Laser spectroscopic study of the electronic states of palladium monoxide (PdO)

Among the group 10 transition metal monoxides, only the palladium monoxide (PdO) radical has hitherto eluded detection in optical spectra. In this study, we report the first optical spectra of the gas-phase PdO molecule using the laser-induced fluorescence excitation and single-vibronic-level (SVL) emission spectroscopies. Eight rotationally resolved excitation spectra were observed in 17 800-23 650 cm-1, allowing the determination of the vibrational frequencies and rotational constants for the ground and five highly excited electronic states. Six low-energy Ω states below 3500 cm-1 were identified from the SVL emission spectra and assigned to six spin-orbit components of the X 3Σ- and A 3Π electronic states. Furthermore, high-level ab initio calculations were performed on numerous Λ-S and Ω electronic states to support the assignments from the experimental observations. The spectral results elucidate the bonding characteristics of PdO and provide support for verifying the significant relativistic effect on the bonding of PtO.