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+(X2A5/2; 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 2A5/2 symmetry. The observed changes in total angular momentum upon photoionization of FeC are IAJ +1 = IJ + - 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 TT-type. The ionization energy, IE = 61243.1 ± 0.5 cm-1 (7.59318 ± 0.00006 eV), for the formation of FeC+ (X2A5/2, v+=0; J+=5/2) from FeC (X3A3, v"=0; J"=3), the rotational constants, Be+ = 0.7015 ± 0.0006 cm-1 and a.e+ = 0.00665 ± 0.00036 cm-1, and the vibrational constants, ft>e+ = 927.14 ± 0.04 cm-1 and (i)e+%e+ = 6.35 ± 0.04 cm-1, for FeC+(X2A5Q) 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. © 2009 American Chemical Society.