Exit-channel dynamics

M. Shapiro Prof. Neumark, in general, I was struck by the similarity between your data on the CH3O radical and the photodissociation of CH3I, which we analyzed many years ago. In particular, it is interesting to note that the degree of excitation of the umbrella mode is similar and appears to increase with increasing excitation of the parent molecule. Obviously the exit channel dynamics dominate here. 

The torque generated by the anisotropy of the upper-state potential energy surface, i.e., the so-called exit channel dynamics. 

As in Chapter 9 we discuss first the elastic limit (no exit channel excitation) in Section 10.1 and subsequently the more interesting inelastic case in Section 10.2. In Section 10.3 we consider the decay of long-lived resonance states and the impact of exit channel dynamics on the product distributions. A simple approximation, the so-called impulsive model, which is frequently employed to analyze experimental distributions in the absence of a PES, is discussed critically in Section 10.4. The chapter ends with a more qualitative assessment of thermal broadening of rotational state distributions in Section 10.5... 

This has very important consequences The alignment of v and j with respect to the space-fixed z-axis is diminished by overall rotation before the excited complex dissociates. A long lifetime destroys the alignment. The correlation of j with v, on the other hand, is not established until the bond breaks and the two fragments recoil such that rotation of the parent molecule prior to dissociation is irrelevant. It represents a new observable which can provide additional information about the bond rupture and the exit channel dynamics other than the final state distributions. 

MP2/6-3H-G direct dynamics was used to study the actual exit-channel dynamics for this reaction. Initial conditions for the trajectories were selected by sampling the [HO-CH3-F] central barrier s 300 K Boltzmann distribution. The simulations showed... 

Another important vector correlation in photodissociation is called v-j correlation, which describes the relationship between the recoil velocity and the angular momentum of one of the fragments. This correlation can give important indications about the kinds of torques in effect between the recoiling products during dissociation. These torques can arise from bending and torsional modes in the parent molecule and from exit channel dynamics. 

RRKM theory to dissociation reactions having no barrier to the reverse process of association. Accordingly one must, in the general case, allow for the influence of exit channel couplings in order to predict the properties of separated products based on the transition state distributions embodied in N E, J). Two ways to accomplish this in approximate fashion are outlined below. One models the effect of exit channel dynamics within the framework of the flexible transition states discussed in chapter 7 and the other handles the exit channel dynamics explicitly using classical dynamics. The former model is referred to as variational RRKM theory with exact channel couplings (VRRKM/ECC). 

Information of a different sort is obtained in a molecular beam experiment, although the means for producing the species undergoing unimolecular decomposition is also chemical activation. Whereas the conventional kinetic studies yield reaction rates for direct comparison with RRKM lifetimes, the beam technique yields product recoil energy distribution which, in principle, contain information regarding exit channel dynamics specifically ignored in RRKM. Comparison of experimental results with RRKM theory is indirect, requiring additional assumptions whose validity must be determined. Fortunately, however, statistical theories of a different sort exist which base their predictions on asymptotic (and therefore measureable) properties of the... 

Direct dynamics simulations based on a high level of electronic structure theory, may be performed to study chemical events that occur in a short time. Thus, though a large amount of compute time is required for each integration step, only a small number of integration steps are required. High-level direct dynamics is practical for simulating the exit-channel dynamics of a chemical reaction from the transition state to products, since this is usually a direct... 

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. 

The desorption flux is so low under these conditions that no gas phase collisions occurred between molecular desorption and LIF probing. Phase space treatments " of final-state distributions for dissociation processes where exit channel barriers do not complicate the ensuing dynamics often result in nominally thermal distributions. In the phase space treatment a loose transition state is assumed (e.g. one resembling the products) and the conserved quantities are total energy and angular momentum the probability of forming a particular flnal state of ( , J) is obtained by analyzing the number of ways to statistically distribute the available (E, J). 

The prototype potential surface invoked in chemical kinetics is a two-dimensional surface with a saddle equilibrium point and two exit channels at lower energies. The classical and quantal dynamics of such surfaces has been the object of many studies since the pioneering works by Wigner and Polanyi. Recent advances in nonlinear dynamical systems theory have provided powerful tools, such as the concepts of bifurcations and chaos, to investigate the classical dynamics from a new point of view and to perform the semiclassical... 

The final rotational state distributions of the products in the fragmentation of a polyatomic molecule contain additional clues about the intra- and intermolecular dynamics, especially about the coupling in the exit channel. In realistic as well as model studies it has been observed that the rotational state distributions of the photodissociation products reflect the angular dependence of the wave function at the transition state and the anisotropy of the PES in the exit channel [4, 9, 10]. HO2 is no exception. 

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