Photoelectron spectroscopy of silicon-fluorine binary cluster anions (Si_nF_m ^-)

Electronic properties of silicon-fluorine cluster anions (Si_nF_m ^-; n=1-11, HZ = 1-3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon rod in an He carrier gas mixed with a small amount of SiF₄ or F₂ gas. The highly abundant clusters are SiF_m ^- (m=3 and 5) and Si_nF^- (n=6, 7, and 10) in their mass spectra. In the photoelectron spectra of SiF_m ^- (m = 1-5), the clusters having odd m have higher electron affinity (EA) than those having even in, indicating that the even/odd alternation in EA is attributed to their electronic structures of a closed/open valence shell. Comparison between photoelectron spectra of Si_nF^- and Si_n ^- (n=4-11) gives the insight that the doped F atom can remove one electron from the corresponding Si_n ^- cluster without any serious rearrangement of Si_n ^- framework, because only the first peak of Si_n ^-, corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping. In some clusters, furthermore, the vibrational structures could be resolved to determine a vibrational frequency and to presume the geometry with ab initio molecular orbital calculations.