Small gas-phase dianions produced by sputtering and gas flooding

Klaus Franzreb, Peter Williams

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Abstract

We have extended our previous experiment [Schauer, Phys. Rev. Lett. 65, 625 (1990)] where we had produced small gas-phase dianion clusters of Cn 2- (n≥7) by means of sputtering a graphite surface by Cs+ ion bombardment. Our detection sensitivity for small Cn 2- could now be increased by a factor of about 50 for odd n. Nevertheless, a search for the elusive pentamer dianion of C5 2- was not successful. As an upper limit, the sputtered flux of C5 2- must be at least a factor of 5000 lower than that of C7 2-, provided that the lifetime of C5 2- is sufficiently long to allow its detection by mass spectrometry. When oxygen gas (flooding with either O2 or with N2 O) was supplied to the Cs+ -bombarded graphite surface, small dianions of OCn 2- (5≤n≤14) and O2 C7 2- were observed in addition to Cn 2- (n≥7). Similarly, Cs+ sputtering of graphite with simultaneous SF6 gas flooding produced SCn 2- (6≤n≤18). Mixed nitrogen-carbon or fluorine-carbon dianion clusters could not be observed by these means. Attempts to detect mixed metal-fluoride dianions for SF6 gas flooding of various Cs+ -bombarded metal surfaces were successful for the case of Zr, where metastable ZrF6 2- was observed. Cs+ bombardment of a silicon carbide (SiC) wafer produced SiCn 2- (n=6, 8,10). When oxygen gas was supplied to the Cs+ -bombarded SiC surface, small dianions of SiOCn 2- (n=4, 6,8) and of SiO2 Cn 2- (n=4, 6) as well as a heavier unidentified dianion (at mz=98.5) were observed. For toluene (C7 H8) vapor flooding of a Cs+ -bombarded graphite surface, several hydrocarbon dianion clusters of Cn Hm 2- (n≥7) were produced in addition to Cn 2- (n≥7), while smaller Cn Hm 2- with n≤6 could not be observed. BeCn 2- (n=4, 6,8,10), Be2 C6 2-, as well as BeC8 Hm 2- (with m=2 and/or m=1) were observed for toluene vapor flooding of a Cs+ -bombarded beryllium metal foil. The metastable pentamer Be9 C4 2- 12 at mz=28.5 was the smallest and lightest dianion molecule that we could detect. The small dianion clusters of SCn 2-, OCn 2-, BeCn 2-, and SiOm Cn 2- (m=0, 1,2) have different abundance patterns. A resemblance exists between the abundance patterns of BeCn 2- and SiCn 2-, even though calculated molecular structures of BeC6 2- and SiC6 2- are different. The abundance pattern of SCn 2- is fairly similar to that of Cn 2-.

Original languageEnglish (US)
Article number224312
JournalJournal of Chemical Physics
Volume123
Issue number22
DOIs
Publication statusPublished - 2005

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ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics

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