Direct reaction collision trajectory

The curve marked ion-dipole is based on the classical cross-section corresponding to trajectories which lead to intimate encounters (9, 13). The measured cross-sections differ more dramatically from the predictions of this theory than previously measured cross-sections for exothermic reactions (7). The fast fall-off of the cross-section at high energy is quite close to the theoretical prediction (E 5 5) (2) based on the assumption of a direct, impulsive collision and calculation of the probability that two particles out of three will stick together. The meaning of this is not clear, however, since neither the relative masses of the particles nor the energy is consistent with this theoretical assumption. This behavior is, however, probably understandable in terms of competition of different exit channels on the basis of available phase space (24). 

Figure 5.13 Plot of a collision trajectory for a reactive coiiision of the "direct" type (the time scale is that for the H + H2 reaction). The collision is direct because of the fast switchover between the old and new bonds. Note also the vibration of the reactant BC molecule prior to the encounter with the reactant atom and the low-amplitude oscillation of the newly formed AB bond, indicating only a minimal product vibrational excitation. To interpret the dynamics recall that the slope of a plot of distance vs. time is the velocity. From Newton s first law, a change in the slope in such a plot indicates that a force is acting.

A perturbation-trajectory method for determining the dynamics of gas-surface collision processes was tested on the collision and subsequent surface reactions of SiH2 on a Si(lll) surface233. The predictions of an exact classical trajectory calculation234 were confirmed the sticking probabilities were unity at all temperatures, and it was found that surface SiH2 can decompose by direct elimination of H2 or by successive dissociation of Si—H bonds. 

Quasiclassical direct dynamics trajectories at the various levels of theory were later calculated to study the central barrier dynamics for the C1 I CH3C1, Cr + C2H5C1, C1- + CH3I, F +CH3C1, OH +CH3C1, and other Sn2 reactions.31,32,47,97 108 The effect of initial reaction conditions, such as energy injection, substrate orientations, and the mode of collision, on the fate of the reaction, product, and energy distribution, was analyzed. Some of these trajectory calculations required serious modification in RRKM and TST for... 

The direct model is characterized by simple, smooth trajectories with little waste motion . For the reaction A+BC - AB+C, B moves from C to A and the reactants separate within a vibrational period. The outcome of the collision depends on the details of the initial conditions, not just on the total energy and angular momentum. 

In principle, we can evaluate Os if by direct counting of collision and reflection events from the molecidar trajectory data for both cases, we observed sufficient number (about 200) of collision events. To estimate Os,.if quantitatively, we have developed an autocorrelation function method. The result is Oseif —90% and there is no qualitative difference among various types of fluids, which suggests that the condensation as a unimolecular chemical reaction is a barricrless process. Calcidation of the local chemical potential also supports this barrieijess picture. ... 

Jaffe and Anderson explored the eno gy requirements for reaction ( -88b) by looking for the products of reaction when acoeteated beams of HI were directed into a long chamber containing DI at low pressure. The mean collision i gy was thereby varied in the range 84—456 kJ mol" but in no case could HD be detected in the products. This result indicated that for reaction to proceed some vibrational - or possibly rotational - excitation of the reactants is necessary. These reactions have also been the subject of theoretical studies - using classical trajectories, although the conclusions to be drawn from these calculations are a matter of some controversy. "... 

A further parameter of the collision is the impact parameter, which defines the orbital angular momentum of the collisioa Ideally, one would like to obtain a set of functions P ml iJ E, b), akin to excitation functions, for which all parameters except b, the impact parameter, are held constant and b is systematically varied. The function is a type of excitation function for which a, the cross section, is replaced by P, a reaction probability per collision or a collision efficiency for reaction. Theoretical predictions for such functions come from trajectory calculations and it would thus be desirable to have data for direct comparison. In practice, of course, the trajectory values of P ml ij E, b) have to be suitably averaged over the whole range of b for comparison with an experimental reactive cross... 

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