Electron impact ionization model

Pyrolysis mass spectroscopy was conducted with a Hewlett-Packard model 5985B gas chromatograph/quadrupole mass spectrometer, operated at sslO- Torr and 70eV electron-impact ionization energy. Samples were introduced into the mass spectrometer via a glass lined direct insertion probe (DIP). The samples were decomposed in the DIP to a nominal temperature of 300°C at a heating rate of 30°C/min. 

Theoretical models of the electron impact ionization process have focused on the calculation of the ionization cross section and its energy dependence they are divided into quantum, semiclassical and semiempirical. Methods for the calculation of the ionization cross section and experimental techniques developed for the measurement of absolute ionization cross sections will be described in more detail below. Cross sections calculated using the semiempirical additivity method developed by Deutsch and Mark (DM) and their coworkers,12-14 the binary-encounter-Bethe (BEB) method of Kim and Rudd,15 16 and the electrostatic model (EM) developed by Vallance, Harland, and Maclagan17,18 are compared to each other and to experimental data. 

A multitude of semiempirical and semiclassical theories have been developed to calculate electron impact ionization cross sections of atoms and atomic ions, with relatively few for the more complicated case of molecular electron impact ionization cross sections. One of the earlier treatments of molecular targets was that of Jain and Khare.38 Two of the more successful recent approaches are the method proposed by Deutsch and Mark and coworkers12-14 and the binary-encounter Bethe method developed by Kim and Rudd.15,16 The observation of a strong correlation between the maximum in the ionization efficiency curve and the polarizability of the target resulted in the semiempirical polarizability model which depends only on the polarizability, ionization potential, and maximum electron impact ionization cross section of the target molecule.39,40 These and other methods will be considered in detail below. 

This expression reproduces the experimentally measured ionization efficiency curves surprisingly well, considering the simplicity of the model on which it is based. There is a discontinuity in the function at the maximum (when X = Xmax) but this affects only a small region of the ionization efficiency curve, and satisfactory values of the cross section are still obtained over this region. A great advantage of this method is that it is very simple to apply, depending on only three parameters the molecular polarizability volume, the ionization potential, and the maximum electron impact ionization cross section. These can be measured or calculated values (from the ab initio EM method described above, for example). 

A DuPont model 21-492B GC/MS with an Incos data system was used to confirm the presence of thiazolidine and nitrosated thiazolidine. A 30 m x 0.24 mm id glass capillary column coated with SP-2250 was used for GC/MS analyses with a 30 1 split ratio (with the larger fraction vented to atmosphere through a charcoal trap). Electron impact ionization was used at 70 eV. The data system generated total ion current chromatograms and recorded mass spectra. 

The electron impact ionization spectrum is given in Figure 2. A LKB model 9000 mass spectrometer was used with ionization potential of 70eV and accelerating voltage of 3.5kv. The mass spectrum shows a molecular ion at 356 (m/e) with two relatively intense peaks at 341 and 233 (m/e). 

Bell et al. [33] proposed an analytical formula, widely known in the literature as the Belfast ionization (BELI) formula [34] that contains the dipole interaction term for the electron-impact ionization of atoms and ions. It has been applied to light atomic and ionic targets with species-dependent parameters. Godunov and Ivanov [34] applied the BELI formula to the El ionization of Ne 1 ions. Here also no generality as to parameters of the formula was provided regarding the species-dependent parameters. Moreover, the BELI formula does not make any allowance for relativistic effects. Haque et al. [35-38] have proposed a modification of this BELI model for evaluating the El K-, L-, and M-shell ionization cross sections of atoms. The relativistic and ionic effects are also incorporated in their modified BELI (MBELL) [35-38] model in addition to generalizing the species-independent... 

M.S. Pindzola, D. Mitnik, F. Robicheaux, T-matrix calculation for the electron-impact ionization of hydrogen in the Tempkin-poet model, Phys. Rev. A 62 (2000) 062718. 

Y.-K. Kim, M.E. Rudd, Binary-encounter-dipole model for electron-impact ionization, Phys. Rev. A 50 (1994) 3954. 

W.M. Huo, Convergent series representation for the generalized oscillator strength of electron-impact ionization and an improved binary-encounter-dipole model, Phys. Rev. A 64 (2001) 042719. 

Hydrocarbon molecules are abundant constituents of planetary atmospheres and major compounds in combustible gas mixtures and in fusion edge plasmas [7-11]. Methane is the simplest of these hydrocarbon molecules. Acetylene, C2H2, is the simplest hydrocarbon molecule that contains 2 carbon atoms. Thus absolute total and partial photon [24-27] and electron [15,28-34] ionization cross-sections and nascent fragment ion energy distributions [19,20,28,36-40] have been studied extensively for these molecules. For the deuterated methane molecule electron impact ionization and dissociative ionization cross-sections were determined for the CD (x=l—4) molecule and radicals applying a fast neutral beam technique [41]. Electron impact total ionization cross-sections have been determined also theoretically applying the BEB (Binary-Encounter-Bethe) model [42], the DM (Deutsch-Mark) method [43] and the JK (Jain-Khare) method [44], Partial electron impact ionization cross-sections were calculated for methane [45,46] as well as total electron impact cross-sections for various CH radicals [47]. The dissocia-... 

