A STUDY OF FECO- AND THE (3)SIGMA(-) AND (5)SIGMA(-) STATES OF FECO BY NEGATIVE-ION PHOTOELECTRON-SPECTROSCOPY

The 488 and 514 nm negative ion photoelectron spectra of FeCO-, obtain ed at an instrumental resolution of 5 meV (40 cm-1), show vibrationall y resolved transitions from the anion ground state to the ground state and a low-lying excited state of the neutral molecule. The ground sta te of FeCO is assigned as the 3SIGMA- state and the excited state, lyi ng 1135 +/- 25 cm-1 higher in energy, as the 5SIGMA- state. The fundam ental vibrational frequencies are v(CO) = 1950 +/- 10, v(FeC) = 530 10 , and v(bend) = 330 +/- 50 cm-1 in the 3SIGMA- state, and v(CO) = 1990 +/- 15, nu(FeC) = 460 +/- 15, and v(bend) = 180 +/- 60 cm-1 in the 5S IGMA- state. Principal force constants are estimated from these result s. Based on a Franck-Condon analysis of the spectrum and other conside rations, the Fe-C bond is determined to be 0. 15 +/- 0.04 angstrom sho rter, and the C-0 bond 0.05 +/- 0.02 angstrom longer, in the 3SIGMA- s tate than in the 5SIGMA- state. These results demonstrate the importan ce of sdsigma hybridization in reducing the sigma repulsion between th e metal 4s electron and the CO 5sigma lone pair, a mechanism that is a vailable only when the electrons in the singly occupied 3dsigma and 4s orbitals are singlet coupled as in the 3SIGMA- state. The FeCO- anion displays a high Fe-C stretching frequency (465 +/- 1 0 cm-1 ), as wel l as an asymptotic Fe-CO bond energy, a bending frequency (230 +/- 40 cm-1) and equilibrium bond lengths intermediate between those in the 3 SIGMA- and 5SIGMA- states. Since the FeCO- ground state is assigned as a 4SIGMA- state in which the extra electron occupies a sigma orbital, these results indicate that the increased sigma repulsion is partiall y offset by stronger metal-CO pi bonding in the anion. The electron af finity of FeCO is measured to be 1.157 +/- 0.005 eV.