State-to-state vacuum ultraviolet photodissociation study of CO2 on the formation of state-correlated CO(X1Σ+; v) with O(1D) and O(1S) photoproducts at 11.95–12.22 eV

Abstract

The state-to-state photodissociation of CO₂ is investigated in the VUV range of 11.94–12.20 eV by using two independently tunable vacuum ultraviolet (VUV) lasers and the time-sliced velocity-map-imaging-photoion (VMI-PI) method. The spin-allowed CO(X¹Σ⁺; v = 0–18) + O(¹D) and CO(X¹Σ⁺; v = 0–9) + O(¹S) photoproduct channels are directly observed from the measurement of time-sliced VMI-PI images of O(¹D) and O(¹S). The total kinetic energy release (TKER) spectra obtained based on these VMI-PI images shows that the observed energetic thresholds for both the O(¹D) and O(¹S) channels are consistent with the thermochemical thresholds. Furthermore, the nascent vibrational distributions of CO(X¹Σ⁺; v) photoproducts formed in correlation with O(¹D) differ significantly from that produced in correlation with O(¹S), indicating that the dissociation pathways for the O(¹D) and O(¹S) channels are distinctly different. For the O(¹S) channel, CO(X¹Σ⁺; v) photoproducts are formed mostly in low vibrational states (v = 0–2), whereas for the O(¹D) channel, CO(X¹Σ⁺; v) photoproducts are found to have significant populations in high vibrationally excited states (v = 10–16). The anisotropy β parameters for the O(¹D) + CO(X¹Σ⁺; v = 0–18) and O(¹S) + CO(X¹Σ⁺; v = 0–9) channels have also been determined from the VMI-PI measurements, indicating that CO₂ dissociation to form the O(¹D) and O(¹S) channels is faster than the rotational periods of the VUV excited CO₂ molecules. We have also calculated the excited singlet potential energy surfaces (PESs) of CO₂, which are directly accessible by VUV excitation, at the ab initio quantum multi-reference configuration interaction level of theory. These calculated PESs suggest that the formation of CO(X¹Σ⁺) + O(¹S) photoproducts occurs nearly exclusively on the 4¹A′ PES, which is generally repulsive with minor potential energy ripples along the OC–O stretching coordinate. The formation of CO(X¹Σ⁺) + O(¹D) photofragments can proceed by non-adiabatic transitions from the 4¹A′ PES to the lower 3¹A′ PES of CO₂via the seam of conical intersections at a near linear OCO configuration, followed by the direct dissociation on the 3¹A′ PES. The theoretical PES calculations are consistent with the experimental observation of prompt CO₂ dissociation and high rotational and vibrational excitations for CO(X¹Σ⁺) photoproducts.