Electron ionization efficiency

Figure 7-7. (a) Electron ionization efficiency curve for CH3OH+ ion (m/z = 32) from an Ar/MeOH expansion. Arrow indicates energy corresponding to the first excited 4s state of Ar (11.55 eV). (b) Electron ionization efficiency curve for CH3OH+ ion (m/z = 32) from an He/MeOH expansion. Arrow indicates onset of ionization (10.8 eV). Reprinted with permission from Vaidyanathan et al. 1991b. Copyright 1991 American Institute of Physics. 

Figure 7-10. Electron ionization efficiency curves of Ar3+ ion in a neat argon expansion at different stagnation pressures (a) 1.2, (b) 2.0, (c) 2.5, and (d) 3.0 atm. Reprinted with permission from Vaidyanathan et al. 1992. Copyright 1992 American Chemical Society.

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

Ionization efficiency. The ratio of the number of ions formed to the number of electrons, photons, or particles that are used to produce ionization... 

The energy of the electron beam (the potential difference between filament g and the area of impact) is adjusted to 70 ev for maximum ionization efficiency, but it can be varied by appropriate choice of potential difference between the heated filament g and anode h ... 

Fig. 4. Electron-impact efficiency curves at m/e = 32 and rri/e = 16 for a supersonic beam of O2. Arrows indicate the literature values72 of the ionization energy of O2 and of the appearance energy of 0+.

Figure 1. Sketch of the ionization efficiency curves and the "70 eV" mass spectrum for the electron impact ionization of carbon monoxide.

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. 

The interaction will become weaker as the electron wavelength becomes greater or less than the molecular diameter with a consequential decrease in the cross section. This leads to an expression for the ionization probability as a function of electron energy, giving the shape of the ionization efficiency curve,... 

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). 

None of the three theories used to calculate electron impact ionization cross sections could be considered to render the others obsolete. The BEB method gives the best fit to the functional form of the ionization efficiency curve for small molecules, it provides a better fit to the experimental data closer to the ionization threshold than the other methods, but it underestimates the maximum ionization cross sections for heavier molecules. The DM method provides a better fit to the ionization efficiency curves for the heavier molecules, especially for electron energies greater than max, but it tends to overestimate the cross sections for heavier molecules and it underestimates E for lighter molecules. The EM method performs as well as the other methods for the value of amax for the light molecules but underestimates the cross sections for heavy molecules by a factor similar to the overestimation of the DM method. The polarizability method outperforms the BEB and the DM methods for the calculation of and when combined with the value from the EM calculation reproduces the ionization efficiency curve as well as the BEB method. 

Another method for finding the W value, called the Fowler equation approach (Inokuti, 1975), is based on three assumptions, some of which can be relaxed. These are (1) that the incident particle is an electron (2) that there is only one ionization potential and (3) that the ionization efficiency is unity— that is, any energy loss E > I results in an ionization with a primary of energy... 

Fig. 1.32. Ionization efficiencies of electrons for some gases as a function of electron energy [16]. 

Fio. 7. Ionization efficiency curve for oxygen (a) obtained using monoenergetic electrons (= + 0-06 e.v.) from an electrostatic velocity selector. The positions of thresholds due to the ground state and vibrationally excited states of the 77g ion are indicated by the arrows (b). (Reproduced with permission from Brion, 1964.)... 

Strictly speaking, every molecular species has an ionization efficiency curve of its own depending on the ionization cross section of the specific molecule. In case of methane, this issue has been studied repeatedly (Fig. 2.3). [18] The ionization cross section describes an area through which the electron must travel in order to effectively interact with the neutral and consequently, the ionization cross section is given in units of square-meters. Ionization cross section graphs are all of the same type exhibiting a maximum at electron energies around 70 eV (Chap. 5.1.3). 

Numerous approaches have been published to improve the accuracy of IE data. However, the uncertainty of electron energy remains, causing the ionization efficiency curves not to directly approach zero at IE. Instead of being linear, they bend close to the ionization threshold and exponentially approximate zero. Even though the electron energy scale of the instrument has been properly calibrated against lEs of established standards such as noble gases or solvents, IE data obtained from direct readout of the curve have accuracies of 0.3 eV (Fig. 2.19a). 

The plateau of the ionization efficiency curve around 70 eV makes small variations in electron energy negligible in practice El works equally well at 60-80 eV. 

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