Photodissociation dynamics of OCS at 248nm: The S(D21) atomic angular momentum polarization

Abstract

The dissociation of OCS has been investigated subsequent to excitation at 248 nm. Speed distributions, speed dependent translational anisotropy parameters, angular momentum alignment, and orientation are reported for the channel leading to S((1)D(2)). In agreement with previous experiments, two product speed regimes have been identified, correlating with differing degrees of rotational excitation in the CO coproducts. The velocity dependence of the translational anisotropy is also shown to be in agreement with previous work. However, contrary to previous interpretations, the speed dependence is shown to primarily reflect the effects of nonaxial recoil and to be consistent with predominant excitation to the 2 (1)A(') electronic state. It is proposed that the associated electronic transition moment is polarized in the molecular plane, at an angle greater than approximately 60 degrees to the initial linear OCS axis. The atomic angular momentum polarization data are interpreted in terms of a simple long-range interaction model to help identify likely surfaces populated during dissociation. Although the model neglects coherence between surfaces, the polarization data are shown to be consistent with the proposed dissociation mechanisms for the two product speed regimes. Large values for the low and high rank in-plane orientation parameters are reported. These are believed to be the first example of a polyatomic system where these effects are found to be of the same order of magnitude as the angular momentum alignment.

The ultraviolet spectrum of OCS from first principles: Electronic transitions, vibrational structure and temperature dependence

“…In the energy interval around 34 000 cm −1 the A and B state cross sections are of the same magnitude. This conclusion is in accord with that of Brouard et al 37 based on an analysis of the angular anisotropy parameter β measured by Kim et al 32 Although the cross sections of the repulsive triplet states a and b are very small in the main part of the spectrum, they exceed the singlet cross sections below 34 000 cm −1 ; they partly fill the gaps between the peaks and cannot be neglected in a quantitative analysis.…”

“…Comparisons between computed and observed 22,23,26,31,34,37,39 angular distribution parameters β suggest that either our A state TDM function is directed too closely along the OCS axis, or we have underestimated the contributions of the B and C states to the electronic absorption for the lower j part of the rotational distribution. The measured alignment parameters A (2) 0 also suggest that the contributions of B and C may be more important than our computed TDM functions indicate.…”

“…20 The details of photofragmentation in the first absorption band have been the focus of many experimental studies. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] The absorption cross section increases significantly with excitation of the bending mode in the ground state. 28,32,36 The fragmentation yields sulfur atoms in both the singlet and triplet states with a branching ratio of about 0.95:0.05 at the absorption peak.…”

“…The product angular distribution has been studied extensively in several laboratories and a strong variation of the β parameter with the CO rotational state has been observed. 23,26,31,34,37,39 In contrast to the many experimental investigations there have been only a few theoretical studies of the photodissociation process. In a joint experimental and theoretical paper Suzuki et al 26 calculated potential energy surfaces (PES) and transition dipole moment (TDM) functions for the lowest singlet electronic states.…”

Ultraviolet photodissociation of OCS: Product energy and angular distributions

“…Brouard et al 3 and Lipciuc et al 4 have provided good recent reviews. The absorption spectrum has a broad Gaussian-like profile with pronounced vibrational structures.…”