Electron impact ionization description

Because the semiclassical theories can be used to calculate differential cross sections with relative ease for close collisions between the incident charged particle and the bound electron, and the Bethe theory provides a straightforward method to describe low-energy electrons ejected in distant collisions, it is only natural to combine the best characteristics of the two approaches to derive a comprehensive description of electron impact ionization. 

The collinear model (Eq. (15)) has been successfully used in the semiclassical description of many bound and resonant states in the quantum mechanical spectrum of real helium [49-52] and plays an important role for the study of states of real helium in which both electrons are close to the continuum threshold [53, 54]. The quantum mechanical version of the spherical or s-wave model (Eq. (16)) describes the Isns bound states of real helium quite well [55]. The energy dependence of experimental total cross sections for electron impact ionization is reproduced qualitatively in the classical version of the s-wave model [56] and surprisingly well quantitatively in a quantum mechanical calculation [57]. The s-wave model is less realistic close to the break-up threshold = 0, where motion along the Wannier ridge, = T2, is important. 

Other topics that have been omitted include the description of scattering processes using Feynman path integrals [18,19] and the description of scattering processes with more than two coupled continua (i.e., where three or more independent particles are produced, as in electron impact ionization [30] or collision induced dissociation) [31]. Our treatment of resonance effects in scattering processes (i.e., the formation of metastable intermediate states) is very brief as this topic is commonly found in textbooks and one monograph is available... 

The first three methods we will discuss are based on variational principles— not minimum principles for the energy, but stationary principles for the scattering amplitude or some related quantity. While these methods are the most elaborate and computationally demanding, they are also potentially the most flexible and the most accurate, in that they make the fewest simplifications and approximations. More approximate methods are also in use, and descriptions can be found elsewhere (e.g., Huo and Gianturco, 1995). Because of its extraordinary utility, we will also briefly consider the method of Kim and Rudd (Kim and Rudd, 1994 Hwang etal, 1996) for obtaining electron impact ionization cross sections, which is based on a very simple model of the electron-target interaction. 

Improvements in experimental technique and analysis will no doubt continue to be made with most emphasis on valence band spectra, and perhaps involving the measurement of other parameters simultaneously with ionization energies. For example, electron impact ionization experiments on gas molecules with determination of the kinematics of incident and emitted electrons can provide information on electron binding energies and on the momentum distribution of valence electrons,< > allowing a very severe test of theoretical descriptions. 

A detailed description of sources used in atmospheric pressure ionization by electrospray or chemical ionization has been compiled.2 Atmospheric pressure has been used in a wide array of applications with electron impact, chemical ionization, pressure spray ionization (ionization when the electrode is below the threshold for corona discharge), electrospray ionization, and sonic spray ionization.3 Interferences potentially include overlap of ions of about the same mass-charge ratio, mobile-phase components, formation of adducts such as alkali metal ions, and suppression of ionization by substances more easily ionized than the analyte.4 A number of applications of mass spectroscopy are given in subsequent chapters. However, this section will serve as a brief synopsis, focusing on key techniques. 

Mass spectrometry (MS) in its various forms, and with various procedures for vaporization and ionization, contributes to the identification and characterization of complex species by their isotopomer pattern of the intact ions (usually cation) and by their fragmentation pattern. Upon ionization by the rough electron impact (El) the molecular peak often does not appear, in contrast to the more gentle field desorption (FD) or fast-atom bombardment (FAB) techniques. An even more gentle way is provided by the electrospray (ES) method, which allows all ionic species (optionally cationic or anionic) present in solution to be detected. Descriptions of ESMS and its application to selected problems are published 45-47 also a representative application of this method in a study of phosphine-mercury complexes in solution is reported.48... 

The theoretical descriptions of the ejected electron spectra for heavy ion impact are basically the same as that for electron impact discussed above, except that the theory is simplified for heavy ions because exchange forces are not an issue. One can write the equivalent of Eq. (17) for the binary encounter approximation to the single differential ionization cross sections for bare heavy ion impact [36] as... 

Some of the gaps in the database can be filled by routine (but tedious and time-consuming) calculations using simplified theoretical models (e.g., BGG-model for electron-impact excitation and ionization of H2(NA v) states, the more involved impact-parameter method for excitation, etc.) For some of above listed processes (such as those mentioned under (d) and (e)), however, it is necessary to develop appropriate theoretical models for description of their dynamics. 

Brief descriptions are given in the following of needed aspects of cross sections, molecular orbitals, and of the more recently devised Stieltjes orbitals that have proved useful in spectral studies. Examples of the use of the Stieltjes formalism In identifying Mulliken valence orbitals in the cross sections of diatomic and polyatomic compounds are reported next. Also indicated are more general aspects of such intravalence transitions as they relate to electron-impact resonances in selected cases. The Importance of dealing with both discrete and continuous spectral intervals on a common basis is emphasized throughout, particularly with reference to the clarification of the positionings of a-xj and tv-ht excitations in molecular photoabsorption and ionization cross sections. 

Electron impact (El) to produce ions in the gas phase with and without plasma formation is another type of positive ion source for SIMS instruments. Electrons are emitted by a cathode and accelerated to an anode where they ionize the gas introduced into the anode. A complete description of El sources is found in references [10, 11]. 

However, quantum chemistry is not restricted to the description of static (or adiabatic) phenomena. Collision dynamics, or the description of systems excited with femtosecond lasers, requires time-dependent approaches. Phenomena like photoionization, excitation, and ionization by electron impact, charge transfer processes [4,5], atomic scattering, and interstellar chemistry, also call for theoretical support. 

Excitation and ionization have a common origin-namely, raising the electronic level of an atom or a molecule from its ground state to a state of higher energy via the impact of charged particles or photons. Nevertheless, their chemical fates can be drastically different. In this chapter, we treat these phenomena descriptively. 

General Description of Method.— The gas is ionized by impact electrons emitted by a hot tungsten filament, and, by means of an electric field the positive ions formed are drawn through a narrow slit into a magnetic field, where they are resolved into constituents of different ratios of charge to mass by a method very similar to that employed by Dempster in his positive ray analysis. The ionization potential necessary to produce each ion is determined by gradually reducing the potential applied to the impact electrons until no trace of the particular ion can be detected. 

Figure 10 demonstrates the consequences of the three-body effect in energy loss. In these calculations only the contribution of ionization was considered. The marked difference with the results of the first-order Bom approach is seen at all A. This is just a consequence of the difference in the energy spectra shown in Fig. 9. The boundary angle for Born approach ( 0.5 mrad) is marked by a noticeable change in behavior of AE (A(p). As is seen in Fig. 10, the SCA approach strongly fails in description of energy loss at small scattering angles. The corresponding impact parameters are too large for electrons to be excited.

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