Ionization efficiency curves, from electron

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. 

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.)... 

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). 

Additional details on some of these methods are described in other sections of this review. Attempts have also been made to determine excited-state populations in single-source mass-spectrometric experiments from an analysis of ionization efficiency curves.38ad There are several difficulties in applying such methods. For instance, it is now known from photoionization studies that ionization processes may be dominated by autoionization. Therefore, the onset of a new excited state is not necessarily characterized by an increased slope in the electron-impact ionization-efficiency curve, which is proportional to the probability of producing that state, as had been assumed earlier. Another problem arises because of the different radiative lifetimes that are characteristic of various excited ionic states (see Section I.A.4). 

Utilizing ionization efficiency curves to determine relative populations of vibrationally excited states (as in the photoionization experiments) is a quite valid procedure in view of the long radiative lifetime that characterizes vibrational transitions within an electronic state (several milliseconds). However, use of any ionization efficiency curve (electron impact, photon impact, or photoelectron spectroscopic) to obtain relative populations of electronically excited states requires great care. A more direct experimental determination using a procedure such as the attenuation method is to be preferred. If the latter is not feasible, accurate knowledge of the lifetimes of the states is necessary for calculation of the fraction that has decayed within the time scale of the experiment. Accurate Franck -Condon factors for the transitions from these radiating states to the various lower vibronic states are also required for calculation of the modified distribution of internal states relevant to the experiment.991 102... 

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 these experiments Ej is obtained from an ionization efficiency curve (i.e., a plot of the ion intensity against the electron energy) as the minimum electron energy required for ionization. The identification of this threshold is difficult, particularly when polyatomic molecules are studied and non-monoenergetic electron beams are used. Thus, unpredictable uncertainties of 10-50 kj mol or more may be associated with many of the early EIMS values of E ... 

MS Monoenergetic electron impact on PH2 (from benzylphosphane) was used, and the ionization efficiency curve was extrapolated to zero ion current. The sharp onset pointed to the adiabatic character of Ej. A measurement with conventional electron energy spread and the usual semilogarithmic method due to [6] yielded Ej = 9.96 eV [2]. 

Ionization efficiency curves can also be easily obtained in the ion cyclotron resonance spectrometer. As mentioned earlier, the most common modulation techniques now center around some form of electron energy or beam modulation. If the electron beam is amplitude-modulated (switched on and off), it is only necessary to sweep the electron energy to obtain such a curve. With the restriction that the ion residence times are much longer (so that some fragmentation patterns may be different from those obtained in more conventional instruments), the ionization efficiency curves thus obtained should be comparable to those obtained in other mass spectrometers. 

If reliable thermochemical data [23,85] is required, the above disturbing effects have to be substantially reduced. [81] One way is to use an electron monochromator (accuracy up to 0.1 eV) [86,87]. An electron monochromator is a device for selecting nearly monoenergetic electrons from an electron beam [88]. Alternatively, photoionization (PI) may be employed instead of EL Photoionization yields even more accurate results ( 0.05 eV) than the electron monochromator [89]. In any case, the half width of the electron or photon energy distribution becomes small enough to detect detailed structural features of the ionization efficiency curves such as electronic transitions. Both techniques have been widely employed to obtain IE data (Table 1.1). 

Apart from the higher resolution relative to similar electron ionization experiments, photoionization has an additional advantage in that there is a finite cross section at threshold, making it easier to detect the actual ionization onset. This also applies to unimolecu-lar fragmentation reactions and is a result of the different threshold laws for the two ionization processes. An ionization efficiency curve produced by photoionization is comparable to a first derivative of its electron-ionization counterpart. 

However, experimental ]V curves often deviate from the ideal /scl- In these cases, the measured current /inj is injection limited caused by a nonohmic contact or poor surface morphology. When the MO interface is nonohmic, carrier injection can be described by the Richardson-Schottky model of thermionic emission the carriers are injected into organic solid only when they acquire sufficient thermal energy to overcome the Schottky barrier ((()), which is related to the organic ionization potential (/p), the electron affinity (AJ, the metal work function (O, ), and the vacuum level shift (A) [34,35]. Thus, the carrier injection efficiency (rj) can be calculated by the following equation ... 

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