Exit channel interaction

D. M. Neumark We indeed take the translational energy distribution from CH3O dissociation to be evidence for exit channel interactions on a repulsive potential-energy surface. This is in contrast to photodissociation of the vinoxy radical, for which very little variation of the CH3 + CO translational energy distribution occurs over a 0.5-eV range of excitation energy. 

There is a much larger difference in OH/ .T excitations between bulk and complexed conditions for the case of NjO-HBr than with CO2-HBr. With C02-HBr, the finite lifetime of a short-lived intermediate such as HOCO can allow enough distance to develop between Br and HOCO before decomposition, so that Br—OH exit-channel interactions may be qualitatively different than in the N20-HBr system. 

Thus, both OH rotational and translational excitations are much lower with N2O-HX complexes than under bulk conditions, while the NH channel shows no such striking change. Why is there such a difference After all, like HO-X, the HN-X interaction is strongly attractive. However, for the case of HN-NO fragmentation, X-NO is also attractive, which is more favorable geometrically than HN-X once the H atom has shifted to the terminal nitrogen. Also, the HNNO lifetime may lead to sufficient HNNO -X distance at the point of fragmentation that HN-X exit channel interactions are lessened. No such unimolecular decomposition lifetime is anticipated for the OH + N2 channel. 

Recently, it has been discovered that insulator surfaces, such as LiF crystals, can exhibit substantially higher negative ion conversion efficiencies than metal surfaces for glancing scattering of atoms, including atomic oxygen (21). This is somewhat suiprising because the band gap of such materials is lO eV. The Ugh efficiency is attributed to the fact that the exit channel interaction now consists of a fuU Coulomb interaction, -... 

All chemical reactions begin as the reagents approach and end as the products separate. The system must traverse both the entrance channel and the exit channel. Our hope is that, by specifying the reagent orientation in the entrance channel and monitoring the appearance of ions in the exit channel, we may learn how orientation affects the transfer of an electron from one species to another. Strictly speaking, we cannot separate the electron transfer, presumed to occur in the entrance channel, from the process of the ions separating, which is in the exit channel. Under certain circumstances, one or the other of these interactions may dominate. We have thus interpreted the data under certain assumptions, but what we really learn is how the... 

Figure 15 Calculated total and state-to-state excitation transfer cross sections in the de-excitation of He(2 P)-Ne. (From Ref. 151.) Both electron exchange and dipole-dipole interactions are included in the coupling matrix elements. The threshold energy into each exit channel is shown on the upper axis.

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