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

The results of experimental and theoretical studies, aiming at a quantitati ve characterization of photoion-induced energy broadening effects in a lase r photoelectron source, are reported. The electron source is based on two-s tep cw laser photoionization of potassium atoms in a collimated beam. In th e experimental studies, the attachment spectra for the formation of (N2O)(9 )O- cluster ions through a narrow vibrational Feshbach resonance (full widt h 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 spat ial 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, a pplied along the atomic beam direction, have been simulated. For our geomet ry (ionization volume about 2 mm(3)) the effective space charge related ene rgy width is found to be about 16 mu eV/pA. (C) 2001 American Institute of Physics.