Excitation by electron impact

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

Vibrational excitation by electron impact of the background neutrals is an important process, because it is a major cause of energy loss for the electrons [reactions SVl (SiH4 stretching mode), SV2 (SIHt bending mode), and HV in Table II]. Moreover, the density of the vibrationally excited molecules has been reported to be important [211]. However, information about reaction coefficients of vibrationally excited molecules is scarce [192]. Here, only the vibrational excitation of SiHa and Ht is included [212, 213]. 

A further technique exists for the determination of triplet energy levels. This technique, called electron impact spectroscopy, involves the use of inelastic scattering of low-energy electrons by collision with molecules. The inelastic collisions of the electrons with the molecules result in transfer of the electron energy to the molecule and the consequent excitation of the latter. Unlike electronic excitation by photons, excitation by electron impact is subject to no spin selection rule. Thus transitions that are spin and/or orbitally forbidden for photon excitation are totally allowed for electron impact excitation. 

A similar conclusion can be drawn from the interaction of metastable nitrogen molecules (the state) with these same surfaces. The relative excitation cross section (the excitation function) for this state is shown in Fig. 36 (see ref. ). Direct excitation by electron impact has a threshold at approximately 6eV and has a maximum at slightly higher energies. De-excitation from the etc. 

Scattering studies with metastable atoms are in many cases easier (and less expensive) than experiments with ground-state atoms, The discussion that follows is mainly concerned with helium, as most of the information is available for this atom. Figure 2 shows a skeletal setup of the experiment. A helium beam from a supersonic nozzle source is excited by electron impact to its two metastable states. The singlet state can be quenched by the 2g radiation from a helium-gas discharge lamp ... 

Purely optical excitation is possible for alkali and alkaline earth atoms. For most other atoms the transition from the ground state to any other level is at too short a wavelength to be useful. To produce Rydberg states of such atoms a combination of collisional and optical excitation is quite effective. A good example is the study of the Rydberg states of Xe by Stebbings et al.24 As shown in Fig. 3.5, a thermal beam of Xe atoms is excited by electron impact, and a reasonable fraction of the excited atoms is left in the metastable state. Downstream from the electron excitation the atoms in the metastable state are excited to a Rydberg state by pulsed dye laser excitation. 

Thiimmel, H.T., Grimm-Bosbach, T., Nesbet, R.K. and Peyerimhoff, S.D. (1995). Rovibrational excitation by electron impact, in Computational Methods for Electron-Molecule Collisions, eds. W.M. Huo and F. Gianturco (Plenum,... 

As shown by Schulz [152,153], 0 may also be excited by electron impact. The cross sections of Schulz have been integrated over a Boltzmann distribution by Ali [78], whose results are displayed in Figure 6.7. 

Figure 6.7 The rate coefficient of 02 vibrational excitation by electron impact as reported by A. W. Ali [78].

Rate Coefficients of Vibrational Excitation by Electron Impact Semi-Empirical Fridman Approximation... 

Vibrational excitation by electron impact is preferably a one-quantum process. Nevertheless, multi-quantum-vibrational excitation is also important. Rate coefficients ev(i i, V2)... 

Table 2-17. Parameters of Multi-Quantum-Vibrational Excitation by Electron Impact...

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... 

Vibrational distributions in non-equilibrium plasma are mostly controlled by W-exchange and VT-relaxation processes, while excitation by electron impact, chemical reactions, radiation, and so on determine averaged energy balance and temperatures. At steady state, the Fokker-Planck kinetic equation (3-116) gives J(E) = const. At E oo = 0,... 

The vibrational energy balance in a plasma-chemical process can be illnstrated by the following simplified one-component equation taking into account vibrational excitation by electron impact, VT relaxation, and chemical reaction (Rnsanov Fridman, 1984 Fridman Kennedy, 2004) ... 

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 ... 

Eqnations (5-29)-(5-32) take into acconnt eV processes of vibrational excitation by electron impact (energy transfer rates Wgy, Wgy, w y) W -exchange between asymmetric and symmetric CO2 modes (energy transfer rates Wyy, Wyy,) as well as between CO2... 

CO2 dissociation. Vibratiorral excitation by electron impact is faster than VT relaxation, and the CO2 dissociation process is energy effective if the ionization degree exceeds its critical value ... 

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