Photoelectron spectroscopy of transition metal-sulfur cluster anions

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The M_nS^-_m cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (n_max) is generally expressed as n_max=2m-1 in each series of the number of S atoms (m). For Fe-S and Mn-S, it was found that the stable cluster ions are the ones with compositions of n = m and n = m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag-S, the EAs of Ag₁S_m are small and around 1 eV, whereas those of Ag_nS_m (n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag₂S unit is preferentially formed with increasing the number of Ag atoms. For Fe-S and Mn-S, the PES spectra of Fe_nS^-_m/Mn_nS^-_m show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to Fe_nS^-_n/Mn_nS^-_n has little effect on the electronic property of Fe_nS_n/Mn_nS_n. The PES spectra imply that the Fe_nS_n cluster is the structural framework of these clusters, as similarly as the determined structure of the Fe_nS_n cluster in nitrogenase enzyme.