Neutral silicon clusters produced by laser ablation in vacuum

Small neutral silicon clusters Si0 ns1–11.produced by pulsed 1.2-eV laser irradiation in vacuum are investigated. n Clusters are photoionized by a 6.4-eV excimer laser and detected by a time-of-flight mass spectrometer. A low fluence of 30–350 mJrcm2 is used for inducing ablation, which is about one third lower than the fluence at visible white plume creation. In this moderate fluence range, the size distribution of photoionized clusters does not change much except for a relative decrease of larger clusters, while the total yield of clusters changes by five orders of magnitude. Velocity distributions for Si1–Si4 and Si9 at 300 mJrcm2 are well fitted by Maxwell–Boltzmann MB. shapes. Evaluated temperatures from the MB distributions depend on cluster size and are too high to reconcile with surface temperature; 4870 K for Si2, 12 420 K for Si4and 47 350 K for Si9. These experimental results suggest that collisional effects at the near surface are small and that the thermal equilibrium theorem cannot be applied to cluster emission. Neutral clusters are considered to be emitted directly from the surface by mechanisms which differ from classical thermal evaporation. q1998 Elsevier Science B.V.