Spin-orbit coupling and vibronic transitions of two Ce(CH) isomers probed by mass-analyzed threshold ionization and relativistic quantum computation.

Ce atom reactions with ethylene, 2-butene, and isobutene are carried out in a pulsed laser vaporization molecule beam source. Ce-containing species are observed with time-of-flight mass spectrometry, and Ce(CH) is characterized with mass-analyzed threshold ionization (MATI) spectroscopy and relativistic quantum chemical calculations. Two structural isomers are identified for Ce(CH): one is the tetrahedronlike Ce[C(CH)] in C symmetry and the other is the five-membered metallocyclic Ce(CHCHCHCH) in C. The MATI spectrum of the C isomer exhibits two vibronic band systems separated by 88 cm, while that of the C isomer displays three split by 60 and 101 cm. The multiple band systems are attributed to spin-orbit splitting and vibronic transitions involving metal-hydrocarbon and hydrocarbon-based vibrations. The splitting in the C isomer arises from interactions of two triplet and two singlet states at the lowest energies, while each splitting in the C isomer involves two triplets and a singlet. Although the Ce atom has ground electron configuration 4f5d6s, Ce valence electron configurations in both isomers are 4f6s in the neutral ground state and 4f in the ion. The remaining Ce 5d electrons in the isolated atom are spin paired in molecular orbitals that are a bonding combination between Ce 5d and hydrocarbon π orbitals.