PNNL

Abstract

We report a photoelectron spectroscopic investigation of a series of monoiron-sulfur clusters FeSn- (n = 1-6) at various photon energies. Vibrationally resolved spectra were measured for FeS- and FeS3-. A wealth of electronic structure information was obtained for FeS and were tentatively assigned, yielding a (5)Delta ground state for FeS and a (7)Sigma(+) and a (5)Delta excited state at 0.675 and 1.106 eV above the ground state, respectively. Franck-Condon factor simulations were performed for the vibrationally resolved (5)Delta ground state and the (5)Delta excited state, yielding an Fe-S bond length of 2.18 and 2.29 Angstrom for the anion ground state and the (5)Delta excited state, respectively, as well as a vibrational temperature of 180 K for the anion. The electron affinities (EA's) of FeSn were measured to be 1.725 +/- 0.10, 3.222 +/- 0.009, 2.898 +/- 0.008, 3.129 +/- 0.008, 3.262 +/- 0.010, and 3.52 +/- 0.02 eV for n = 1-6, respectively. A significant EA increase was only observed from FeS to FeS2, whereas all larger species FeSn (n = 3-6) possess EA's similar to that of FeS2 within +/-0.3 eV. By comparing the trend of EA in FeSn to that of FeOn, we proposed that all the FeSn- (n > 1) species take (S-m(2-))Fe3+(S-n-m(2-)) type structures, in which Fe assumes its favorite +3 formal oxidation state. Preliminary density functional calculations were carried out and the obtained structures support the proposed structural evolution of the FeSn clusters.