NEUTRAL SILICON CLUSTERS PRODUCED BY LASER-ABLATION IN VACUUM

Small neutral silicon clusters Si-n(0) (n = 1-11) produced by pulsed 1 .2-eV laser irradiation in vacuum are investigated. Clusters are photo ionized by a 6.4-eV excimer laser and detected by a time-of-flight mas s spectrometer. A low fluence of 30-350 mJ/cm(2) is used for inducing ablation, which is about one third lower than die fluence at visible w hite plume creation. In this moderate fluence range, the size distribu tion of photoionized clusters does not change much except for a relati ve decrease of larger clusters. while the total yield of clusters chan ges by five orders of magnitude. Velocity distributions for Si-1-Si-4 and Si-9 at 300 mJ/cm(2) are well fitted by Maxwell-Boltzmann (MB) sha pes. Evaluated temperatures from the MB distributions depend on cluste r size and are too high to reconcile with surface temperature; 4870 K for Si-2, 12420 K for Si-4 and 47350 K for Si-9. These experimental re sults suggest that collisional effects at the near surface are small a nd that the thermal equilibrium theorem cannot be applied to cluster e mission. Neutral clusters are considered to be emitted directly from t he surface by mechanisms which differ from classical thermal evaporati on. (C) 1998 Elsevier Science B.V.