Franzreb Klaus, Williams Peter
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA.
J Chem Phys. 2005 Dec 8;123(22):224312. doi: 10.1063/1.2136154.
We have extended our previous experiment [Schauer et al., Phys. Rev. Lett. 65, 625 (1990)] where we had produced small gas-phase dianion clusters of C(n) (2-)(n > or =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 N(2)O) was supplied to the Cs(+)-bombarded graphite surface, small dianions of OC(n) (2-)(5< or =n < or =14) and O(2)C(7) (2-) were observed in addition to C(n) (2-)(n > or =7). Similarly, Cs(+) sputtering of graphite with simultaneous SF(6) gas flooding produced SC(n) (2-)(6< or =n< or =18). Mixed nitrogen-carbon or fluorine-carbon dianion clusters could not be observed by these means. Attempts to detect mixed metal-fluoride dianions for SF(6) gas flooding of various Cs(+)-bombarded metal surfaces were successful for the case of Zr, where metastable ZrF(6) (2-) was observed. Cs(+) bombardment of a silicon carbide (SiC) wafer produced SiC(n) (2-) (n=6,8,10). When oxygen gas was supplied to the Cs(+)-bombarded SiC surface, small dianions of SiOC(n) (2-) (n=4,6,8) and of SiO(2)C(n) (2-) (n=4,6) as well as a heavier unidentified dianion (at mz=98.5) were observed. For toluene (C(7)H(8)) vapor flooding of a Cs(+)-bombarded graphite surface, several hydrocarbon dianion clusters of C(n)H(m) (2-)(n> or =7) were produced in addition to C(n) (2-)(n> or =7), while smaller C(n)H(m) (2-) with n< or =6 could not be observed. BeC(n) (2-) (n=4,6,8,10), Be(2)C(6) (2-), as well as BeC(8)H(m) (2-) (with m=2 and/or m=1) were observed for toluene vapor flooding of a Cs(+)-bombarded beryllium metal foil. The metastable pentamer (9)Be(12)C(4) (2-) at mz=28.5 was the smallest and lightest dianion molecule that we could detect. The small dianion clusters of SC(n) (2-), OC(n) (2-), BeC(n) (2-), and SiO(m)C(n) (2-) (m=0,1,2) have different abundance patterns. A resemblance exists between the abundance patterns of BeC(n) (2-) and SiC(n) (2-), even though calculated molecular structures of BeC(6) (2-) and SiC(6) (2-) are different. The abundance pattern of SC(n) (2-) is fairly similar to that of C(n) (2-).
我们扩展了之前的实验[绍尔等人，《物理评论快报》65, 625 (1990)]，在该实验中，我们通过用Cs⁺离子轰击石墨表面产生了C(n)(2⁻)(n≥7)的小气相双负离子团簇。现在，对于奇数n，我们对小C(n)(2⁻)的检测灵敏度提高了约50倍。然而，寻找难以捉摸的C(5)(2⁻)五聚体双负离子的尝试并未成功。作为上限，假设C(5)(2⁻)的寿命足够长以允许通过质谱检测，那么C(5)(2⁻)的溅射通量必须至少比C(7)(2⁻)的低5000倍。当将氧气（用O₂或N₂O进行吹扫）供应到Cs⁺轰击的石墨表面时，除了C(n)(2⁻)(n≥7)之外，还观察到了OC(n)(2⁻)(5≤n≤14)和O₂C(7)(2⁻)的小双负离子。类似地，在同时用SF₆气体吹扫的情况下，对石墨进行Cs⁺溅射产生了SC(n)(2⁻)(6≤n≤18)。通过这些方法未观察到混合的氮 - 碳或氟 - 碳双负离子团簇。对于各种Cs⁺轰击的金属表面用SF₆气体吹扫来检测混合金属 - 氟化物双负离子的尝试，在Zr的情况下取得了成功，观察到了亚稳态的ZrF₆(2⁻)。对碳化硅(SiC)晶片进行Cs⁺轰击产生了SiC(n)(2⁻)(n = 6, 8, 10)。当将氧气供应到Cs⁺轰击的SiC表面时，观察到了SiOC(n)(2⁻)(n = 4, 6, 8)和SiO₂C(n)(2⁻)(n = 4, 6)的小双负离子以及一个较重的未识别双负离子（质荷比为98.5）。对于用甲苯(C₇H₈)蒸气吹扫Cs⁺轰击的石墨表面，除了C(n)(2⁻)(n≥7)之外，还产生了几个C(n)H(m)(2⁻)(n≥7)的烃类双负离子团簇，而未观察到n≤6的较小C(n)H(m)(2⁻)。对于用甲苯蒸气吹扫Cs⁺轰击的铍金属箔，观察到了BeC(n)(2⁻)(n = 4, 6, 8, 10)、Be₂C(6)(2⁻)以及BeC₈H(m)(2⁻)(m = 2和/或m = 1)。质荷比为28.5的亚稳态五聚体(⁹)Be₁₂C₄(2⁻)是我们能够检测到的最小且最轻的双负离子分子。SC(n)(2⁻)、OC(n)(2⁻)、BeC(n)(2⁻)和SiO(m)C(n)(2⁻)(m = 0, 1, 2)的小双负离子团簇具有不同的丰度模式。BeC(n)(2⁻)和SiC(n)(2⁻)的丰度模式之间存在相似性，尽管BeC₆(2⁻)和SiC₆(2⁻)的计算分子结构不同。SC(n)(2⁻)的丰度模式与C(n)(2⁻)的相当相似。
