Electron reaction cross-section

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. 

The total reaction cross-sections of the individual primary and secondary species were derived and are compared in Table III with reactivities determined from previous electron impact studies (10, 31). 

Subsequently, new PESs were constructed using ab initio calculations of the electronic energy for reactions in the CHj, BePE and NH t systems. In each case, classical mechanics was used to examine the apparent convergence of the reaction cross-sections as a function of the size of the data set. In each case convergence was demonstrated. 

Electron impact studies10 of the appearance potentials and reaction cross-sections for the decomposition of W(CO)6 have shown that this decomposition occurs by a cascade mechanism... 

In Equation 3, e and m are the impinging electron energy and mass, (e) is the reaction cross section, and / (e) is the electron energy distribution function. Of course, if an accurate expression for fie) and if electron collision cross sections for the various gas phase species present are known, k can be calculated. Unfortunately, such information is generally unavailable for the types of molecules used in plasma etching. 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

The photoionization of a molecule to yield an electron and an unfragmented ion may be considered to be the simplest of all photodissociation reactions, and therefore also one of the simplest of the radiationless processes in an isolated molecule. In addition, because the products are charged, a combination of mass spectrometric and photometric data yields information about photoionization reactions not now available for molecular fragmentation reactions. For example, the reaction cross sections for generation of specific charged products and the total photon absorption cross section may be measured and compared, thereby yielding the residual cross section corresponding to radiationless processes other than photoionization. From this information we can deduce some of the consequences of the competition between several radiationless processes in an isolated molecule. 

Harpoon reactions of alkaline metal atoms with halogen molecules in the gas phase seem to be the first instance of the observation of chemical electron transfer reactions at distances somewhat exceeding gas-kinetic diameters. Actually, as far back as 1932, Polanyi, while studying diffusion flames found for these reactions cross-sections of nR2, somewhat exceeding the gas-kinetic cross-sections [69]. Subsequently, more precise measurements which were carried out in the 1950s and 1960s with the help of the molecular beam method, confirmed the validity of this conclusion [70],... 

These reactions, called inverse (3 decay, were obtained by adding the antiparticle of the electron in the normal (3 decay equation to both sides of the reaction. When we did this we also canceled (or annihilated) the antiparticle/particle pair. Notice that other neutrino-induced reactions such as ve + n —> p+ + e do not conserve lepton number because an antilepton, ve, is converted into a lepton, e. Proving that this reaction does not take place, for example, would show that there is a difference between neutrinos and antineutrinos. One difficulty with studying these reactions is that the cross sections are extremely small, of order 10-19 bams, compared to typical nuclear reaction cross sections, of order 1 barn (10—24 cm2). 

Valuable insight, particularly with regard to the effects of electronic excitation on reaction cross sections and reaction dynamics, has also been achieved without accurate knowledge of the actual potential surfaces, through the use of molecular-orbital correlation diagrams. Adiabatic correlation rules for neutral reactions involving polyatomic intermediates were developed by Shuler 478 These were adapted and extended for ion-neutral interactions by Mahan and co-workers.192,45 479,480 Electronic-state correlation diagrams have been used to deduce the qualitative nature of the potential surfaces that control ion-neutral reaction dynamics. The dynamics of the reaction N+(H2,H)NH+ and in particular the different behavior of the N + (3P) and N + ( Z)) states,123 for example, have been rationalized from such considerations (see Fig. 62). In this case the... 

This alkalilike behavior of metastable noble-gas atoms effectively transforms the excitation energy of the metastable noble-gas atom into electronic energy of a rare-gas halide molecule with large reaction cross section. Because the electronically excited noble-gas halides have short radiative lifetimes and the ground-state noble-gas halides are not strongly bound, the process of formation of electronically excited noble-gas halides from metastable noble-gas atoms has been shown to be ideal for the operation of the electronic transition laser and has been successfully used in high-efficiency rare-gas halide lasers in recent years.21"23... 

Figure 7 is a graphic description of the kinetic energy required by a deuteron to produce D-D, D-T, and D-helium-3 nuclear reactions. The bottom of the chart depicts the required deuteron kinetic energy level in thousands of electron volts. The x-axis coordinate is labeled from 10° to 103 kilo-electronvolts. The y axis is labeled in terms of the nuclear reaction cross section. Three types of nuclear reaction curves are depicted. Note that each curve rises to a maximum and then decreases in value. The D-D curve is shown with its maximum value at about 1000 keV. Considering the use of a typical ion accelerator, electric potentials ranging from about 10 to 106 keV are used. 

As the positron energy is raised above the positronium formation threshold, EPs, the total cross section undergoes a conspicuous increase. Subsequent experimentation (see Chapter 4) has confirmed that much of this increase can be attributed to positronium formation via the reaction (1.12). Significant contributions also arise from target excitation and, more importantly, ionization above the respective thresholds (see Chapter 5). In marked contrast to the structure in aT(e+) associated with the opening of inelastic channels, the electron total cross section has a much smoother energy dependence, which can be attributed to the dominance of the elastic scattering cross section for this projectile. 

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