Zero-kinetic-energy photoelectron spectroscopy of Ar(2p)-excited argon clusters

Zero-kinetic-energy photoelectron (ZEKE) spectra of atomic argon and argon clusters are recorded in the Ar(2p) excitation regime. Characteristic shifts of the Ar(2p) ionization energies are observed as a function of cluster size. The maximum of the zero-kinetic-energy photoelectron band (Ar(2p32)) is found at 248.9±0.1 eV, whereas the corresponding band in the ZEKE spectrum of a cluster beam of average cluster size of approximately 750 atoms occurs at 247.94±0.01 eV. An enhanced ZEKE yield in the Ar(2p) continuum is attributed to inelastic scattering of the photoelectron within individual clusters. These results are compared to Ar(2p) spectroscopic studies of argon atoms, argon clusters and solid argon. Based on our analysis of the Ar(2p) ZEKE spectrum of large clusters, the Ar(2p32) ionization energy of solid argon is predicted to be 247.7±0.1 eV.