Investigation of the stabilities of neutral and ionic lead and lead-antimony clusters under single and multiphoton ionization conditions

Abstract

The fluence dependence of lead cluster ion distributions at 222 nm and 308 nm reveal markedly different behavior. Results obtained at 308 nm display a simple uniform increase in intensity with higher laser fluence with tittle change in relative intensities. At 222 rim, however, a significant transformation is found from a markedly different low fluence distribution to a high fluence pattern, which is essentially indistinguishable from that observed at 308 nm. It is concluded that mass spectra obtained at 308 nm, regardless of fluence, or at 222 nm and high fluence contain appreciable contributions from fragmentation. Hence, under these conditions the mass spectra are found to be dominated by cluster ion stabilities. Magic numbers observed at both high and low fluence correspond well to those obtained using electron-impact ionization, and in many instances parallel the magic numbers characteristic of rare-gas clusters. This suggests the stabilities of both neutral and monovalent cationic lead clusters are largely determined by closepacking considerations, and are not appreciably influenced by electronic structure. Similar preferences for close-packed structures are also found for mixed lead-antimony clusters containing one or two antimony atoms that are ionized using high fluence 308 nm excitation.

Isomers of smallPbnclusters(n=2–15): Geometric and electronic structures based onab initiomolecular dynamics simulatio

“…Therefore, total energy calculations including the spin-orbit effects are extremely important for these clusters. For smaller size clusters ͑Pb 2 and Pb 3 ͒ although the binding energies obtained from MP2 method are closer to the experimental values 26,27 ͑0.42 and 0.77 eV/atom for Pb 2 and Pb 3 , respectively͒ however, they overestimate as the size increases. This is evident from the fact that for n = 15 the binding energy obtained from MP2 is already within 97% of the bulk.…”

“…Experimental studies have been carried out for the fragmentation behavior of neutral and charged clusters of lead. 15,22,26 The results demonstrated the importance of electronic shell effects on the stability of neutral and charged clusters.…”

Kinetic study of neutral lead cluster reactions

“…Previous studies [15] of lead clusters have invariably concluded the anomalous abundance observed for Pb~ results from exceptional stability attributable to an icosahedral structure, similar to that observed for raregas clusters. Though thermodynamic data are lacking, it is easy to imagine that Pb + might undergo a facile fragmentation to Pb + under severe ionization conditions.…”

“…The contents of the flow tube are sampled on axis through a 1.6 cm hole drilled into the apex of a 45 ° cone. The apparatus is described more fully elsewhere [15].…”

“…The shift to smaller cluster sizes at high fluence is indicative of fragmentation due to multiphoton processes. Under such conditions, it has been shown that the distribution is largely determined by cluster ion stabilities [15]. The most stable ions are relatively resistant to subsequent fragmentations and appear prominently in the mass spectrum.…”