Reactive Scattering of Oxygen Atoms with Iodine Molecules

Reactive scattering of O atoms with I2 molecules has been studied at an initial translational energy E 43 kJ tool-1 using cross-correlation time-of-flight analysis with resolutiolnimprovedoverpreviousmeasurements. Thebroadcentre-of-mass differential cross section favours the backward hemisphere with a product translational energy distribution lying above the distribution observed at lower initial translational energy but below the microcannonical RRHO distribution. However, a minor component with very low product translational energy favours the forward hemisphere. The observed scattering is attributed to a strongly bent OII triplet complex intermediate with a lifetime shorter than the period of overall rotation of the axis of the heavy II diatomic but long compared with the period of vibrational and rotational motion of the light O atom.


INTRODUCTION
The reactive scattering of a light atom with a heavy diatomic has recently attracted considerable interest due to the propensity for migratory dynamics [1][2][3][4] in such reactions and the possibility of signifi- cant correlation effects being observed between the final angular momenta.5 In the preceding paper 6 a model for the breakdown of long-lived complex dynamics in a light plus heavy-heavy (L + HH) reaction has been applied to O + Br2 reactive scattering in order to rationalise the increased prominence of the backward scattering observed at high initial translational energy.Previous measurements 7 on the O + 12 reaction indicate that similar behaviour is also apparent for this reaction at high initial translational energy.However, a recent RRKM analysis 8 of the laboratory angular distribution of OI scattering suggests that the reaction dynamics may be adequately represented by a long-lived complex model.In order to examine the dynamics of O + 12 reaction more closely, reactive scattering has been remeasured using a supersonic beam of O atoms seeded in He buffer gas 9 with improved resolution of the OI product velocity distributions.

EXPERIMENTAL
The apparatus and experimental conditions were the same as those used in previous measurements 7 but with the time-of-flight channel width reduced to -8/zs and a cross-correlation chopper x bearing five pseudorandom sequences rotating at 400 Hz.The velocity distribution of the supersonic beam of 12 molecules seeded in N2 buffer gas peaked at Vpk 700 rn s -1 with a full width at half maximum intensity Vwd 190 rn s -1 corresponding to a Mach number M 7.

RESULTS
The laboratory angular distribution of OI number density shown in Figure 1 and the laboratory velocity distributions of OI flux shown in Figure 2 reveal features in the region close to the laboratory centroid which were poorly resolved in the previous measurements.The trans- formation of these laboratory data to centre-of-mass coordinates employed the stochastic method of Entemann 11 with the differential reaction cross section expressed as a sum of two components each consisting of a product of an angular function T(O) and a velocity distribution Ui(u) I(O,u) This two component function proved essential to the analysis of the features now clearly resolved in the experimental velocity distribu- tions.The angular and velocity distributions in Figure 3, show the principal component scattering over the full angular range but favouring the backward hemisphere with a higher product transla- tional energy than the minor component which favours the forward hemisphere with a very low product translational energy.The fits of the stochastic analysis are shown by a solid curve in Figure 1 and broken curves in Figure 2. The contributions of the separate com- ponents to the laboratory angular distribution are shown by broken curves in Figure 1.While the principal component makes the major contribution to the OI number density over the full angular range, the minor component makes an important contribution in the angular range close to the laboratory centroid.This accords with the features observed in the laboratory velocity distributions lying close to the laboratory centroid.The peak E'pk and average E'av product transla- tional energies are listed in the table for each component together with the initial translational energy E and the reaction exoergicity ADo.The TRANSLATIONAL ENERGY0 E'/kJ moL- ire 3 Component angula Iunctions T(8) and tanslafional energy distributions P(E') for 0 + I at an initial translational energy E 43 kJ tool-.ajor component is shown by a solid cu, minor component by a broken curve and the distribution of itial tanslationa] energy by a dot-dash ce.backward scattering shown by solid curves.Angular distribution of Eq. (4) for short- lived complex and microcannonical RRHO prediction of Eq. ( 5) for the product translational energy distribution are shown by broken curves.
translational energy distribution or backward scattering are shown in Figure 4.The composite angular function is similar to that for the principal component but with slightly increased scattering in the forward direction.The composite product translational energy dis- tribution for forward scattering differs from that of the backward scattering only at very low translational energy.Indeed the contri- bution of the minor component to the total reaction cross section may be determined by integration over the centre-o-mass angular and This gives an estimate of only -0.2% for the minor component, and its prominence in the laboratory scattering may be attributed to the kinematic enhancement of OI product with very low centre-of-mass velocity.

