Photodissociation dynamics of CO + hv → CO(XΣ) + O(D) via the 3PΠ state.

The vacuum ultraviolet (VUV) photodissociation of CO is important to understand the primary photochemical processes of CO induced by solar VUV excitation in the Earth's atmosphere. Here, we report a detailed study of vibrational-state-specific photodissociation dynamics of the CO(XΣ) + O(D) channel via the 3PΠ state by using the time-sliced velocity-mapped ion imaging apparatus combined with the single VUV photoionization detection scheme. By recording the sliced images of the O(D) photoproducts formed by VUV photoexcitation of CO to the individual vibrational structure of the 3PΠ state, both the vibrational state distributions of the counterpart CO(XΣ) photoproducts and the vibrational-state-specific product anisotropy parameters (β) are determined. The experimental results show that photodissociation of CO at 108.22, 107.50, 106.10, and 104.76 nm yields less anisotropic (β > 0) and inverted distributed CO(XΣ, v) photoproducts. The possible dissociation mechanism for the CO(XΣ) + O(D) channel may involve the non-adiabatic transition of excited CO from the initially prepared state to the 3A' state with potential energy barriers. While at 108.82 and 107.35 nm, the vibrational distributions are found to have the population peaked at a low vibrational state, and the anisotropy parameters turn out to be negative. Such variation indicates the possibility of another non-adiabatic dissociation pathway that may involve Coriolis-type coupling to the low-lying dissociative state. These observations show sclear evidence of the influence of the initially vibrational excitations on the photodissociation dynamics of CO via the 3PΠ state.