D.J. Smith

Reactive scattering of a supersonic oxygen atom beam: O + Br2

Reactive scattering of O atoms with Br2 molecules has been studied at an initial translational energy E = 40 kJ mol-1 using a supersonic beam of O atoms seeded in He, and at E = 15 kJ mol-1 using O atoms seeded in Ne. Velocity distributions of OBr product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross-section have been obtained. For O atoms seeded in Ne, there is peaking in the forward direction with a lower intensity 0·6 ± 0·1 in the backward direction. The product translational energy distribution agrees with that predicted by a long-lived collision complex model with a tight linear transition state for dissociation. For O atoms seeded in He, the scattering in the backward direction is increased to 1·2 ± 0·1 and has a higher product translational energy than the scattering in the forward direction. The reactive scattering observed at the lower initial translational energy indicates a short-lived O-Br-Br collision complex with a lifetime a...

Reactive scattering of a supersonic chlorine atom beam: Cl + CH3I

Reactive scattering of Cl atoms with CH3I molecules has been studied over the range of initial translational energy E = 62-23 kJ mol-1, using a supersonic beam of Cl atoms seeded in He and Ne buffer gas. Laboratory angular and velocity distributions of ICl product were measured which indicate that reaction occurs via a long-lived collision complex with forward scattering appearing only at the highest collision energy. The dependence of the total reaction cross section on initial translational energy was also determined. The observed reactive scattering arises from an endoergic dissociation of the collision complex. The results are interpreted by a model which accounts for the effect of angular momentum conservation on this endoergic dissociation compared with the favoured dissociation of the complex back to reactants.

Reactive scattering of a supersonic oxygen atom beam O + I2

Reactive scattering of O atoms with I2 molecules has been studied at an initial translational energy E = 43 kJ mol-1 using a supersonic beam of O atoms seeded in He and E = 18 kJ mol-1 using O atoms seeded in Ne. Velocity distributions of OI product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross section were obtained which show predominantly rebound scattering at both initial translational energies. Scattering in the forward direction has a product translational energy distribution similar to that predicted for a long-lived collision complex but scattering in the backward direction has a much higher product translational energy. The greater predominance of rebound scattering observed for O + I2 compared with O + Br2 may be attributed to the greater exoergicity of the O + I2 reaction. However, comparison with the O + IBr reaction which exhibits a long-lived collision complex mechanism indicates that the predominance of backward scattering fo...

Angular distributions of reactive scattering arising from persistent complexes with dissociation at arbitrary angles to the inertial axes of the transition state

Angular distributions of reactive scattering arising from a long-lived collision complex dissociating via a transition state that approximates to a linear rotator are predicted for H atom displacement occurring over a distribution of bending angles β with respect to the rotator axis. A conical angular distribution may be generated when only a narrow range of bending angles contributes to the reactive scattering. However, the angular distribution may take on quite different character as the distribution of bending angles becomes broader. Thus, an isotropic angular distribution is generated by a bending angle distribution which follows the spherical polar weighting P(β) ∝ sin β. Forward and backward scattering is generated by bending angle distributions favouring less strongly bent displacement, while sideways scattering is generated by a bending angle distribution favouring more strongly bent displacement. The experimental angular distribution for OH + CO reactive scattering which shows mild forward and ba...

Reactive scattering of a supersonic methyl radical beam: CH3 + ICl, IBr, I2, Br2

Reactive scattering of CH3 radicals with ICl, IBr, I2 and Br2 molecules has been studied at an initial translational energy E ∼ 30kJ mol-1, using a supersonic beam of CH3 radicals seeded in He buffer gas. Laboratory angular and velocity distributions of CH3I and CH3Br product have been measured which show a gradation from short-lived complex dynamics for ICl to rebound dynamics for I2 and Br2 with intermediate behaviour for IBr. This variation in dynamics is attributed to the form of the potential energy surface which shifts from a shallow hollow for CH3 + ICl to repulsion in the exit valley for CH3 + I2, Br2, with both features being present for the intermediate case of CH3 + IBr.

