Ionization cross section spectrometer

A particular strength of Equation (7) is that the intensity ratio is formed between mea-surements of the same X-ray energy in both the unknown and standard. This procedure has significant advant es First, there is no need to know the spectrometer s efficiency, a value that is very difficult to calibrate absolutely, since it appears as a multiplicative factor in both terms and therefore cancels. Second, an exact knowledge of the inner shell ionization cross section or fluorescence yields is not needed, since they also cancel in the ratio. 

A number of excellent reviews and books have included consideration of the fundamental electron impact ionization process, and the attention afforded the experimental measurement of ionization potentials and Augment ion appearance energies over the years is reflected in the comprehensive database of ionization potentials and gas phase ion enthalpies of formation published through the National Bureau of Standards in printed and electronic forms. In contrast, few absolute ionization cross sections have been measured. The most comprehensive compilation of molecular ionization cross sections are relative values measmed with a modified commercial electron impact mass spectrometer ion source using the cross section for Ar as a reference. ... 

Compounds were quantified by comparing the computer calculated area for the brominated compound with the integrated response for a known amount of octachlo-ronaphthalene. Differences in ionization cross-section, which affect the sensitivity of the mass spectrometer to a given compound, were compensated for by determining the relative molar response (RMR) of authentic compounds to octachloronaphthalene. 

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... 

A further note is that for purposes of mass identification and detection, the neutral clusters are ionized. The ionized distribution of cluster sizes may not faithfully represent the neutral size distribution in a one-to-one correspondence due to fragmentation upon ionization, ion stability, ionization cross-sections, and mass discrimination in the spectrometer. For most of the clusters described here which involve hydrogen bonding, ionization of the neutral cluster usually leads to a protonated cluster via a reaction typified by... 

LiF(g) which is consistent with the literature values listed in the table below. Due to lack of knowledge of the fragmentation patterns, the relative ionization cross sections of the ionic species formed in the mass spectrometer, and the molecular constants of the LiF polymers, the evaluation of equilibrium constants used to calculate A H" involves considerable uncertainty. In order to solve this problem more research work seems necessary. 

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. 

In the following sections of this article, we describe the principles of ionization cross-section measurements, including a brief description of the fast-beam apparatus and the high-resolution double-focusing mass spectrometer employed in the present studies. A comprehensive review of semiempirical calculations of total ionization cross sections is given. Comparisons between these calculated cross sections and the experimental results are presented. The decomposition of the various molecules in a low-temperature plasma is discussed on the basis of the measured ionization-cross-section data, and comparisons are made with the results of in situ plasma diagnostics studies using mass spectrometric techniques. 

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. 

In both the fast-beam apparatus and the double-focusing mass spectrometer, absolute cross sections can be determined with uncertainties of 15% for the parent ionization cross sections and 18% for the dissociative ionization cross sections. These error margins include statistical and all known systematic uncertainties and are typical for ionization-cross-section measurements carried out with this apparatus (Tamovsky and Becker, 1992 Tamovsky and Becker, 1993). 

Electron impact ionization of the parent molecule is only one of several important ion formation processes in nonthermal plasmas. Secondary processes such as electron impact ionization of neutral fragments produced by dissociation of the parent molecule and ion-molecule reactions are other mechanisms contributing to the formation of plasma ions. It is interesting to compare ion abundances in a realistic plasma with the ion abundances predicted from electron impact ionization cross sections measured under single-collision conditions. Although mass spectrometry of plasma ions is a known and well-developed diagnostic method (Osher, 1965 Drawln, 1968 Schmidt et al., 1999), its application to plasmas for thin-film deposition is not very common. The main reasons are deleterious effects of insulating deposits on the ion collection orifice (which connects the mass spectrometer to the plasma) and on the ion transfer optics, which render it... 

The El ionization cross section starts at zero at the ionization threshold, increases approximately linearly (Wannier, 1953), and finally peaks at an electron energy of about 70 eV. The latter is the standard electron energy used in mass spectrometers. The vanishing cross section at the ionization threshold is a particularly unfortunate property of electron ionization. 

Other Observations The assumption that azides decompose with partial release of nitrogen in the form of free atoms is confirmed by L vov [47] by direct mass-spectrometric analysis of the primary decomposition products of NaNs. Analysis of the initial mass-spectrometric patterns reported by Walker [49], taking into account partial dissociation of N2 molecules in the ionizing chamber of a quadrupole mass spectrometer, and the differences in the ionization cross sections of N and N2 and in the solid angles of the inlet of these species into the ionizing chamber, showed that the ion current ratio /(N+)//(Nj) in Walker s experiments [49] was 0.86, which is consistent with the estimate based on thermochemical data (Table 16.21). 

The central KEMS equation can be derived now that the Knudsen cell vapor source and mass spectrometer have been described. This follows directly from the vapor flux in the molecular beam selected from the distribution of material effusing from the Knudsen cell (molecular beam flux equation) and the definition of the ionization cross section (Equation 48.18). However, in accordance with the aim of identifying factors that affect the measured ion intensity and that are unrelated to sample temperature and composition, it useful to rewrite Equation 48.18 in terms of the number of ions produced per second in the elementary volume dv in the region defined by the intersection of the molecular and electron beams, ni(E) [71,80] (this is prior to the formation of the ion beam) ... 

The mass spectrometer is the only technique which allows the vapor composition to be determined, and this can be done rapidly and with great sensitivity. Beyond that, its usefulness for pressure measurements depends on how accurately the apparatus can be calibrated. In this regard, it has been common practice to use the known vapor pressure of Ag, Au, or sometimes Mo combined with the measured or calculated ionization cross sections for the various species. While this method and its variations can give satisfactory answers, a comparison with an absolute determination using the system of interest is better. For example, the pressure of many metallic elements is known or can be determined by the Langmuir technique. If the elements of the system of interest fall into this category. 

These range measurements were integral in nature. However, also in the early sixties, the first measurements pertaining to single collisions were published. Hansen et al. [2.10] and Hansen and Flammersfeld [2.11] used a p spectrometer to velocity-select electrons and positrons stemming from radioactive sources. With these particles, they measured the K-shell ionization cross section for a variety of heavy atoms. Due to the low projectile intensity, they had to use several days ( ) of data-collection time per point to obtain a reasonable accuracy. In general, the result was that no systematic difference between o-K(e ) and 0- (6 ) could be observed. However, as seen in fig. 2.1, where their results for an Ag target are shown, at the lowest projectile velocity, there is an indication that ) I discussed in section 4, additional, more accurate, experiments,... 

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatterings with the electrons of the material, and it s energy is continuously reduced as a frmction of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizii an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. 

In this connection investigations are to be mentioned in which a mass-spectrometric analysis has been made of neutral radicals, e.g., CHjCO, split off from acetone by u.v. photons in the ordinary range.27-28 In the first a flash lamp has been used and the radicals were ionized as usual by electron impact. In the second the same radical ionized at a field emission electrode. Recently, several alkyl radicals generated by pyrolysis have been studied. Their values of lv and of the photoionization cross sections could be obtained in the mass spectrometer under monochromatic vacuum u.v. irradiation.29... 

---