Energy distributions of pulsed-laser field-desorbed gaseous ions and field-evaporated metal ions: A direct time-of-flight measurement

Energy distributions of pulsed-laser field-desorbed gaseous ions and field-evaporated metal ions have been obtained by directly measuring their flight times in the pulsed-laser atom-probe field-ion microscope. Energy distributions of gaseous ions show the same critical energy deficit, I-φ, and the same general characteristics, namely a sharp rise at the high-energy side and a slowly decaying tail with resonant secondary peaks at the low-energy side, as those found in field ionization. However, the width of the main peak does not widen with increasing field as in field ionization. Energy distributions of field-evaporated metal ions, in general, are more symmetrically shaped with no prominent low-energy tails. A small low-energy tail can be seen for highly charged ions if the field-evaporated ions consist of another lower-charge-ion species of nearly equal abundance. The critical-energy deficit of these ions is consistent with the expression Λ+Σi^Iᵢ-nφ-Q. No surface-plasmon excitations are evident in field ion emission. The energy distribution of pulsed-laser field-desorbed H₃⁺ ions resembles more closely those of field-evaporated metal ions. Based on these observations, mechanisms of pulsed-laser field desorption and field evaporation of different types of ions have been proposed.