Electron impact measurements, ionization energies

A large nnmber of the measured ionization energies for stable neutral molecules come from electron impact appearance energy studies, but it has also been adapted for the direct study of reactive neutral molecules. If the reactant molecule is RH, then the appearance energy for R+ from RH, designated AE(R+, RH),... 

The rate coefficient for electron impact ionization depends very sensitively on the ionization energy, as is demonstrated in Fig. 8. We calculated the ratio of the rate coefficients for total ionization of silane for different electron temperatures using the total ionization cross sections of Chatham et al. (1984) and Krishnakumar and Srivastava (1995). The primary difference between the two cross-section data sets at low energies is a 0.6-eV difference in the measured ionization energy [with Chatham et al. (1984) reporting the lower value]. As expected, the... 

Dissociation energies for C—H bonds have been obtained from electron impact measurements both by the direct and indirect method. The dissociation energy D CH3—H) in methane was deter-niinecD by measuring the appearance potential of the CHI ion when methane was fed into the ionization chamber. This is taken to be the energy of the process... 

In addition, the enthalpies of atomization were calculated from the appearance energies of R ions measured experimentally (Chervonnyi and Chervonnaya, 2004b,e,f Evdokimov et al., 1984 Sapegin, 1984 Sapegin et al., 1982a, 1984) under the conditions of electron-impact-induced ionization of RCI3 molecules. 

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

In this chapter we focus attention on the efficiency of ionization, the ionization cross section, and consider some recent experimental measurements and theoretical studies of the ionization process. A sketch of electron impact ionization curves, the variation of the ionization cross section as a function of the electron energy, using CO as an example, are shown in Figure 1. The mass spectrum, collected at the electron energy corresponding to the maximum in the ionization cross section, is also shown, although there will be no further discussion of fragmentation in this... 

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. 

Fio. 12. Fhotoelectron spectrum of methanol vapour using the helium resonance line (21-21 e.v.). Ionization energy increases from left to right. The adiabatic ionization potentials measured (Al-Jobomy and Turner, 1964) are indicated by vertical arrows, and can be compared with (probably) vertical I.P. values derived from electron impact appearance potentials by Collin (1961) (dotted arrows). 

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