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

Abstract

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+(X2delta5/2; 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 2delta5/2 symmetry. The observed changes in total angular momentum upon photoionization of FeC are |deltaJ+| = |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+ (X2delta5/2, v+=0; J+=5/2) from FeC (X3delta3, v"=0; J"=3), the rotational constants, Be+ = 0.7015 +/- 0.0006 cm(-1) and alphae+ = 0.00665 +/- 0.00036 cm(-1), and the vibrational constants, omegae+ = 927.14 +/- 0.04 cm(-1) and omegae+chie+ = 6.35 +/- 0.04 cm(-1), for FeC+(X2delta5/2) 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.