Exit channel effects

Although exit channel effects are capable of producing a range of non-Boltzmann population distributions, the wavelength dependence of the kinetic energy provides an indication that nonthermal activation is responsible for the fast component of the desorption signal. The activation mechanism responsible for this desorption process is not determined from these experiments, but will be re-addressed in section 4.6. 

As interesting as these special "exit-channel" effects are in their own right, it has been shown that because of a cancellation, they have no bearing on the MIF phenomenon [15]. We stress this point, since occasionally it is assumed in the literature that the special exit channel effect in the ratios is a key to understanding the MIF. Instead, the mass-independent effect of "scrambled" systems and the anomalously large mass-dependent effect for reactions of the type Q -F OO QOO QOO and QO -F O, have very different origins and are unrelated. Perhaps these remarks may seem paradoxical. The various rate constants for these "isotopically unscrambled" reactions can be used to compute the observables for the isotopically scrambled system, and so compute and 5 0. However, the detailed analysis [15] showed that there is much cancellation, summarized below, and that the theoretical expression for the MIF conditions is now simpler than would appear from fhe expression for the MIF in terms of the individual rate constants [15]. In particular, the zero-point energy effect, important for the individual isotope rate constants, disappears when the combination of them that determines the MIF is calculated. 

Another recent advance in electron-molecule resonances is their role in molecular autoionization and photoionization. Here they show up as an exit-channel effect. The increasing availability of synchrotron sources and the proliferation of high-resolution laser spectroscopic techniques are leading to expanded interest in these processes because of the necessity to interpret the resonance features for a greater variety of molecules of chemical interest. Electron-molecule shape resonances are also responsible for structure in inner-shell electron energy-loss spectra in the region around the core ionization threshold acting as a final-state interaction, the same resonances... 

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. 

Fig. 9.27 Effect of translation and vibration on collisions, when saddle point (x) is in the exit channel.

Flavone has been shown to be effective at 170 /iM concentration to inhibit HDAC-1 activity, and also to induce histone H3 acetylation in NB4 APL cell lines. Flavone s mechanism of HDAC inhibition is currently unknown, but it has been proposed that similar to the inhibitors of kinases, it might occupy the exit channel or an alternative, shallow binding site found in HDAC with possible effects of causing allosteric modulation. This blockage or allosteric modulation would promote inhibition. ... 

Independent support for these conclusions regarding the orientation effects in the exit channel is provided by the experiments of Kalamarides et al. " Reaction of K atoms in high Rydberg states with CF3I, K + CF3I -> + F + CFj, was studied... 

A very small torque in the exit channel implies a comparatively large effect of overall rotation of the parent molecule on final rotational state distributions. 

This suggests that the rotational excitation may be associated entirely with the extended transition state rather than the presence of a molecular well with a different orientation in the exit channel. In summary, N2 recombination from Cu(l 11) follows a very similar pattern of energy release to that of H2 on the same surface, albeit with a much larger energy release. Desorption is dominated by N2 repulsion from the surface with little evidence for inelastic effects. 

Some physical insight into how this cancellation arises is considered next. In particular, we comment on why the dramatic and large exit channel mass-dependent isotope effect observed [18] under "unscrambled" experimental conditions is not relevant when the experimental conditions are, instead, the usual ones... 

One of the first attempts to compare the effect of different forms of energy on gas-phase reactions was done in a very general way by John Polanyi [70, 73]. When the reaction barrier is in the entrance or the exit channel of the reaction, translational or vibrational energy, respectively, is more efficient at promoting the reaction. These rules, known as Polanyi s rules, are not linked specifically to electron-transfer reactions. On the contrary, they were derived without reference to a specific reaction. As an illustration of these rules in an electron-transfer reaction, vibrational excitation of HCl gives easier access than translation to a late barrier in the K-hHCl reaction [123, 124]. 

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