Construction of an apparatus for the investigation of inelastic electron scattering processes at high energy resolution and experimental determination of appearance energies of various atoms, molecules and small van-der-Waals clusters

A new experimental setup is presented, which enables the investigation of inelastic electron scattering processes at high energy resolution with dE< 100 meV. This setup is a typical molecular beam apparatus and it consists of a pulsed nozzle, a trochoidal electron monochromator electron source and quadrupole mass-selection/detection system. An additional personal-computer, extended with a multi channel scaler and a multi lab card, allows either time correlated data acquisition or direct mass scans. The first measurements were done on ionization cross sections close to threshold; furthermore appearance energies for various atoms, molecules and small van-der-Waals clusters have been determined. By taking into account the energy spread of the electron beam, the ionization cross-sections show in general a linear increase from the appearance energy. All data, except the appearance energies of the rare gas clusters, are in good agreement with the results of photoionization experiments and give detailed information about the electronic states of the target ions. In contrast to photoionization data, only vertical ionization has been observed for rare gas clusters. The proposed trimer ionic core for Arn+ cluster ions, n>3, is supported by these measurements