Threshold Ionization Spectroscopy and Theoretical Calculations of LnO (Ln = La and Ce)

Mass-analyzed threshold ionization (MATI) spectroscopy was used to measure the vibronic spectra of LnO (Ln = La and Ce). Single-reference coupled cluster and relativistic multireference configuration calculations were carried out to compare with the measured vibronic energies. The spectrum of LaO displays a single vibronic band system, while that of CeO shows multiple ones. The ionization energies of LaO and CeO are measured as 5.2446(6) and 5.3332(6) eV, respectively, which are a 100-fold improvement over the literature values. The vibrational energies of the neutral molecule and corresponding ion reveal the charge effect on the metal-oxygen bond of both species. The single band system in the spectrum of LaO arises from the transition of the ground state of the neutral molecule with the La(6s¹)O(2p⁶) valence configuration to the ground state of the singly charged ion with the La(6s⁰)O(2p⁶) configuration. The multiple band systems in the spectrum of CeO are attributed to the spin-orbit coupling for the Ce(4f¹6s¹)O(2p⁶) configuration of the neutral molecule and an excited state for the Ce(4f¹)O(2p⁶) configuration of the ion.