Pulsed field ionization–photoelectron bands for CO2+(A 2Πu and B 2Σu+) in the energy range of 17.2–19.0 eV: An experimental and theoretical study

Abstract

The vacuum ultraviolet (VUV) pulsed field ionization–photoelectron (PFI–PE) spectra for CO2 have been measured in the energy range of 17.2–19.0 eV, showing complex vibronic structures for the CO2+(A 2Πu and B 2Σu+) states. The PFI–PE spectra for CO2+(A 2Πu and B 2Σu+) are dominated by the v1+ (symmetric stretching) vibrational progressions, and weak bands due to excitation of both even and odd quanta of the v2+ (bending) and v3+ (antisymmetric stretching) modes are observed in the VUV–PFI–PE spectra. The simulation of rotational contours resolved in the PFI–PE vibronic bands associated with excitation to CO2+(A 2Π3/2,1/2u;v1+=0–5,v2+=0,v3+=0) and CO2+(B 2Σu+;0,0,0) has yielded accurate ionization energies for the formation of these states from CO2(X 1Σg+). Three-dimensional potential energy functions (PEFs) for CO2+(B 2Σu+) have also been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Based on these PEFs, vibrational energy levels for CO2+(B 2Σu+), together with the Franck–Condon factors for their formation from CO2(X 1Σg+), have been calculated. With the guide of these theoretical predictions, the vibrational bands resolved in the PFI–PE spectrum for CO2+(B 2Σu+) have been satisfactorily assigned. This assignment reveals the nature of many vibrational PFI–PE bands as originated from anharmonic resonance interactions and members of Fermi polyads.