Electrons impact excitation

In this set of elementary reactions, k,-, k, and k represent the specific reaction rates for electron-impact excitation, electron-impact ionization, homonuclear associative ionization, and radiative decay of the reactive state X to an unreactive one X respectively. It is assumed in the treatment that only one excited state is involved. 

There are significant differences between tliese two types of reactions as far as how they are treated experimentally and theoretically. Photodissociation typically involves excitation to an excited electronic state, whereas bimolecular reactions often occur on the ground-state potential energy surface for a reaction. In addition, the initial conditions are very different. In bimolecular collisions one has no control over the reactant orbital angular momentum (impact parameter), whereas m photodissociation one can start with cold molecules with total angular momentum 0. Nonetheless, many theoretical constructs and experimental methods can be applied to both types of reactions, and from the point of view of this chapter their similarities are more important than their differences. 

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

The second excitation mechanism, impact scattering, involves a short range interaction between the electron and the molecule (put simply, a collision) which scatters the electrons over a wide range of angles. The usefiil feature of impact scattering is that all vibrations may be excited and not only the dipole active ones. As in Raman spectroscopy, the electron may also take an amount of energy hv away from excited molecules and leave the surface with an energy equal to Eq + hv. 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

Electrons and photons do not impact molecules or atoms. They interact with them in ways that result in various electronic excitations, including ionization. For this reason it is recommended that the terms electron impact and photon impact be avoided. 

Ozone can be destroyed thermally, by electron impact, by reaction with oxygen atoms, and by reaction with electronically and vibrationaHy excited oxygen molecules (90). Rate constants for these reactions are given ia References 11 and 93. Processes involving ions such as 0/, 0/, 0 , 0 , and 0/ are of minor importance. The reaction O3 + 0( P) — 2 O2, is exothermic and can contribute significantly to heat evolution. Efftcientiy cooled ozone generators with typical short residence times (seconds) can operate near ambient temperature where thermal decomposition is small. 

In two other implementations of electron impact SNMS, a plasma is generated in the ionizer volume to provide an electron gas sufFiciendy dense and energetic for efficient postionization (Figure 2c). In one instrument, the electrons are a component of a low-pressure radiofrequency (RF) plasma in Ar, and in the second, the plasma is an electron beam excited plasma, also in Ar. The latter type of electron-gas SNMS is still in the developmental stages, while the former has been incorporated into commercial instmmentation. 

Table I. Distribution of Excited H2+ Produced by Franck-Condon Electron Impact Processes with 50-Volt Ionizing Electrons...

Ne is metastable neon produced by electron impact. Ne transfers its excitation to hydrogen molecules. The hydrogen molecules participating in these energy transfer collisions are produced in highly excited preionized states which ionize after a time lag sufficient to permit the initial neon and hydrogen collision partners to separate. The hydrogen ion is formed in the v = 5 or 6 quantum states and reacts with a second neon... 

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

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. 

A review of the early experimental works can be found in references [56-58]. More recently, Chutjian recorded the electron-impact excitation spectrum of formaldehyde [59,60] and reported transition energies that are taken as reference values in many other works. So are the experimental values compiled by Robin [61]. 

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]. 

FIG. 15. Electron impact reaction rales as a function of the average electron energy. A I 1 mixture of SiHa and H2 was used, at a total pressure of 83 Pa. (a) Reaction rates for SiHa, (b) reaction rates for Si2H6 (dotted lines) and H2 (solid lines). Abbreviations are ion. ionization dis, dissociation vib, vibrational excitation att, attachment. See Table II for details and references. (Adapted from G. J. Nienhuis, Ph.D. Thesis. Universileit Utrecht. Utrecht, the Netherlands. 1998.)... 

The power dissipated at two different frequencies has been calculated for all reactions and compared with the energy loss to the walls. It is shown that at 65 MHz the fraction of power lost to the boundary decreases by a large amount compared to the situation at 13.56 MHz [224]. In contrast, the power dissipated by electron impact collision increases from nearly 47% to more than 71%, of which vibrational excitation increases by a factor of 2, dissociation increases by 45%, and ionization stays approximately the same, in agreement with the product of the ionization probability per electron, the electron density, and the ion flux, as shown before. The vibrational excitation energy thresholds (0.11 and 0.27 eV) are much smaller than the dissociation (8.3 eV) and ionization (13 eV) ones, and the vibrational excitation cross sections are large too. The reaction rate of processes with a low energy threshold therefore increases more than those with a high threshold. 

Optical emission is a result of electron impact excitation or dissociation, or ion impact. As an example, the SiH radical is formed by electron impact on silane, which yields an excited or superexcited silane molecule (e + SiHa SiH -t-e ). The excess energy in SiH is released into the fragments SiH SiH -I-H2 + H. The excited SiH fragments spontaneously release their excess energy by emitting a photon at a wavelength around 414 nm. the bluish color of the silane discharge. In addition, the emission lines from Si. H, and H have also been observed at 288, 656, and 602 nm, respectively. 

Matsuda and Hata [287] have argued that the species that are detectable using OES only form a very small part (<0.1%) of the total amount of species present in typical silane deposition conditions. From the emission intensities of Si and SiH the number density of these excited states was estimated to be between 10 and 10 cm", on the basis of their optical transition probabilities. These values are much lower than radical densities. lO " cm . Hence, these species are not considered to partake in the deposition. However, a clear correlation between the emission intensity of Si and SiH and the deposition rate has been observed [288]. From this it can be concluded that the emission intensity of Si and SiH is proportional to the concentration of deposition precursors. As the Si and SiH excited species are generated via a one-electron impact process, the deposition precursors are also generated via that process [123]. Hence, for the characterization of deposition, discharge information from OES experiments can be used when these common generation mechanisms exist [286]. 

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