Fission of Polyanionic Metal Clusters

Size-selected dianionic lead clusters ${\mathrm{Pb}}_n^{2-}$, $n=34–56$, are stored in a Penning trap and studied with respect to their decay products upon photoexcitation. Contrary to the decay of other dianionic metal clusters, these lead clusters show a variety of decay channels. The mass spectra of the fragments are compared to the corresponding spectra of the monoanionic precursors. This comparison leads to the conclusion that, in the cluster size region below about $n=48$, the fission reaction ${\mathrm{Pb}}_n^{2-}\to {\mathrm{Pb}}_{n-10}^{-}+{\mathrm{Pb}}_{10}^{-}$ is the major decay process. Its disappearance at larger cluster sizes may be an indication of a nonmetal to metal transition. Recently, the pair of ${\mathrm{Pb}}_{10}^{-}$ and ${\mathrm{Pb}}_{n-10}^{-}$ were observed as pronounced fragments in electron-attachment studies [S. König et al., Int. J. Mass Spectrom. 421, 129 (2017)]. The present findings suggest that this combination is the fingerprint of the decay of doubly charged lead clusters. With this assumption, the dianion clusters have been traced down to ${\mathrm{Pb}}_{21}^{2-}$, whereas the smallest size for the direct observation was as high as $n=28$.

Figure 1

Abundance spectra of ${\mathrm{Pb}}_{41}^{-}$ and ${\mathrm{Pb}}_{41}^{2-}$ after size selection of monoanions ${\mathrm{Pb}}_{41}^{-}$ (top left) and additional photoexcitation (bottom left), after electron bath of ${\mathrm{Pb}}_{41}^{-}$ followed by size selection of the dianions ${\mathrm{Pb}}_{41}^{2-}$ (top right) and additional photoexcitation (bottom right). (Data of left column taken from the studies reported in [23].)

Figure 2

Abundance spectrum of size selected and photoexcited ${\mathrm{Pb}}_{44}^{2-}$ clusters (label of the precursor underlined, as also in the following spectra).

Figure 3

Abundance spectrum of ${\mathrm{Pb}}_{44}^{-}$ (top), ${\mathrm{Pb}}_{34}^{-}$ (bottom), and ${\mathrm{Pb}}_{34}^{2-}$ (middle) after photoexcitation. (Data for top and bottom taken from Ref. [22].)

Figure 4

Intensity plot of the abundance of size-selected and photoexcited lead-cluster dianions $n/2$. The color represents the signal height with logarithm scaling. Each horizontal line is normalized to the highest signal. The most prominent line at $m/z=n/2$ is due to the remaining precursor clusters ${\mathrm{Pb}}_n^{2-}$ (label $n/2$, underlined).

Figure 5

Relative abundance of the sum of ${\mathrm{Pb}}_{8,\dots ,13}^{-}$ (blue) and ${\mathrm{Pb}}_{(n+1)/2,\dots ,n-10}^{-}$ (red) for each precursor ${\mathrm{Pb}}_n^{2-}$.

Figure 6

Abundance spectrum of ${\mathrm{Pb}}_{38}^{2-}$ after photoexcitation. The dashed lines and double arrows I and II indicate integration borders for fragments ${\mathrm{Pb}}_{8,\dots ,13}^{-}$ and ${\mathrm{Pb}}_{(n+1)/2,\dots ,n-10}$ (for details, see text).

Figure 7

Abundance spectra after photoexcitation of ${\mathrm{Pb}}_{35}^{2-}$ (top), after electron interaction of ${\mathrm{Pb}}_{35}^{-}$ [23] (middle), and after electron interaction of ${\mathrm{Pb}}_{25}^{-}$ [23] (bottom).