Revisiting the electronic structure of PtO: Cryogenic photoelectron spectroscopy and relativistic insights.

We investigate the electronic structure of PtO via cryogenic anion photoelectron spectroscopy combined with relativistic multireference calculations, clarifying long-standing ambiguities in the assignment of its low-lying electronic manifold. Vibrationally resolved spectra covering the low-lying states are obtained via 193 nm photodetachment and 400 nm velocity map imaging (VMI) of PtO- anions cooled to 13 K. The adiabatic and vertical detachment energies are measured to be 2.16 and 2.29 eV, respectively, in good agreement with relativistic SOC-NEVPT2 predictions within 0.12 eV. High-resolution VMI reveals a fine-structure splitting of 1008 ± 30 cm-1 in the X3Σ- ground state, quantitatively reproduced by SOC-NEVPT2 as a 959 cm-1 second-order spin-orbit splitting. The two components display nearly identical vibrational structure, confirmed by Franck-Condon simulations. At higher binding energy, an intense σ-detachment feature is observed at 3.27 eV, providing a decisive spectroscopic anchor for reassessing the electronic manifold. Together with theoretical results, this feature establishes that the band at 2.73 eV corresponds to the A1Δ singlet state rather than to a widely split 3Π-state manifold, as previously proposed. These results establish a revised, experimentally benchmarked electronic structure of PtO and demonstrate that cryogenic cooling combined with high-energy photodetachment is essential for resolving spin-orbit-coupled manifolds in heavy transition-metal oxides.