Slow electrons vibrational excitation

For the benzene derivatives here considered no dissociated methyl or amino radicals could be found in the mass spectra, either under the action of the photons alone, or in conjunction with a bombardment with slow electrons. As mentioned above this method of combined excitation has been attempted for the detection of neutral radicals produced under illumination.26 The fact that no such radicals have been detected for the compounds studied here, would mean, as generally assumed, that the benzene ring is an efficient energy sink, degrading the excess of vibrational energy imparted by the photon to the linkage between the substituent and ring. 

In those cases when no sharp groups of slow electrons are observed, the unselective deformation of the distribution curve can be ascribed to the excitation of intramolecular and intermolecnlar vibrations in the lattice. The appearance of sharp groups of slow electrons is to be ascribed to the electronic excitation of the molecular ions formed, as was the case for the photoionization of similar compounds in the gas phase, dealt with in the previous sections. As recently found, energy losses of 0.3-0.4 e.v. may be due to the excitation of electrons from traps. 

The mechanism of the photoelectron emission is evidently the same as in the hydrocarbons. The loss in sharpness of the maxima and a relative abundance of slow electrons may have an explanation, if the sequence of the molecular orbitals in these pigments is more narrowly spaced than are those of the hydrocarbons. Possibly, there is an excitation of molecular vibrations. 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

Theoretical calculations of the dissociative recombination of Hj using the same approach as for the predissociation problem give cross sections that are smaller than all experimental results given in Table 1. However, these are only restricted, two-dimensional theoretical results, and cannot be used to draw definitive conclusions. Three-dimensional calculations, including all relevant electronic states, will be needed in the future. What seems clear at this point is that direct dissociative recombination, (X Ui) + e H3 (X E ) H + H + H or H + H2, is very slow. " For HeH" " it was found that the indirect mechanism, which involves vibrationally excited Rydberg states as intermediates, enhances the cross section.According to the preliminary calculations, the effect of the indirect mechanism is even more pronounced for Hj. Model calculations have shown that channel mixing effects can enhance the DR cross section substantially when the indirect mechanism is taken into... 

This relation (Landau Teller, 1936) demonstrates the adiabatic behavior of vibrational relaxation. Usually the Massey parameter at low gas temperatures is high for molecular vibration cox ox k which explains the adiabatic behavior and results in the exponentially slow vibrational energy transfer during the VT relaxation During the adiabatic collision, a molecule has enough time for mai vibrations and the oscillator can actually be considered stractureless, which explains such a low level of energy transfer. An exponentially slow adiabatic VT relaxation and intensive vibrational excitation by electron impact result in the unique role of vibrational excitation in plasma chemistry. Molectrlar vibrations for gases... 

This mechanism is the most effective channel for CO2 dissociation in plasma. First of all, the major portion of the discharge energy is transferred from plasma electrons to CO2 vibration at electron temperature typical for non-thermal discharges (7 1 eV) (see Fig. 5-5). The rate coefficient of CO2 vibrational excitation by electron impact in this case reaches maximum values of about ev = 1-3 x 10 cm /s. Vibrational energy losses through vibrational-translational (VT) relaxation at the same time are mostly related to symmetric vibrational modes and they are relatively slow ... 

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