Many-body calculation of the magnetic, optical, and charge-transfer spectra of solid oxygen in the α and β phases

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The electronic spectra of {α} and {β} solid O₂ were calculated in a full many-body approach for a cluster consisting of four O₂ molecules with periodic-boundary conditions. By including only the partially occupied {π} orbitals (16 spin orbitals, 8 electrons) the basis set consists of 12 870 many-electron states. The resulting spectra contains three basic types of excitations at very different energy ranges: (a) 81 states corresponding to the ground state and magnetic excitations (magnons) within 100 meV; (b) 1215 states of neutral molecular excitations (excitons) with energies from about 1 to 8 eV; and (c) 11 574 charge-transfer states with excitation energies up to 60 eV. Analysis of the properties of the ground states in both {α} and {β} solid O₂ has been carried out. The lowest many-body states, those that correspond to neutral, unexcited molecules, describe accurately the magnetic excitations of both solids. The two phases have very different spectra resulting, even at very low temperatures, in a sizeable difference in the (magnetic) entropy. The calculated entropy difference at the observed {α}-{β} phase transition temperature agrees surprisingly well with the experimentally measured heat of transformation.