Vacuum ultraviolet photodissociation dynamics of CO2 near 133 nm: The spin-forbidden O(3P<i>j</i>=2,1,0) + CO(X1Σ+) channel

Abstract

Understanding vacuum ultraviolet (VUV) photodissociation dynamics of CO₂ is of considerable importance in the study of atmospheric chemistry and planetary chemistry. Yet, photodissociation dynamics of the spin-forbidden O(³P_j=2,1,0) + CO(X¹Σ⁺) channel has not been clearly understood so far. Here, we study the O(³P_j) + CO(X¹Σ⁺) dissociation processes in the VUV photodissociation of CO₂ 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(³P_j=2,1,0) photofragment, the total kinetic energy releases, the CO(X¹Σ⁺) 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 ³P_j spin-orbit states (j = 2, 1, 0) exhibit a broad CO(X¹Σ⁺) 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(³P_j=2,1) channels or β = 0.25 for the O(³P_j=0) 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(³P_j=2,1,0) + CO(X¹Σ⁺) products, with strong photolysis wavelength dependence.