Energy broadening due to photoion space charge in a high resolution laser photoelectron source

Abstract

The results of experimental and theoretical studies, aiming at a quantitative characterization of photoion-induced energy broadening effects in a laser photoelectron source, are reported. The electron source is based on two-step cw laser photoionization of potassium atoms in a collimated beam. In the experimental studies, the attachment spectra for the formation of (N2O)9O− cluster ions through a narrow vibrational Feshbach resonance (full width at half maximum 2.3 meV) were measured as a function of the photocurrent. The theoretical studies involved Monte Carlo simulations of the broadening effects and were based on potential distributions caused by realistic spatial distributions of the photoions. Using the corresponding electric field distribution, trajectories were calculated for a representative ensemble of electrons, and effective electron energy distributions were obtained from averages over the electron trajectories in the volume relevant for electron attachment. Furthermore, the effects of additional weak electric fields, applied along the atomic beam direction, have been simulated. For our geometry (ionization volume about 2 mm3) the effective space charge related energy width is found to be about 16 μeV/pA.