Influence of the collision energy on the O(1D) + RH --> OH(X2Pi) + R (RH=CH4, C2H6, C3H8) Reaction Dynamics. A laser Induced Fluorescence and Quasiclassical Trajectory Study

The influence of the collision energy (ET) on the O(1D) + RH OH(X2H) + R (RH = CH4, C2H6, and C3H8) reaction dynamics has been studied, using the N2O photodissociation at 193 nm as O(1D) precursor (ET = 0.403 eV) and probing the OH v = 0 and 1 levels by LIF. A triatomic QCT study of the reaction with CH4 on a fully ab initio based analytical PES has also been performed, and a quite good agreement with the experimental OH rovibrational distributions has been obtained. Our experimental results are similar to those obtained when the O3 photodissociation is used to produce O(1D) (ET = 0.212 eV), as expected on the basis of the available energy in products and also from the QCT calculations. The P(v=0)/P(v= 1) populations ratio values reported for C2H6 and C3H8 in a very recent work (Wada and Obi, J. Phys. Chem. A 1998, 102, 3481), where the N2O was also used to generate O(1D), are probably largely underestimated. The rotational distributions obtained are similar to those obtained in other experiments, and a quite good agreement has been obtained for the spin-orbit and A-doublet populations. The reaction takes place near exclusively through the insertion of the O(1D) atom into a C-H bond below 0.6 eV, and the mechanism may be direct or nondirect (mainly through short-lived (CH3)OH collision complexes) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect one leads to a noninverted distribution. At higher ET, the abstraction mechanism also contributes appreciably to reactivity. © 2000 American Chemical Society

Atomic and molecular data for spacecraft re-entry plasmas

The modeling of atmospheric gas, interacting with the space vehicles in re-entry conditions in planetary exploration missions, requires a large set of scattering data for all those elementary processes occurring in the system. A fundamental aspect of re-entry problems is represented by the strong non-equilibrium conditions met in the atmospheric plasma close to the surface of the thermal shield, where numerous interconnected relaxation processes determine the evolution of the gaseous system towards equilibrium conditions. A central role is played by the vibrational exchanges of energy, so that collisional processes involving vibrationally excited molecules assume a particular importance. In the present paper, theoretical calculations of complete sets of vibrationally state-resolved cross sections and rate coefficients are reviewed, focusing on the relevant classes of collisional processes: resonant and non-resonant electron-impact excitation of molecules, atom-diatom and molecule-molecule collisions as well as gas-surface interaction. In particular, collisional processes involving atomic and molecular species, relevant to Earth (N2, O2, NO), Mars (CO2, CO, N2) and Jupiter (H2, He) atmospheres are considered

Theoretical characterization of transition state dynamical resonances in heavy-light-heavy reactions

The resonant reactivity of three elementary Heavy-Light-Heavy reactions is presented and discussed. Collinear reactivity, in which a vibrational adiabatic model is constructed, is used for a detailed analysis of resonance phenomena, which appear as a direct consequence of transition state metastable states in the strong interaction region of the potential energy surface. Their influence on the detailed mechanism of the elementary process is also discussed. The shape of the resonant peak, and the phase and the Argand plot of the S-matrix are used for a further characterization.Three-dimensional approximate calculations are used to test the evolution of the energy dependent structure present in detailed quantities when sums and integrations over all partial waves contributing to reaction are taken into account to obtain the usual averaged global quantities such as integral state-to-state cross sections

Atomic and molecular data for spacecraft re-entry plasmas

The modeling of atmospheric gas, interacting with the space vehicles in re-entry conditions in planetary exploration missions, requires a large set of scattering data for all those elementary processes occurring in the system. A fundamental aspect of re-entry problems is represented by the strong non-equilibrium conditions met in the atmospheric plasma close to the surface of the thermal shield, where numerous interconnected relaxation processes determine the evolution of the gaseous system towards equilibrium conditions. A central role is played by the vibrational exchanges of energy, so that collisional processes involving vibrationally excited molecules assume a particular importance. In the present paper, theoretical calculations of complete sets of vibrationally state-resolved cross sections and rate coefficients are reviewed, focusing on the relevant classes of collisional processes: resonant and non-resonant electron-impact excitation of molecules, atom-diatom and molecule-molecule collisions as well as gas-surface interaction. In particular, collisional processes involving atomic and molecular species, relevant to Earth (N-2, O-2, NO), Mars (CO2, CO, N-2) and Jupiter (H-2, He) atmospheres are considered

Quasiclassical trajectory study of molecular alignment effects on the dynamics of the reactions of Cl, Br and I whith H2

The X + H2 (X = Cl, Br, and I) reactions may be taken as models for endoergic triatomic reactions with heavy-light-light kinematics and collinear saddle point. The dependence of scalar and two-vector properties, angular distributions (k, k), (k, j), (k, j), and (I, j), on (ET, v, j), as well as the effect of considering initial parallel (II), perpendicular (), and random (null) k-j alignment has been studied using the quasiclassical trajectory (QCT) method. The threshold energy for II alignment is always higher than the ones for and null alignments, but for high enough ET values (II) becomes larger than () and (null), and the same occurs for the j-dependence. In the v range of values explored (II) is in general equal or larger than () and (null). The expression 1/3(II) + 2/3() provides a very good estimate to (null) if the system is not in the vicinities of the threshold region, and some useful relations to simplify the QCT calculations for II alignment have also been given. For the two-vector properties considered, the results obtained for alignment are in general closer to the ones for null alignment than the results obtained for II alignment. The angular correlations that result from the calculation are not a trivial result coming from a kinematic constraint; being particularly remarkable the role played by the rotation of the H2 molecule. These results may be explained taking into account the saddle point properties, the "effective molecular size" of the rovibrationally excited H2 molecule, and the geometrical implications of the alignments

Collision energy effects on the dynamics of the reaction O(3P) + CH4(X 1A1) --> OH(X2P) + CH3(X2A").

A study of the collision energy effects on the dynamics of the title reaction was performed using the quasi-classical trajectories (QCT) method and an analytical triatomic potential energy surface recently derived by our group. Scalar and two-vector properties of the reaction were analysed in terms of the collision energy. The results obtained can be rationalised in terms of the coexistence of reactive trajectories with rebound and non-rebound features, both corresponding to an abstraction reaction mechanism. Future work should account for both the full dimensionality of the system and the possibility of quantum effects. © 2001 Elsevier Science B.V