Vacuum ultraviolet photodissociation dynamics of CO near 133 nm: The spin-forbidden O(P) + CO(XΣ) channel.

Understanding vacuum ultraviolet (VUV) photodissociation dynamics of CO is of considerable importance in the study of atmospheric chemistry and planetary chemistry. Yet, photodissociation dynamics of the spin-forbidden O(P) + CO(XΣ) channel has not been clearly understood so far. Here, we study the O(P) + CO(XΣ) dissociation processes in the VUV photodissociation of CO at the photolysis wavelengths between 129.02 and 134.67 nm by using the time-sliced velocity-mapped ion imaging technique. From the vibrational-resolved images of the O(P) photofragment, the total kinetic energy releases, the CO(XΣ) cofragment vibrational state distributions, and the product angular distributions have been derived, respectively. The experimental observations show that the total kinetic energy releases for the three P spin-orbit states (j = 2, 1, 0) exhibit a broad CO(XΣ) vibrational energy distribution with significant inverted characteristics, especially at short photoexcitation wavelengths, indicating that the VUV photodissociation could take place in a relatively linear geometry of the triplet state, with one C-O bond extended and the other compressed. Furthermore, a notable photolysis wavelength dependent feature has also been found in the product angular distributions of all three spin-orbit channels (j = 2, 1, 0): Only the vibrational-state specific anisotropy parameter β values at 130.18 nm behave more anisotropic, while all those at other photolysis wavelengths are near the value β = 0.5 for O(P) channels or β = 0.25 for the O(P) channel, with small fluctuations. This anomalous phenomenon suggests that the different nonadiabatic interactions, such as singlet-triplet coupling, may play a key role in the formation of O(P) + CO(XΣ) products, with strong photolysis wavelength dependence.