Zinc oxide and its anion: A negative ion photoelectron spectroscopic study

We have recorded, assigned, and analyzed the photoelectron spectrum of ZnO-. The adiabatic electron affinity (E.A.(a)) of ZnO and the vibrational frequencies of both ZnO and ZnO- were determined directly from the spectrum, with a Franck-Condon analysis of its vibrational profile providing additional refinements to these parameters along with structural 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 measured 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 value of D-0(ZnO), gives a value for D-0(ZnO-) of 2.24 eV. Since the extra 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 provided by the calculations of Bauschlicher and Partridge, which are presented in the accompanying paper. Their work showed that our experimental findings can be understood in terms of the a (3)Pi state of ZnO dissociating to its ground-state atoms, while the X(1)Sigma(+) state of ZnO formally dissociates to a higher energy atomic asymptote. (C) 1998 American Institute of Physics. [S0021-9606(98)01843-1].