DISCUSSION
The geometry of the OII triplet complex is predicted by Walsh molecular orbital theory 2 to be more strongly bent than the OBrBr complex due to the lower electronegativity of the central I atom compared with the Br atom.As in the interbond angle fl decreases, the model presented in the preceding paper 6 for the angular distribution of reactive scattering generated by precession of a bent long-lived com- plex 13 T(O) (2/r) arcsin (sin),/sin 0) for sin 0 > sin ), T(O) 1 for sin0<sin has progressively broader forward and backward peaking.However, the limit of an isotropic angular distribution with ), 90 is reached only when the preferred geometry of the complex has a symmetrical isosceles triangular configuration IOI.The composite angular function T(O) of Figure 4 shows OI scattering over the full angular range favouring the backward direction.If it is assumed that precessing complexes have an exponential distribution of lifetimes according to the osculating complex model, 14-17 the density of complexes declines over successive half periods of rotation.If it is also assumed that precession commences with a scattering angle 0 for complexes formed in small impact parameter collisions and that the angular distribution for a very long complex lifetime approximates to isotropic, then the angular distribution generated for a finite lifetime may be written T(O) e -('-/* + e -(,+)/* + e -(3r-)/0* + e -(3:r+)/* + 2 cosh (O/O*)(e -/* + e -3zt/O* + ...) Normalising to unity at 0 r yields where 0* 2rr/T and r denotes the complex lifetime and T the rotational period.The angular distribution corresponding to Eq. ( 4) with 0* 200 , shown by a broken curve in Figure 4 agrees quite well with the composite angular distribution shown by a solid curve, although the shape of the broken curve is concave upward compared with the convex shape of the experimental curve.This discrepancy in shape may well arise from the approximations invoked in deriving the simple limiting form of Eq. ( 4).In particular the discussion of back- ward scattering for O + Br2 in the preceding paper 6 indicates that the distribution of initial angles for complex precession in small impact parameter collisions may be closer to a cosine distribution than the single value 0 r used in deriving Eq. ( 4).The nominal lifetime r 0.56 T corresponding to the parameter 0* 200 indicates that OII complexes precess for roughly one half period of overall rotation of the principal axis of the complex which approximates to the axis of the heavy I-I diatomic.In the case of the O + Br2 reaction 6 the product translational energy distribution for scattering in the backward direc- tion was found to be well represented by the microcannonical RRHO prior distribution TM P(f') (33/2/2)f'1/2(1 f') (5) where F' E'/(E + ADo).The RRHO distribution shown by a broken curve in Figure 4, predicts rather higher product translational energy than that exhibited by the experimental distribution shown by a solid curve.
The rate constant k 8.4 x 101 dm 3 mo1-1 s -1 for the O + 12 19 reaction measured at 298 K corresponds to a total reaction cross section Q 25/2 which is considerably greater than that Q 2.5/2 for O + Br2.Indeed RRKM calculations 2 indicate that there is no energy barrier in the entrance valley of the potential energy surface for O + 12 but that reaction is still confined 8 to quite small impact parameters b 3 ]k.If bond lengths of r(OI) = 2.0 A and r(I2) ---2.8/ are adopted for an OII triplet complex with an interbond angle fl 120 , as has recently been proposed for the preferred geometry of the FII transition state in the F + 12 reaction, then a collision radius R 3/ is predicted by the collision complex model in the preceding paper. 6This preferred geometry for the OII triplet complex yields ), 36 which gives an angular function T(O) according to Eq. ( 3) with quite pronounced forward and backward peaking.Such an angular distribution is in accord with the experimental distributions deter- mined TM at lower initial translational energy but not with the more nearly isotropic distribution determined in the present experiments at higher initial translational energy.Similarly the product translational energy distribution determined TM at low initial translational energy peaks at a low product translational energy corresponding to the maximum centrifugal barrier 22 B'm (IS,/la,')E (6)   where/x,/z' denote the initial and final reduced masses.At the higher initial translational energy of the present experiments the product translational energy distribution for the major component peaks well above the centrifugal barrier Bh 10 kJ mo1-1 predicted by Eq. ( 6), but the minor component peaks at much lower energy.
The experimental data for O + I2 at high initial translational energy follow the short-lived complex model proposed in the preceding paper 6 where the complex lifetime is shorter than the period of overall rotation of the symmetry axis of the OII triplet complex which is approximately the I--I bond axis, but the lifetime is long compared with the period of O atom vibrational and rotational motion.However, the OII complex is more strongly bent than the OBrBr complex and the OI reactive scattering depends both on impact parameter and on the initial angle of inclination between the 12 molecule axis and the plane of collision.Consequently the angular distribution of reactive scattering is not so directly related to impact parameter as is the case for the O + Br2 scattering.Thus the product translational energy distribution for the principal component of the O + 12 scattering does not vary with scattering angle and peaks at an energy intermediate between the maximum centrifugal barrier and the prediction of the microcannonical RRHO distribution.The minor component of the O + I2 reactive scattering, which favours the forward hemisphere and has a very low product translational energy, may be related to the limiting configuration of large impact parameter collisions with the I2 molecule axis initially strongly inclined to the plane of collision as illustrated in Figure 5.In this configuration the component of the initial orbital angular momentum perpendicular to the I--I bond axis L sin ), gives a centrifugal barrier 23 B' B sinEy (7) which is much lower B' 3.5 kJ mo1-1 than the maximum centrifugal barrier B;n given by Eq. ( 6) for the in plane scattering and is compar- able to the product translational energy for the minor component of the OI reactive scattering given in the table.
The reaction dynamics for O + Br2 and O + I2 have been discussed in terms of motion over the triplet potential energy surface 24 but these surfaces are underlain by a singlet surface with a deep minimum corresponding to the symmetrical isosceles triangular configuration.2s Thus singlet character induced by strong spin-orbit interaction may contribute to the wavefunction of the potential energy surface for strongly bent configuration of the triplet complex shown in Figure 5.

