A series of cluster anions CnX- are produced from laser ablation of appropriate samples, where X is selected as a group-VB element. The recorded mass spectra of these cluster anions display a drastic even/odd alternation on ion intensities: For CnN-, only anions with odd n can be observed; For CnP- and CnAs-, cluster anions with even n are produced but with lower signal intensities; For CnSb-, the signal intensity of clusters does not show even/odd alternation; Finally, for CnBi, the intensities of cluster anions with even n are higher than those with odd n. This parity effect can be attributed to the linear structure of the cluster anions, and the parity reversal of CnX- from CnN- to CnBi- can be explained from the electronegativity decreasing of the heteroatom X as it descends in the group. The Hfickel model was applied to account the structural feature of these clusters.

本文介绍一台能够同时检测激光等离子体中的正负离子的飞行时间质谱仪，它不仅具有较高的分析测试效率，而且能够研究激光等离子体中正负离子在组分、丰度等方面的相关性。初步测试显示了该仪器对正负离子都具有很高的灵敏度和质量分辨能力。