Core excitation and deexcitation in argon multilayers: Surface- and bulk-specific transitions and autoionization versus Auger decay

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Abstract
By photoabsorption in the Ar 2p range of Ar multilayer condensates, distinct contributions of surface and bulk excitations can be distinguished in the characteristic core resonances detected by partial-electron-yield near-edge x-ray-absorption fine-structure spectra. Compared to the corresponding bulk/gas–phase features, the surface excitations are shifted by ∼0.5 eV towards lower/higher photon energy. Deexcitation of core-to-bound 4s resonances in these condensates leads to distinct spectator-shifted Auger features (2h1e final states). Decay of the 3d,5s resonances, however, is dominated by normal Auger processes to 2h states; 2h1e features are weak. This differs from the behavior of free atoms; it can be understood by the lowering of the ionization threshold to below the 3d,5s resonances by screening in the condensate and by efficient ionization of the core excitonic states during core lifetime, i.e., within a few femtoseconds. Detailed analysis reveals differences for the decay spectra of the lowest resonance at the surface and in the bulk, and shows that the screening energies for the various 2h1e states differ from those of 1h states, as well as among each other. Evidence for the existence of weak participator decay channels (1h final states) at certain photon energies is also found.