The collinear models are also useful close to the two-electron break-up threshold. In 1994 Rost was able to obtain the correct Wannier exponent by a semiclassical treatment of electron impact ionization of hydrogen, another important quantum problem which involves nonintegrable three-body dynamics (see also Rost (1995)). 

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. 

The direct knockout mechanism had earlier been evoked by Samson [36, 37], who pointed out that the variation of some double photoionization cross sections as a function of energy closely resembles the form of electron impact ionization cross sections of ions. This is, of course, the expected behavior if direct knockout dominates, which is expected in the energy range relatively near threshold. Further consideration of these model mechanisms is left for a later section where the role of initial state electron correlation is discussed. 

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. 

In this section, we briefly present a few examples that illustrate recent applications of some of the theoretical methods discussed above. The first example is electronically elastic scattering by nitrous oxide, N2O the second is electronically inelastic scattering by ethylene, C2H4. These examples have been chosen not because of any special relevance to plasma processing, but because it is possible in these cases to compare theoretical results obtained by different methods both to each other and to experimental data. We conclude the section with an example application of the BEB model to electron impact ionization of SFs-... 

This article describes recent advances in the experimental determination of electron impact ionization cross sections for silane (SiH4) its radicals, SiH. (x = 1 to 3) and the Si-organic molecules tetramethylsilane (TMS), Si(CH3)4 tetraethoxysilane (TEOS), Si(0-CH2-CH3)4 and hexamethyldisiloxane (HMDSO), (CH3)3-Si-0-Si-(CH3)3, which is one of the simplest siloxane compounds. These are model substances, and the results obtained for these species may be used in efforts to predict the ionization properties of other, more complex Si-organic molecules. The ionization cross sections of the stable compounds were measured using a high-resolution double-focusing mass spectrometer. The cross-section data for the radicals were obtained in a fast-neutral-beam apparatus. 

The total and selected partial electron impact ionization cross sections of TMS are shown in Fig. 10. Also shown in Fig. 10 are the calculated total single TMS ionization cross sections from the modified additivity rule and from the BEB model of Kim and coworkers (Ali et al., 1997). There is reasonably good agreement between the two calculated cross sections and between the calculated cross sections and the measured cross section of Basner et al. (1996) (at least for impact energies below about 80 eV). The cross section of McGinnis etal. (1995) is considerably smaller than the two calculated cross sections and the measured cross section of Basner et al. (1996). 

In preparative-scale (10 M 1-naphthol) experiments, an Ace-Hanovia 450-W medium-pressure mercury arc was used with a Pyrex filter sleeve and a magnetically stirred Ace water-cooled reaction vessel. In experiments involving rose bengal as a sensitizer, tungsten lamp illumination was used. Ultraviolet spectral changes were measured with a Perkin-Elmer model 552A spectrophotometer. For GC-MS analysis, a Hewlett-Packard 5985A instrument was used with a fused silica capillary column coated with a bonded nonpolar polymethyl silane phase introduced directly into the electron Impact ionization source. Compounds were tentatively identified by comparison to published spectra and confirmed where possible with authentic standard materials. 

Evaluated In a photoionization mass spectrometric study from the difference in the thresholds for PHJ from PH3 and PHJ from PHg [1 ]. This value is only 4 kJ/mol higher than the almost coincident values calculated with the 4th-order Moller-Plesset perturbation procedure [1, 8] and with the generalized valence bond model [9]. - From the electron impact ionization of PH3 and PHg. - From the fluorescence excitation of PH3 photolysis fragments. - From the appearance potential of PHJ in electron impact studies of PH3 and the ionization potential of PHg. - From the appearance potential of PH2 from PH3 (2.2,2.3 eV) and the electron affinity of the PH2 radical (1.25 eV, see p. 62) [5] for earlier results (D<326 kJ/mol), see [10]. - From the highest populated rotational-vibrational level of HF, which is produced in a hydrogen abstraction reaction of PH3 with F atoms in a flowing afterglow experiment [6] for earlier results, see [11]. - Literature value based on the upper limit D<326 kJ/mol. 

Mass spectral (MS) identification of the oxidation products was carried out on a HP Model 5971 series mass selective detector (MSD) interfaced to a HP Model 5890 gas chromatograph. Mass spectra were obtained by electron impact ionization at 70 eV and a source temperature of 250°C. The capillary colunm and GC conditions were the same as described above. 

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