The ultraviolet negative ion photoelectron spectrum of Pdminus;2is presented for electron binding energies up to 3.35 eV. The anion is prepared by sputtering in a flowing afterglow ion source. Multiple lowhyphen;lying electronic states of Pd2, all unidentified previously, are observed with resolved vibrational structure. The spectrum shows two strong electronic bands, each with similar vibrational progressions. Franckndash;Condon analyses are carried out on the two transitions and molecular constants are extracted for the anion and the two neutral electronic states. With the help of simple molecular orbital arguments andabinitiocalculations, these two electronic bands are assigned as the triplet ground state (3Sgr;+u) and a singlet excited state (1Sgr;+u). The adiabatic electron affinity is E.A.(Pd2)=1.685plusmn;0.008 eV and the singlet excitation energyT0(1Sgr;+u) is 0.497plusmn;0.008 eV (4008plusmn;65 cmminus;1). The bonding in the palladium dimers is discussed and we find that the anion bond strength is 1.123plusmn;0.013 eV stronger than that of the neutral. Related studies of Pdminus;yield an improved electron affinity of E.A.(Pd)=0.562plusmn;0.005 eV.
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