High-pressure optical spectra of condensed oxygen

Optical-absorption spectra (1.5-3.5 eV) of the fluid and three solid phases of O₂ have been recorded up to 40 GPa near 298 K. The principal feature of the spectra of the fluid and rhombohedral β-O₂ is the 2¹Δg←2³Σg^- band; in β-O₂, it is polarized perpendicular to the c axis of the hexagonal cell. The energies of this and other transitions among the ³Σg^-, ¹Δg, and ¹Σg⁺ states increase with increasing pressure. The spectra and visual observations of the transformation between orthorhombic and ε-O₂, whose structure has not been determined, suggest that ε-O₂ has a layered structure. Absorption spectra of orthorhombic and ε-O₂ for light propagating perpendicular to the layers are dominated by a moderately strong band which moves from the uv through the visible with increasing pressure and a very strong ultraviolet band. These bands can be assigned as excitations from π⁴-π*2 states to π³-π*3 states which are mixed with intermolecular-charge-transfer excitations. Both bands move toward longer wavelengths at high pressures. At pressures above 13 GPa, absorption features for light propagating in the planes of the ε-phase structure can be assigned as 2¹Δg←2³Σg^-. However, between 10 and 13 GPa, this feature is absent, and the pressure dependence of the Raman spectrum also is unusual. Both observations suggest that, between these pressures, the molecules reorient with respect to the crystallographic axes.