Photoelectron Spectroscopy of Nickel, Palladium, and Platinum Oxide Anions

The 364-nm negative ion photoelectron spectra of XO and OXO molecules (X=Ni, Pd, and Pt) are reported. The spectra yield the electron affinities (EAs): EA(NiO)=1.455±0.005 eV; EA(PdO)=1.672±0.005 eV; EA(PtO)=2.172±0.005 eV; EA(ONiO)=3.043±0.005 eV; EA(OPdO)=3.086±0.005 eV; EA(OPtO)=2.677±0.005 eV. In addition, for the diatomics, transitions from the anion X?²Π_3/2 and X?′²Π_1/2 states into neutral X?³Σ⁻, ³Π, and for NiO and PdO, ¹Π, are assigned. Several states have been reassigned from those in the existing literature. Anion ²Π_3/2-²Π_1/2 spin-orbit splittings are measured, as are neutral ³Π₂-³Π₁ spin-orbit splittings: the XO ³Π ₂-³Π₁ splittings increase from 405±30 cm⁻¹ (NiO) to 805±30 cm⁻¹ (PdO) to 3580±40 cm⁻¹ (PtO). A bond length shortening of 0.03±0.01 Å is measured upon electron detachment from NiO⁻, resulting in an anion bond length of 1.66±0.01 Å. The bond length does not change upon electron detachment from PdO⁻ using 3.4-eV photons. The Pt-O bond length decreases by 0.035±0.010 Å in the ³Π₁←²Π_3/2 transition. The spectrum of OPtO displays a significantly more extended vibrational progression than those of ONiO or OPdO, and the O-Pt bond length is found to decrease by 0.07±0.01 Å upon electron detachment. The spectra support the view that the Ni-O bond is largely ionic, the Pd-O bond is somewhat less so, and the Pt-O bond displays a substantial covalent character.