Partial ionization cross sections

In order to measure cross sections, a beam of electrons of known energy is directed through a gas sample of known pressure and the resulting ion and electron currents measured.63 If mass selective ion detection is used, then partial ionization cross sections oz may be determined. These cross sections correspond to the production of z electrons and an ion or ions having total charge +ze. Some instruments allow the counting cross section oc, also known as the ion production cross section, to be determined ... 

Since the summation method allows absolute partial ionization cross sections to be determined, it is straightforward to extract the counting or ion production cross section from the data. Since this is the quantity which is given by many of the current... 

At energies higher than 10 keV, ionization cross sections were calculated using Seltzer s formula [183]. Seltzer s formula gives the partial ionization cross sections for five molecular orbitals of water. The total ionization cross sections in the energy range of 10 keV to 10 MeV were obtained by summing up all the partial cross sections (Fig. 8). 

FIGURE 3. Total ionization cross section of diethylzinc and partial ionization cross sections for the production of the Zn-containing ions. Reproduced by permission of Elsevier from Reference 27... 

In this section we discuss the more important experimental results for continuum oscillator strengths measured by electron spectroscopy that have been reported up to mid 1978. The discussion is divided on the basis of target species rather than the type of experiment since this stresses the interrelation and complementary nature of many of the experiments. As the experimental work is far from complete in many cases, only a limited picture of the overall breakdown processes is available at present. In particular, a very limited amount of work has been reported for inner shells. More data are generally available for mass fragmentation (photoionization mass spectrometry) than for partial ionization cross sections (photoelectron spectroscopy). 

The 2s partial ionization cross section for neon has been studied from threshold out to about 130 eV by Codling et al.131 using synchrotron radiation and two types of photoelectron spectrometer (127° and CMA). The results are more closely represented by R-matrix,128 or RPAE,55 theory than by the Hartree-Fock calculations.132 Wuilleumier and Krause137 have studied Ne(2s) ionization with a few soft-X-ray lines, and these results, together with a single measurement near threshold by Samson and Gardner136 are compatible with the synchrotron measurements.131... 

The perfect agreement to the partial cross-sections that were performed with instruments that were specially designed for cross-section measurements for fragment ions that have initial kinetic energies larger than 5eV clearly demonstrates the success of the present correction method. Furthermore, this work clearly demonstrates that commercial instruments can be applied for the determination of partial ionization cross-sections. As in the studies by Stebbings and co-workers [14] and Tian and Vidal [15] the present method... 

The determination of the total ionization cross section of a molecule requires in principle a careful measurement of all quantities in Eq. (2). Partial ionization cross sections can be obtained if the detection of the ion current is restricted to a particular product ion. In the latter case, a mass selective device—e.g., a mass spectrometer—has to be employed. The most commonly used mass spectrometers for this purpose are magnetic, radio-frequency, quadrupole, and time-of-flight mass spectrometers. In all cases, the detection sensitivity of the instrument may vary with the mass of the detected ions and must be known accurately. 

A detailed description of the fast-beam apparatus and of the experimental procedure employed in the determination of absolute partial ionization cross sections has been given in previous publications (Wetzel et al, 1987 Freund et al., 1990 Tamovsky and Becker, 1992 Tamovsky and Becker, 1993). For the measurements of the cross sections of silane radicals, a dc discharge biased at typically 2 to 3 kV through SiD4 served as the primary ion source. Deuterated rather than protonated target species were used in these studies to facilitate a... 

Measured Appearance Energies and Partial Ionization Cross Sections at 70 eV Impact Energy for the Various Fragment Ions Produced by Dissociative Ionization OF Silane... 

Measured Appearance Energies and Partial Ionization Cross Sections at 70 eV for Various Fragment Ions Produced by Dissociative Electron Impact Ionization of TMS. The Fragment Ions are Arranged According to Their Decomposition Route, H loss by CH Decay, Removal of A Complete Methyl Group, and H Transfer from CH3 Groups, Respectively... 

Fig. 11. Absolute ionization cross section of TEOS as a function of electron energy. The circles ( ) refer to the total ionization cross section of Basner et at. (1999), and the dotted line represents the total ionization cross section of Holtgrave el al. (1993), The inverted triangles ( ) denote the cross section obtained by Basner el al. (1999) for only those ions that were measured by Holtgrave et al. (1993). Also shown is a calculated cross section using the modified additivity rule (dashed line). Several partial ionization cross sections trom Basner et al. (1999) are also shown diamonds ( ), m/z — 193 crosses (x), m/z = 149 triangles (A), m/z = 163 and stars ( ), m/z = 208.

Partial ionization cross sections for the formation of the ion dXm/z = 147, for the ion and the total single HMDSO ionization cross section are presented in Fig. 12. The cross section for the dominant ion (m/z = 147) reaches a... 

Fig. 14. Comparison of mass spectrometrically obtained ion abundances in TMS-, TEOS-, and HMDSO-containing rf discharges (open bars) with calculated ion foimation rates using measured partial ionization cross sections and a Maxwellian enetgy distribution of the plasma electrons corresponding to an average energy of 3 eV (full bars).

In this equation, N is the number of atoms present in the analytical volume, / is the intensity above background of the relevant K edges, and is the partial ionization cross-section. This relationship applies specifically to K edges, but similar expressions have been derived for L and M edges. This equation is analogous to that discussed previously for XEDS quantification, in that the concentration of the two elements is related directly to the ratio of the intensities adjusted by a sensitivity factor, which, in this case is the ratio of the two partial ionization cross-sections (a). 

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