Photodissociation dynamics of the OCS + <i>hv</i> → CO(1Σ+) + S(3P <i>J</i> =2, 1, 0) in the first UV absorption band

Abstract

The OCS photodissociation dynamics of the minor triplet S(3PJ=2, 1, 0) channels across the first UV absorption band (202–248 nm) have been studied using the time-sliced velocity-mapped ion imaging technique. Wavelength-dependent dynamical features are directly revealed from the sliced images of the S(3PJ=2, 1, 0) photofragments. The internal energy distributions of the counter CO fragments exhibit a vibrational progression, superimposed by a dominant high-energy feature characterized by low vibrational but high rotational excitation. In addition, a minor fraction of extremely high rotationally excited CO(v = 0, 1) is observed near the limitation of available energy. The relative populations of these components show pronounced dependences on both photolysis wavelength and spin–orbit state. The internal-energy-dependent anisotropy parameters β(Eint), combined with insights from potential energy surfaces, indicate distinct dissociation mechanisms. The large positive β(Eint) values for both the dominant highly rotationally excited CO and a substantial population of vibrationally excited CO support a dissociation mechanism dominated by intersystem crossing from the initially excited 21A′ (and minor 11A″) state to the dissociative triplet 23A″ state. In contrast, the minor component of extremely high rotationally excited CO is generated via a two-step nonadiabatic pathway involving internal conversion from the 21A′ state to the ground state followed by intersystem crossing to the triplet 13A′ or 13A″ state, which yields near-zero or small negative β(Eint) values due to the nonaxial recoil effect. Furthermore, an abrupt drop in β(Eint) for low vibrationally excited CO fragments near 223 nm suggests an additional contribution from direct excitation to the triplet 23A″ state and its subsequent nonadiabatic dissociation.