Measurement of the charge state distribution of field evaporated ions: Evidence for post-ionization

Charge state distributions of field evaporated Si, Ni, Mo, Rh, W, Re, Ir and Pt ions have been measured as a function of electric field strength using the pulsed laser atom-probe. The results are compared to previously published theoretical calculations based on the post-ionization model of field evaporation. The agreement between theory and experiment is sufficient to establish the general validity of the post-ionization model. Measurements of the charge state distributions as a function of evaporation rate at constant temperature (increasing field) and constant field (increasing temperature) are also presented for W, Mo and Si. The observation that the fractional abundances of different charge states for the same material do not change with changing temperature indicates that the activation energies of desorption are the same for the different charge states and provides further support for the post-ionization model. The anomalous field evaporation behavior observed at high temperatures (e.g., desorption from localized areas on the surface and the occurrence of ionic clusters) is also discussed.