Charge transfer and electron-vibron coupling in dense solid hydrogen Original Research Article

We examine the role of charge transfer (CT) interactions in dense molecular hydrogen in relation to recently observed spectroscopic properties at megabar pressures. Specifically, we consider virtual H2+H2− states in which an electron is transferred to a neighbor. The Mulliken CT integral t admixes H2+H2− fluctuations of overlapping molecules. The amplitude of the charge fluctuations is the ionicity γ, which is found in H2 dimers and lattices with nonoverlapping valence and conduction bands for t small compared to the CT excitation energy ωCT. Vibronic coupling within a dimer is found using a Herzberg-Teller expansion and gives explicit expressions for vibron shifts in Raman and IR spectra and for IR oscillator strength due to electron-vibron coupling. We estimate linear electron-vibron coupling constants and other parameters required to interpret the vibrational data at and above the 150-GPa transition of dense hydrogen within a CT model. We discuss important structural implications of equal IR and Raman shifts at the transition and relate anisotropic CT processes to the orientational state of the dense molecular solid.