Reactive scattering of a supersonic chlorine atom beam Cl + I2

Reactive scattering of Cl atoms with I2 molecules has been studied at an initial translational energy E = 61 kJ mol-1 using a supersonic beam of Cl atoms seeded in He and at E = 26 kJ mol-1 using Cl atoms seeded in Ne. Velocity distributions of ICl product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross section have been obtained which show sharp peaking in the forward direction at both initial translational energies. The product translational energy distributions are independent of scattering angle and show that a substantial fraction f′pk ∼ 0·35, f′av ∼ 0·5 of the total available energy is disposed into translation. These results are interpreted in terms of the stripping model due to Herschbach, which identifies two components in the reactive scattering; wide angle scattering arising from smaller impact parameter collisions and forward scattering arising from a narrow range of the largest impact parameters. The transition state for forward...

Reactive scattering of O(3P) atoms with alkyl and allyl iodide molecules

Abstract Differential cross sections have been measured for the reactions of O( 3 P) atoms with C 2 H 5 I, (CH 3 ) 2 CHI, (CH 3 ) 3 CI and C 3 H 5 I molecules at an initial translational energy E ≈ 36 kJ mol −1 using a supersonic beam of O atoms seeded in He buffer gas. The product angular distribution for O+C 2 H 5 I shows mild peaking in the forward and backward direction with slightly favours the backward direction, and a product translational energy distribution in accord with the predictions of phase space theory. The propensity toward backward scattering and the fraction of the total available energy disposed into product translation both increase progressively along the series of reactions. This trend is attributed to reaction at small impact parameters b ⩽2.5 A over a weakly attractive triplet potential energy surface, which splits into 3 A′ and 3 A″ Renner—Teller components in bent OIR configurations.

Reactive scattering via a persistent collision complex as a function of impact parameter

Abstract A recently proposed microcanonical transition state theory for angular distributions of reactive scattering arising from a persistent collision complex is extended to the full range of impact parameters. At larger impact parameters the angular distribution is governed by the precessional motion of the symmetric top transition state for complex dissociation while at smaller impact parameters the angular distribution is determined by the disposition of the product orbital angular momentum estimated from phase space theory. The extended theory predicts an angular distribution for F+C 2 H 5 I reactive scattering which is in excellent agreement with recent experimental measurements at an initial translational energy E ≈ 38 kJ mol −1 .

Reactive scattering of a supersonic oxygen atom beam O+ICl

Reactive scattering of O atoms with ICl molecules has been studied at an initial translational energy E = 40 kJ mol-1 using a supersonic beam of O atoms seeded in He and at E = 15 kJ mol-1 using O atoms seeded in Ne. Velocity distributions of OI product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross-section have been obtained which show peaking almost equally in the forward and backward directions at both initial translational energies. The product translational energy distributions are consistent with a long-lived O-I-Cl collision complex dissociating via a loose transition state. The stability of the O-I-Cl complex is attributed to the low electronegativity of the central I atom compared with the peripheral atoms. This electronegativity ordering rule also determines the stability of the intermediates in the other reactions of oxygen atoms with halogen molecules. The mild peaking of the product angular distributions for O + ICl and IBr ind...

Transition state dynamics of F + ICl, C2H5I reactive scattering

Reactive scattering of F atoms with ICl and C2H5I molecules has been studied at an initial translational energy E ∼ 12 kJ mol-1 using a supersonic beam of F atoms seeded in Ne buffer gas. The centre-of-mass angular distribution of IF scattering for F + ICl, measured previously at E ∼ 34 kJ mol-1, shows peaking mainly in the forward direction, which is consistent with reaction via a short-lived complex with a lifetime of half a rotational period. The microcanonical theory for dissociation of a persistent complex via a prolate symmetric top transition state accounts for the higher energy F + ICl scattering arising from the full range of impact parameters. The angular distribution for F + C2H5I shows peaking in the forward and backward directions which is consistent with a complex lifetime greater than one rotational period. The extended microcanonical theory accounts for scattering by a range of transition state configurations generated by internal rotation about the extended C-I bond for collisions at the ...