Formation of silicon and water cluster ions in pulsed-laser stimulated field desorption

Multiatom cluster ions of 3 to 16 atoms/cluster have been observed in pulsed-laser stimulated field desorption of silicon emitters. In a time-of-flight mass spectrum fine structures can be found in the mass line of every ion species, which are produced by isotope mixing in the clusters, and which will facilitate a correct identification of the charge state of the ions. The most abundant cluster ion species are Si2+4, Si2+5, and Si2+6. The numbers 4–6 may be called the magic numbers of the desorption process since they correspond to highly symmetric small units of atoms existing in Si surface layers. Most of the energy distributions of the ion species contain an exponentially decaying low energy tail which is produced by spontaneous dissociation of larger cluster ions, possibly due to photodissociation of larger cluster ions by the same laser pulses used for desorption. Water cluster ions of the form (H2O)mH+ (m=1–12) can be observed in pulsed-laser field desorption of ‘‘field condensed’’ water on a metal emitter surface. The appearance energy decreases monotonically with the cluster size, and also depends on the condition of the surface.