Valence Photoelectron Spectroscopy of Non-Volatile Organometallic Molecules

Ultraviolet photoelectron spectroscopy (UPS) has been instrumental in developing modern molecular electronic structure theory. The energy difference between the neutral molecule ground state and the low-lying cationic states, measured as ionization energies by UPS, is the closest measure of orbital energies described by molecular orbital theory.1 In addition, the ionizations independently provide information on electron configurations, charge potentials, bond strengths and other properties that relate to chemical reactivity in molecular systems.1, 2 Historically, high-quality photoelectron spectroscopy has been most informative for molecular species in the gas phase. However, many molecular systems of interest are not sufficiently stable or volatile for gas-phase UPS investigations. In an effort to provide high quality electronic structure data for such systems we are investigating methods for obtaining photoelectron spectra of molecules in thin films in which the surface-molecule and intermolecular interactions are relatively weak; thus approaching the gas phase limit. Previously we reported the ionizations of molecules containing metal-metal quadruple bonds3 and the first ever valence photoeleciron spectrum of C₆₀,4, 5 and found that the valence ionizations of these molecules in thin films closely resemble their gas-phase photoeleciron spectra. The differences (and similarities) between thin film and gas-phase data provide additional information about intermolecular interactions and electron relaxation in bulk materials. Here we present the UPS of ferrocene tethered to a gold surface via an alkanethiol chain. Ferrocene-terminated alkanethiols are known to form stable monolayers and have been characterized extensively.6, 7