Jahn--Teller effect in the photoelectron spectrum of iron pentacarbonyl

High quality photoelectron spectra of gaseous Fe(CO)5 excited by HeI, HeII, and ArI photons have been obtained. Major attention is focused on the primarily metal 3d ionizations, which occur in the binding energy region from 7 to 11 eV. Ionization to the 2E' positive ion state (centered at 8.6 eV) clearly shows the effects of Jahn--Teller distortions in the positive ion. This ionization results in two ionization bands of approximately equal intensity and shape separated by 0.38 eV at room temperature. These bands broaden and the splitting increases to 0.47 eV at 473 0K. Ionization to the 2E'' positive ion state, centered at 9.9 eV, is much less influenced by the Jahn--Tellar effect. There is no discernable splitting of this band at room temperature. These observations are discussed in terms of the electronic structure and bonding of Fe(CO)5. Simple model calculations of the energies of the doubly degenerate electronic states in relation to the appropriate doubly degenerate normal vibrational modes are used to investigate the splitting. The magnitudes of the 2E' and 2E'' splittings are reproduced very well, and the temperature dependence is reproduced within experimental error. It is found that the vibrational motion primarily responsible for the splitting is a low frequency OC--Fe--CO bending in the equatorial plane. This bending is found in two normal modes, one of which is closely related to the Berry pseudorotation process. The splitting of the 2E' ionization is clear experimental evidence in the gas phase that the preferred geometry of d7 M(CO)5 is not D/sub 3h/ in this case