Dynamics of the Reaction O(3P) + H2S → OH + SH. 2. State-Resolved Differential Cross Sections and Angular Momentum Correlations

The Dynamics of Reactions of O(3P) Atoms with Saturated Hydrocarbons and Related Compounds

We review the experimental and theoretical work which has been carried out on the dynamics of reactions of O(3P) with saturated hydrocarbons and related systems. We concentrate primarily on gas phase reactions, but also cover in less detail the more limited work on condensed phase and interfacial reactions. Although O(3P) + saturated alkane reactions are the primary focus, the dominant features of their dynamics are compared and contrasted with those of unsaturated alkanes, functionalised alkanes, and inorganic hydrides (including silanes, germanes, H2S, and hydrogen halides). The principal experimental techniques are reviewed. The experimentally determined quantities are identified, including excitation functions, OH rovibrational and fine-structure partitioning, the rather limited equivalent results for the organic radical co-product, and differential cross-sections. The dynamical conclusions that have been inferred are discussed and compared with the predictions of various levels of theory from semi-empirical models through to rigorous ab initio treatments. For many organic systems, most of the evidence points to OH being formed via a direct abstraction mechanism in which the O(3P) atom attacks along an isolated C–H bond. Outstanding problems with this basic interpretation and gaps in the current knowledge base are identified.

Differential scattering cross-sections for CNA2Π+Ar

We present the first results from a novel experimental approach to the measurement of state-to-state differential scattering cross-sections for inelastic scattering of electronically excited CN A(2)Pi with Ar. Photodissociation of ICN with linearly polarized 266 nm radiation generates CN X(2)Sigma(+) (upsilon(")=0,J(")) with a near mono-energetic speed distribution and large anisotropy. Saturated optical pumping of the nascent CN X(2)Sigma(+) transfers this speed distribution without distortion to selected rotational quantum states of the A(2)Pi (upsilon(')=4) level. The products of rotational energy transfer within the A(2)Pi (upsilon(')=4) level into the J(')=0.5, F(2), f, state are probed using frequency modulated stimulated emission spectroscopy on the A-X (4,2) band with a single frequency external cavity tunable diode laser. Doppler profiles of transitions from individual rotational, spin-orbit and lambda doublet specific levels are acquired for different geometrical arrangements of photolysis polarization and probe propagation directions. The resulting Doppler profiles, which for this J(')=0.5 state cannot display a rotational angular momentum alignment, are combined to yield composite Doppler profiles depending on speed and translational anisotropy, which are analyzed to determine fully state-to-state resolved differential scattering cross-sections.

Efficiencies of state and velocity-changing collisions of superthermal CN A2Π with He, Ar, N2and O2

Polarized laser photolysis of ICN is combined with saturated optical pumping to prepare state-selected CN Alpha(2)Pi (nu' = 4, J = 0.5, F(2), f) with a well-defined anisotropic superthermal speed distribution. The collisional evolution of the prepared state is observed by Doppler-resolved Frequency Modulated (FM) spectroscopy via stimulated emission on the CN Alpha(2)Pi-Chi(2)Sigma(+) (4,2) band. The phenomenological rate constants for removal of the prepared state in collisions with He, Ar, N(2) and O(2) are reported. The observed collision cross-sections are consistent with attractive forces contributing significantly for all the colliders with the exception of He. The collisional evolution of the prepared velocity distribution demonstrates that no significant back-transfer into the prepared level occurs, and that any elastic scattering is strongly in the forward hemisphere.

Frequency modulated spectroscopy as a probe of molecular collision dynamics

We describe the application of frequency modulated spectroscopy (FMS) with an external cavity tuneable diode laser to the study of the scalar and vector properties of inelastic collisions. CN X(2)Sigma(+) radicals are produced by polarized photodissociation of ICN at 266 nm, with a sharp velocity and rotational angular momentum distribution. The collisional evolution of the distribution is observed via sub-Doppler FMS on the A(2)Pi-X(2)Sigma(+) (2,0) band. He, Ar, N(2), O(2) and CO(2) were studied as collider gases. Doppler profiles were acquired in different experimental geometries of photolysis and probe laser propagation and polarization, and on different spectroscopic branches. These were combined to give composite Doppler profiles from which the speed distributions and specific speed-dependent vector correlations could be determined. The angular scattering dynamics with species other than He are found to be very similar, dominated by backward scattering which accompanies transfer of energy between rotation and translation. The kinematics of collisions with He are not conducive to the determination of differential scattering and angular momentum polarization correlations. Angular momentum correlations show interesting differences between reactive and non-reactive colliders. We propose that this reflects differences in the potential energy surfaces, in particular, the nature and depth of attractive potential wells.

Crossed beam studies of the reactions of atomic oxygen in the ground 3P and first electronically excited 1D states with hydrogen sulfide

The reactions of both ground, (3)P, and electronically excited, (1)D, oxygen atoms with hydrogen sulfide, H(2)S, have been investigated by means of the crossed molecular beams method with mass spectrometric detection at different collision energies. Amongst the possible reaction channels those leading to HSO+H for the O((3)P) reaction and to HSO/HOS+H and SO+H(2) for the O((1)D) reaction have been identified and investigated. The dynamics of the channels leading to HSO/HOS+H are elucidated for the reactions of both states and the trend with increasing the collision energy analyzed. Noteworthily, the formation of SO+H(2) products appears to be an open channel for the O((1)D) reaction, at least for the highest collision energy investigated (11.8 kcal/mol). Finally, the recent experimental and theoretical estimates of the enthalpy of formation of the HSO radical have been critically analyzed to evaluate their conformity with the present experimental data.