Unimolecular decay pathways of state-selected CO2+ in the internal energy range of 5.2-6.2 eV:: An experimental and theoretical study

The vacuum ultraviolet pulsed field ionization (PFI)-photoelectron (PFI-PE) spectrum of CO2 has been measured in the energy region of 19.0-20.0 eV. The PFI-PE vibrational bands resolved for CO2+(C (2)Sigma(g)(+)) are overwhelmingly dominated by the origin band along with weak vibrational bands corresponding to excitations of the nu(1)(+) (symmetric stretching), nu(2)(+) (bending), and nu(3)(+) (antisymmetric stretching) modes. The simulation of the rotational contour resolved in the origin PFI-PE band yields a value of 19.3911+/-0.0005 eV for the ionization energy of CO2 to form CO2+(C (2)Sigma(g)(+)). A PFI-PE peak is found to coincide with each of the 0 K dissociation thresholds for the formation of O+(S-4)+CO(X (1)Sigma(+)) and CO+(X (2)Sigma(+))+O(P-3). This observation is tentatively interpreted to result from the lifetime switching effect, arising from the prompt dissociation of excited CO2 in high-n (ngreater than or equal to100) Rydberg states prior to PFI. We have also examined the decay pathways for state-selected CO2+ in the internal energy range of 5.2-6.2 eV using the PFI-PE-photoion coincidence scheme. The coincidence TOF data show unambiguously the formation of O+(S-4)+CO(X (1)Sigma(+);nu(')=0,1) and CO+(X (2)Sigma(+);nu(+)=0,1)+O(P-3). Analysis of the kinetic energy releases of fragment ions suggests that the dissociation of excited CO2+ involved is nonstatistical and proceeds with an impulsive mechanism. Potential energy functions (PEFs) for the CO2+(C (2)Sigma(g)(+)) state and the lowest quartet states of CO2+, together with their spin-orbit interactions, have been calculated using the complete active space self-consistent field and internal contracted multireference configuration interaction methods. Based on these PEFs, vibrational levels for CO2+(C (2)Sigma(g)(+)) have been also calculated using a variational approach. With the aid of these theoretical calculations, vibrational bands resolved in the PFI-PE spectrum for CO2+(C (2)Sigma(g)(+)) have been satisfactorily assigned, yielding a nu(3)(+) value of 2997 cm(-1). The theoretical calculation also provides a rationalization that the predissociation for CO2+(C (2)Sigma(g)(+)) to form O+(S-4)+CO(X (1)Sigma(+)) and CO+(X (2)Sigma(+))+O(P-3) most likely proceeds via the repulsive a (4)Sigma(g)(-) and b (4)Pi(u) (or B-4(1) in a bent geometry) states. (C) 2003 American Institute of Physics.