The chemiionization reactions Ce + O and Ce + O2: Assignment of the observed chemielectron bands

Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemiionization reactions Ce + O → CeO+ + e− and Ce + O2 → CeOmath image + e−. Selected spectroscopic constants for CeOn and CeOmath image (n = 1, 2), as well as reaction enthalpies of the chemiionization reactions of interest, have been computed and compared with experimental values. In contrast to the lanthanum case, for both Ce + O2(X3Σmath image) and Ce + O2( a1Δg), the Ce + O2 → CeOmath image + e− reaction is shown to be exothermic, and thus, contributes to the experimental chemielectron spectra. The apparent discrepancy between the computed reaction enthalpies and the high kinetic energy offset values measured in the chemielectron spectra is rationalized by arguing that chemielectrons are produced mainly via two sequential reactions (Ce + O2 → CeO + O, followed by Ce + O → CeO+ + e−) as in the case of lanthanum. For Ce + O2 (a1Δg), a chemielectron band with higher kinetic energy than that recorded for Ce + O2( X3Σmath image) is obtained. This is attributed to production of O( 1D) from the reaction Ce + O2( a1Δg) → CeO + O( 1D), followed by chemiionization via the reaction Ce + O( 1D) → CeO+ + e−. Accurate potential energy curves for the ground and a number of excited states of CeO and CeO+ have been computed, and a mechanism for the chemiionization reactions investigated experimentally was proposed.