The charge-state distribution of field-evaporated tungsten ions has been measured as a function of electric field using the pulsed-laser atom probe. The results confirm the recent post-ionization model of Haydock and Kingham and give the first experimental evidence for multiple post-ionization. At low field strengths (< 3.0 V/Å) and high surface temperatures (> 1000 K), field-evaporated triply charged dimers are detected, and built-up regions which significantly enhance the local electric field at the desorption site are observed.

• Received 29 September 1980

DOI:https://doi.org/10.1103/PhysRevB.24.1848