Core excitons in the soft X-ray region: Solid argon

A theoretical calculation of core excitons is performed for the L_II,III soft X-ray threshold of solid argon at ～ 245 eV. The binding energies and the relative transition amplitudes for the lowest allowed exciton states are computed by formulating the problem in terms of Wannier functions and solving the resulting integral equation in the one-site approximation. The results obtained allow to locate the onset of interband transitions at an energy a few eV above previous theoretical determinations. Therefore, sharp structure previously interpreted in terms of conduction band density of states is attributed to discrete excitonic transitions, as strongly suggested by the close analogy with the atomic absorption spectrum. A comparison with the fundamental excitonic absorption in the vacuum ultra violet region is carried out in terms of the ratio of the electron-hole exchange interaction to the spin-orbit splitting of the hole states.