Singly- and doubly-negative carbon clusters in sputtering: Energy spectra, abundance distributions and unimolecular fragmentation

Abstract

The emission of singly- and doubly-charged negative cluster ions in sputtering of graphite by 14.5 keV Cs+ ion bombardment was investigated by mass spectrometry. Specifically, for anionic Cn− (n⩽23) and CsCn− (n⩽11) and dianionic Cn2− (n⩽39) species the emission-energy spectra were recorded and their abundance distributions as a function of cluster size n were determined. The energy spectra provided evidence for cluster decomposition in the ion accelerating region of the spectrometer corresponding to a time scale from some 10−10 s to several 10−8 s. The abundance of these fragment ions are similar to those of the parent ions in terms of the dependence on the size n and their absolute magnitudes converge with increasing cluster size. Due to energetic ejection events, the clusters are sputtered with high internal energies; they cool by unimolecular decomposition. The most probable fragmentation process for Cn− appears to be by evaporation of a neutral C2 molecule. For these decay reactions, the fragmentation-time distributions were derived from the appropriate parts of the energy spectra; they were found to scale exponentially with time. From these data the average lifetimes τ for these unimolecular decompositions were determined. For Cn− the lifetimes slightly increase with n: τ∼8×10−9 s at n=6 to τ∼5×10−8 s at n=25. Similar values are found for CsCn−, whereas for the dianionic clusters Cn2− they are shorter, τ∼(5–7)×10−9 s for n=12–18. Estimates of internal energies, Eint, of sputtered Cn− clusters were derived from these lifetimes, employing statistical theories of unimolecular decomposition; values of Eint increase with cluster size n for 5⩽n⩽25, whereas the average internal energy per constituent atom, Eint/n amounts to ∼1 eV in that range.