Chemisorption of benzene on metal dimer anions : a study by photoelectron detachment spectroscopy

Photoelectron detachment spectra of M-2(C6H6)(-) (M = Pt, Pd, Pb) have been measured in the gas phase using photon energies of a Nd:YAG laser. The vibrationally resolved ground state transition from the anion to the neutral reveals an adiabatic electron affinity of (2.01 +/-0.05) eV and (0.88 +/-0.05) eV for Pt-2(C6H6) and Pd-2(C6H6), respectively. A ground state vibrational energy of (24.2 +/-1) meV has been resolved for Pt-2(C6H6). The corresponding vibrational energy of Pt-2(C6H6)(-) amounts to (19.0 +/-1.0) meV. The ground state vibrational energies of Pd-2(C6H6) and Pd-2(C6H6)(-) are (20.3 +/-1.0) meV and (18.0 +/-2.0) meV, respectively. The small vibrational frequencies suggest a perpendicular coordination (C-6v-symmetry) of the benzene-adsorbed transition metal dimers. Pb-2, on the other hand, is bound parallel to the benzene plane (C-2v-symmetry). A closed shell ground state electron configuration is postulated for Pb-2(C6H6) in contrast to the triplet ground state of unreacted Pb-2. The vertical electron affinity of Pb-2(C6H6) is (1.95 +/-0.05) eV. (C) 2001 American Institute of Physics