L of collision
CoS'to Figure 5 Proposed triplet complex for the O + I2 reaction.Principal axes of the complex arc denoted, xl, x2 with x3 perpendicular to the plane of the complcx.The initial orbital angular momentum L is perpendicular to the plane of collision.The reactive scattering 5 arising from such a symmetrically bound singlet intermediate would necessarily exhibit symmetry about 0 90 and because of the long-range attractive interaction induced by the oxygen atom lying between the halogen atoms on the singlet surface, would give rise to in plane scattering with low product translational energy.However, there is little evidence for sharply forward-backward peaked scattering in the O + I2 reaction and for the O + Br2 reaction the observed sharp forward peaking may be attributed to a triplet intermediate which is not strongly bent. 6Hence the dynamics of the O + Br2 and O + I2 reactions appear to be confined mainly to the triplet potential energy surface, which must therefore be separated from the singlet surface by an energy barrier in strongly bent configurations.

Figure 1
Figure1Laboratory angular distribution (number density) of OI reactive scattering at an initial translational energy E 43 kJ mo1-1.The overall fit of the Entemann kinematic analysis shown by solid curve, the contributions of the separate components by broken curves.

Figure 4
Figure 4 Composite angular distribution and translational energy distribution for

Table I
Reaction energetics; initial translational energy E, peak E'pk and average product translational energies and reaction exoergicity ADo.