Threshold Ionization and Spin–Orbit Coupling of Ceracyclopropene Formed by Ethylene Dehydrogenation

Spin-orbit coupling of electrons on separate lanthanide atoms of Pr2O2 and its singly charged cation

Although it plays a critical role in the photophysics and catalysis of lanthanides, spin-orbit coupling of electrons on individual lanthanide atoms in small clusters is not well understood. The major objective of this work is to probe such coupling of the praseodymium (Pr) 4f and 6s electrons in Pr2O2 and Pr2O2+. The approach combines mass-analyzed threshold ionization spectroscopy and spin-orbit multiconfiguration second-order quasi-degenerate perturbation theory. The energies of six ionization transitions are precisely measured; the adiabatic ionization energy of the neutral cluster is 38 045 (5) cm-1. Most of the electronic states involved in these transitions are identified as spin-orbit coupled states consisting of two or more electron spins. The electron configurations of these states are 4f46s2 for the neutral cluster and 4f46s for the singly charged cation, both in planar rhombus-type structures. The spin-orbit splitting due to the coupling of the electrons on the separate Pr atoms is on the order of hundreds of wavenumbers.

Probing La and Ce Excited-State Reactivity with Resonant Two-Photon Ionization Spectroscopy

Dehydrogenation and C-C bond cleavage of 1-butyne by the excited states of La and Ce atoms are investigated in laser-ablation metal molecular beams. The excited states of the metal atoms are prepared by resonant excitation, detected by resonant two-photon ionization spectroscopy, and the reaction products are monitored by photoionization time-of-flight mass spectrometry. The reactivities of La* [5d²(³F)6p (⁴G_5/2°)] and Ce* [4f5d(³F°)6s6p(³P°) (⁵H₅)] excited states are observed to be higher than those of the initial states of the corresponding metal atoms. The higher reactivities of the excited states are attributed to their higher energies and favorable electron configurations to form two covalent bonds of the metal-insertion intermediates. Although both excited La and Ce atoms show increased reactivities, the enhancement for Ce is much more pronounced than that of La, which cannot be explained by electron configurations alone. The larger reactivity enhancement from the initial states to the excited state of the Ce atom than that of La is due to the longer lifetime of the Ce excited state.

Spin-orbit coupling and vibronic transitions of Ce(C3H4) and Ce(C3H6) formed by the Ce reaction with propene: Mass-analyzed threshold ionization and relativistic quantum computation

A Ce atom reaction with propene is carried out in a pulsed laser vaporization molecule beam source. Several Ce-hydrocarbon species formed by the C-H and C-C bond activation of propene are observed by time-of-flight mass spectrometry, and Ce(C₃H_n) (n = 4 and 6) are characterized by mass-analyzed threshold ionization (MATI) spectroscopy and density functional theory, multiconfiguration, and relativistic quantum chemical calculations. The MATI spectrum of each species consists of two vibronic band systems, each with several vibronic bands. Ce(C₃H₆) is identified as an inserted species with Ce inserting into an allylic C-H bond of propene and Ce(C₃H₄) as a metallocycle through 1,2-vinylic dehydrogenation. Both species have a C_s structure with the Ce 4f¹6s¹ ground valence electron configuration in the neutral molecule and the Ce 4f¹ configuration in the singly charged ion. The two vibronic band systems observed for each species are attributed to the ionization of two pairs of the lowest spin-orbit coupled states with each pair being nearly degenerate.

Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by association and carbon-carbon bond cleavage of isoprene

La atom reaction with isoprene is carried out in a laser-vaporization molecular beam source. The reaction yields an adduct as the major product and C-C cleaved and dehydrogenated species as the minor ones. La(C₅H₈), La(C₂H₂), and La(C₃H₄) are characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of all three species exhibit a strong origin band and several weak vibronic bands corresponding to La-ligand stretch and ligand-based bend excitations. La(C₅H₈) is a five-membered metallacycle, whereas La(C₂H₂) and La(C₃H₄) are three-membered rings. All three metallacycles prefer a doublet ground state with a La 6s¹-based valence electron configuration and a singlet ion. The five-membered metallacycle is formed through La addition and isoprene isomerization, whereas the two three-membered rings are produced by La addition and insertion, hydrogen migration, and carbon-carbon bond cleavage.