Vibrationally Resolved Photoelectron Spectroscopy of MgO- and ZnO- and the Low-Lying Electronic States of MgO, MgO-, and ZnO

Vibrationally resolved photoelectron spectra of MgO- and ZnO- have been recorded at several photon energies under varied experimental conditions. Peaks in these highly structured spectra have been assigned to photodetachment transitions from the MgO- and ZnO- ground state (X 2Σ+) to vibrational progressions in the ground and several low lying neutral excited states. In addition, a high-temperature MgO- spectrum shows spectral features due to photodetachment from an excited electronic state of the MgO- anion, which has been assigned to an A2Π anionic state. From the MgO- spectra, the electron affinity of the MgO ground state (XΣ+) is determined to be 1.630 (0.025) eV. Four electronic excited states of MgO, a3Π, A1Π, b3Σ+, and B1Σ+, were found to lie 2510, 3390, 8390, and 20 000 cm-1 above the X1Σ+ neutral ground state, respectively. The excited MgO- AΠ anion state was found to lie 4791 cm-1 above the MgO- X2Σ+ anion ground state. The photoelectron spectra of ZnO-, presented here at higher photon energies, extend a previous photoelectron study by Fancher et al. to the first two excited neutral states, aΠ and A1Π, which have been found to lie 2460 and 4960 cm-1 above the X1Σ+ ground state, respectively. From Franck−Condon analyses of the well-resolved vibrational progressions for each electronic transition, equilibrium internuclear distances and fundamental vibrational frequencies of the MgO and ZnO neutral electronic states were determined. Moreover, because the sources employed produced vibrationally hot anions, the bond length and vibrational frequencies of both the MgO- ground and excited states were found from the vibrational hot band transitions.