Small gas-phase dianions produced by sputtering and gas flooding

We have extended our previous experiment [Schauer , Phys. Rev. Lett. 65, 625 (1990)] where we had produced small gas-phase dianion clusters of C-n(2-)(n >= 7) by means of sputtering a graphite surface by Cs+ ion bombardment. Our detection sensitivity for small C-n(2-) could now be increased by a factor of about 50 for odd n. Nevertheless, a search for the elusive pentamer dianion of C-5(2-) was not successful. As an upper limit, the sputtered flux of C-5(2-) must be at least a factor of 5000 lower than that of C-7(2-), provided that the lifetime of C-5(2-) is sufficiently long to allow its detection by mass spectrometry. When oxygen gas (flooding with either O-2 or with N2O) was supplied to the Cs+-bombarded graphite surface, small dianions of OCn2-(5 <= n <= 14) and O2C72- were observed in addition to C-n(2-)(n >= 7). Similarly, Cs+ sputtering of graphite with simultaneous SF6 gas flooding produced SCn2-(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 ZrF62- was observed. Cs+ bombardment of a silicon carbide (SiC) wafer produced SiCn2- (n=6,8,10). When oxygen gas was supplied to the Cs+-bombarded SiC surface, small dianions of SiOCn2- (n=4,6,8) and of SiO2Cn2- (n=4,6) as well as a heavier unidentified dianion (at m/z=98.5) were observed. For toluene (C7H8) vapor flooding of a Cs+-bombarded graphite surface, several hydrocarbon dianion clusters of CnHm2-(n >= 7) were produced in addition to C-n(2-)(n >= 7), while smaller CnHm2- with n <= 6 could not be observed. BeCn2- (n=4,6,8,10), Be2C62-, as well as BeC8Hm2- (with m=2 and/or m=1) were observed for toluene vapor flooding of a Cs+-bombarded beryllium metal foil. The metastable pentamer (BeC42-)-Be-9-C-12 at m/z=28.5 was the smallest and lightest dianion molecule that we could detect. The small dianion clusters of SCn2-, OCn2-, BeCn2-, and SiOmCn2- (m=0,1,2) have different abundance patterns. A resemblance exists between the abundance patterns of BeCn2- and SiCn2-, even though calculated molecular structures of BeC62- and SiC62- are different. The abundance pattern of SCn2- is fairly similar to that of C-n(2-). (c) 2005 American Institute of Physics.