OBSERVATION OF SMALL MULTIPLY-CHARGED CLUSTERS IN A LASER-PRODUCED PLASMA

The observation of what are believed to be small multiply charged clus ters generated in a laser-produced plasma is reported and discussed, a nd a possible formation mechanism is proposed. Homonuclear gold cluste r ions comprising three atoms and, carrying charges of up to +5, and o f four atoms carrying charges of up to +3, or possibly +4, were observ ed. With a carbon sample, cluster ions of charge up to +6 and of mass slightly greater than that of four carbon atoms were also observed. Th e observation of such non-integer cluster ion to single atom ion mass ratios may indicate the generation of heteronuclear cluster ions forme d between the atoms of the target material and vacuum contamination. N on integer mass ratios were also obtained for gold. It is proposed tha t the duster ions form through the influence of a small background gas pressure which enables neutrals, generated through recombination, to aggregate on to single atom ions. The observation of Au-3(2+), Au-4(3) and of small multiply charged homonuclear carbon cluster ions has al so been reported by other workers using different sources. The observa tion of Au-3(4+) and Au-3(5+), and what appears to be small multiply c harged heteronuclear carbon cluster ions is new, (C) 1997 by John Wile y & Sons, Ltd.