Core-level excitation in argon clusters

Argon clusters prepared in a supersonic-jet expansion are photoionized with monochromatized synchrotron radiation in the L3/L2 regime (240–280 eV). The cluster cations and their fragments are mass selected in a quadrupole mass spectrometer. The photoionization-efficiency curves of small cluster ions already show similar features which are known from X-ray absorption of solid argon. These solid-like features become more dominant when the cluster-size distribution of the neutral precursors is increased. The spectra are analysed in terms of near-edge features (NEXAFS) and extended fine structure (EXAFS). The NEXAFS regime shows cluster-specific features which are different compared to the atom and the solid. In the energy regime above 260 eV, EXAFS-like structures have been identified. Possible fragmentation mechanisms for core-level-excited argon clusters are discussed.