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

Photoelectron detachment spectra of M(2)(C(6)H(6))(-) (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)(C(6)H(6)) and Pd(2)(C(6)H(6)), respectively. A ground state vibrational energy of (24.2 +/-1) meV has been resolved for Pt(2)(C(6)H(6)). The corresponding vibrational energy of Pt(2)(C(6)H(6))(-) amounts to (19.0 +/-1.0) meV. The ground state vibrational energies of Pd(2)(C(6)H(6)) and Pd(2)(C(6)H(6))(-) 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)(C(6)H(6)) in contrast to the triplet ground state of unreacted Pb(2). The vertical electron affinity of Pb(2)(C(6)H(6)) is (1.95 +/-0.05) eV. (C) 2001 American Institute of Physics.