Angular distributions of inner-shell photoelectrons from rare-gas clusters

The photoionization of Ar, Kr, and Xe rare-gas clusters was investigated using angle-resolved time-of-flight photoelectron spectroscopy. Fine-structure-resolved photoelectron angular distributions were determined for the Ar $2p$, Kr $3d$, and Xe $4d$ core levels and the Ar $3s$ inner-valence level as a function of photon energy for two average cluster sizes. Overall, the angular distributions of inner-shell photoelectrons from the cluster and the free atom are found to be very similar, except for the first 10--150.3em0exeV above the respective photoionization thresholds, where the angular anisotropy of the atomic photoelectrons is high, while our measurements demonstrate considerably less anisotropy in the cluster photoelectron emission. The angular distribution of Ar $3s$ inner-valence photoelectrons from the cluster is substantially less anisotropic than for the atomic photoelectrons throughout the covered photon energy range, but the difference decreases for increasing photon energy.