high densities, molecular bonding has long been predicted to give way to a monatomic and presumably metallic lattice(3). At intermediate densities, intermolecular interactions are expected to increase; however, the relative strengths of the intermolecular and intramolecular interactions, and their effect on physical and chemical properties, have received comparatively little attention theoretically. Recent diamond-anvil-cell studies have revealed a range of unexpected phenomena in solid hydrogen at these densities(4). Here we show that marked changes in the infrared and Raman spectra of the intramolecular stretching modes (vibrons)(5) with increasing pressure can be interpreted in a manner analogous to the behaviour of organic charge-transfer salts at ambient pressure, including those exhibiting pressure-induced neutral-to-ionic transitions(6,7) Increased molecular overlap in dense hydrogen leads to symmetry breaking, which makes possible charge-transfer states between adjacent H-2 molecules. The consequent changes in bond strength and in vibron frequencies are evident in the spectra. These findings present a new picture of dense hydrogen and highlight the advantages of a localized, 'chemical' description of the bonding.