Core excitation in free clusters: Ionization, relaxation, and fragmentation

Cluster-size-dependent spectroscopic changes are reported for the Ar(2p) core ionization, electronic relaxation via Auger decay, and subsequent fragmentation of free argon clusters with sizes up to 700 atoms. Excitation spectra recorded using zero kinetic energy (ZEKE) photoelectrons, Auger electrons, as well as multicoincidence ion mass spectra, are reported. It is found that core ionization energies decrease as the cluster size increases from the atom to the condensed phase. Evidence for thermalization of photoelectrons via intracluster scattering is found, indicating that ZEKE photoelectrons are sensitive to the bulk of clusters. In contrast, Auger electron yield spectra are more sensitive to the surface of the clusters. Ion pair and triplet signals from dissociative double and triple ionization are used to investigate the final stabilization step of core-excited clusters. Fragmentation mechanisms are deduced for different cluster sizes.