Exit channels

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). 

The high energy behavior of the cross-section for this reaction— i.e., its rapid fall-off—can be explained by assuming that different exit channels compete on the basis of available phase space (24), The low energy behavior presents a greater mystery. Further possible reasons for this behavior are discussed below. 

Figure 1.5. Femtosecond spectroscopy of bimolecular collisions. The cartoon shown in (a illustrates how pump and probe pulses initiate and monitor the progress of H + COj->[HO. .. CO]->OH + CO collisions. The huild-up of OH product is recorded via the intensity of fluorescence excited hy the prohe laser as a function of pump-prohe time delay, as presented in (h). Potential energy curves governing the collision between excited Na atoms and Hj are given in (c) these show how the Na + H collision can proceed along two possible exit channels, leading either to formation of NaH + H or to Na + H by collisional energy exchange.

The problem of controlling the outcome of photodissociation processes has been considered by many authors [63, 79-87]. The basic theory is derived in detail in Appendix B. Our set objective in this application is to maximize the flux of dissociation products in a chosen exit channel or final quantum state. The theory differs from that set out in Appendix A in that the final state is a continuum or dissociative state and that there is a continuous range of possible energies (i.e., quantum states) available to the system. The equations derived for this case are... 

Reactions without wells can also exhibit multiple pathways due to deviation from the MEP. While many trajectories may follow the MEP over a saddle point, alternative pathways arise when forces on the PES steer away from the saddle point, typically into relatively flat regions of the PES, before finding an additional path to the same exit channel. The roaming mechanisms recently elucidated in the photodissociation of formaldehyde and acetaldehyde, and the reaction of CH3 + O, are examples of this phenomenon, and are discussed in Section V. 

Reactions occurring on two (or more) electronic states can lead to the same product asymptote. These pathways may occur if more than one electronic state correlates adiabatically to the same asymptote (e.g., single or triplet coupling of two approaching species), or if nonadiabatic transition(s) move population from one state to another. Here, I make the distinction that products of the same structural formula do not represent the same exit channel if they are produced in different electronic states. For example, in the reaction... 

Figure 4. Schematic potential energy surface for the reaction of FeO" " with methane. The sohd line indicates the sextet surface the quartet surface is shown with a dotted line, in each case leading to the production of Fe + CH3OH. The dashed line leads to formation of FeOET + CH3. The pathway leading to the minor FeCH2" + H2O channel is not shown. Schematic structures are shown for the three minima the [OFe CHJ entrance channel complex, [HO—Fe—CH3] insertion intermediate, and Fe" (CH30H) exit channel complex. See text for details on the calculations on which the potential energy surface is based.

Based on this physical view of the reaction dynamics, a very broad class of models can be constructed that yield qualitatively similar oscillations of the reaction probabilities. As shown in Fig. 40(b), a model based on Eckart barriers and constant non-adiabatic coupling to mimic H + D2, yields out-of-phase oscillations in Pr(0,0 — 0,j E) analogous to those observed in the full quantum scattering calculation. Note, however, that if the recoupling in the exit-channel is omitted (as shown in Fig. 40(b) with dashed lines) then oscillations disappear and Pr exhibits simple steps at the QBS energies. As the occurrence of the oscillation is quite insensitive to the details of the model, the interference of pathways through the network of QBS seems to provide a robust mechanism for the oscillating reaction probabilities. 

Among the early second-row transition metals, the chemistry is richest for yttrium. In our early studies of Y + C2H2123 and C2H6,124 several competing product channels were observed, while only H2 elimination was observed for Zr, Nb, or Mo.122 Likewise, in reactions with larger molecules (containing three or more carbons for example), yttrium always led to the greatest number of unique product channels. A likely cause for this result is the presence of an exit-channel barrier for H2 elimination in reactions with Y. Because H2 elimination is usually the most thermodynamically... 

The reaction of C(3P) with acetylene also exhibits chemical branching, with competing exit channels ... 

Herbst and Dunbar" have investigated the effects of exit channel barriers on association reactions of type 43 and have shown that, depending on the size of the barrier, the efficiency of radiative association reactions as a function of N can be strongly curtailed. For example, at 10 K and a nonpolar neutral reactant, they found for a system with a well depth of 2 eV and an exothermic channel barrier of 1.0 eV, N = 130 atoms for 100% sticking efficiency, approximately 10 times the corresponding value of N in the absence of a competitive exothermic channel. 

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