ZINC-OXIDE AND ITS ANION - A NEGATIVE-ION PHOTOELECTRON SPECTROSCOPICSTUDY

We have recorded, assigned, and analyzed the photoelectron spectrum of ZnO-. The adiabatic electron affinity (E.A.(a)) of ZnO and the vibrat ional frequencies of both ZnO and ZnO- were determined directly from t he spectrum, with a Franck-Condon analysis of its vibrational profile providing additional refinements to these parameters along with struct ural information. As a result, we found that E.A.(a)(ZnO) = 2.088 +/- 0.010 eV, omega(e)(ZnO) = 805+/-40 cm(-1), omega(e)(ZnO-)= 625 +/- 40 cm(-1), and that r(e)(ZnO-)>r(e)(ZnO) by 0.07 Angstrom. Since our meas ured value of E.A.(a)(ZnO) is 0.63 eV larger than the literature value of E.A.(O), it was also evident, through a thermochemical cycle, that D-0(ZnO-) >D-0(ZnO) by 0.63 eV. This, together with the literature va lue of D-0(ZnO), gives a value for D-0(ZnO-) of 2.24 eV. Since the ext ra electron in ZnO- is expected to occupy an antibonding orbital, the combination of D-0(ZnO-)>D-0(ZnO), omega(e)(ZnO-)<omega(e)(ZnO), and r (e)(ZnO-)>r(e)(ZnO) was initially puzzling. An explanation was provide d by the calculations of Bauschlicher and Partridge, which are present ed in the accompanying paper. Their work showed that our experimental findings can be understood in terms of the a (3)Pi state of ZnO dissoc iating to its ground-state atoms, while the X(1)Sigma(+) state of ZnO formally dissociates to a higher energy atomic asymptote. (C) 1998 Ame rican Institute of Physics. [S0021-9606(98)01843-1].