Rovibronically Selected and Resolved Two-Color Laser Photoionization and Photoelectron Study of the Iron Carbide Cation

摘要

By using a two-color laser excitation—photoionization scheme, we have obtained rovibronically selected and resolved state-to-state pulsed field ionization—photoelectron (PFI-PE) bands for FeC~+(X~2Δ_(5/2); v~+=0-2 J~+) allowing unambiguous rotational assignments for the photoionization transitions. The finding of the J~+ = 5/2 level as the lowest rotational state confirms that the ground FeC+ ion state is of ~2Δ_(5/2) symmetry. The observed changes in total angular momentum upon photoionization of FeC are |ΔJ~+|= |J + — J′|≤ 3.5, indicating that the photoelectron orbital angular momentum is limited to l≤ 3. This observation is also consistent with the conclusion that the photoionization involves the removal of an electron from the highest occupied molecular orbital of the π-type. The ionization energy, IE = 61243.1 ± 0.5 cm~(-1) (7.59318 ± 0.00006 eV), for the formation of FeC~+ (X~2Δ_(5/2), v~+=0; J~+=5/2) from FeC (X~3Δ_3, v"=0; J"=3), the rotational constants, Be~+ = 0.7015 ± 0.0006 cm~(-1) and α_e~+ = 0.00665 ± 0.00036 cm~(-1), and the vibrational constants, ω_e~+ = 927.14 ± 0.04 cm~(-1) and ω_e~+X_e~+ = 6.35 ± 0.04 cm~(-1), for FeC~+(X~2Δ_(5/2)) determined in the present study are compared to the recent state-of-the-art ab initio quantum chemical calculation at the C-MRCI+Q level of theory. The large deviation (0.49 eV) observed between the present experimental IE value and the C-MRCI+Q theoretical IE prediction highlights the great need for the further development of ab initio quantum theoretical procedures for more accurate energetic predictions of transition metal-containing molecules.