Photoelectron spectroscopy of iron-sulfur cluster anions

Iron-sulfur cluster anions (Fe_nS_m^-, n=1-6, m=1-6) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle-type time-of-flight electron spectrometer. The Fe_nS_m^-. cluster anions were formed in a laser vaporization cluster source. It was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. The electron affinities were measured from the onsets of the PES spectra. Three low-lying electronic states were observed for FeS. The PES spectra of Fe_nS^- (n=1-4) series show a unique similarity, indicating that the Fe atom addition to FeS^- has little effect on the electronic property of FeS. The PES spectra of Fe_nS₂^- series show a similarity among the cluster anions with n=2-4, showing that Fe₂S₂^- is the structural framework of these clusters. For Fe_nS₃^- series, Fe₃S₃^- is proposed to be the structural framework. The electronic properties and geometrical structures of the clusters are discussed.