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

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-2 Delta(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 2 Delta(5/2) symmetry. The observed changes in total angular momentum upon photoionization of FeC are |Delta J(+)| = |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 pi-type. The ionization energy, IE = 61243.1 +/- 0.5 cm(-1) (7.59318 +/- 0.00006 eV), for the formation of FeC+ (X2 Delta(5/2), v(+)=0; J(+)=5/2) from FeC (X-3 Delta(3), v ''=0; J ''=3), the rotational constants, B-e(+) = 0.7015 +/- 0.0006 cm(-1) and alpha(+)(e) = 0.00665 +/- 0.00036 cm(-1), and the vibrational constants, omega(+)(e) = 927.14 +/- 0.04 cm(-1) and omega(+)(e)chi(+)(e) = 6.35 +/- 0.04 cm(-1), for FeC+(X-2 Delta(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.