Ionization and Appearance Potentials

The minimum energy I required for ionization is called the (first) ionization potential. The W value is the average energy required to produce a pair of ions in the medium. Experimentally this value is obtained by dividing the absorbed dose by the total number of collected ions. The W value depends primarily on 

Franck and Hertz (1913) first demonstrated that an electron has to acquire a minimum energy before it can ionize. Thus, they provided an operational definition of the ionization potential and showed that it is an atomic or molecular property quite free from experimental artifacts. However, this kind of experiment does not tell anything about the nature of the positive ion for this, one needs a mass spectrometric analysis. Although Thompson had demonstrated the existence of H+, H2+, and H3+ in hydrogen discharge, it seems that Dempster (1916) was the first to make a systematic study of the positive ions. 

FIGURE 4.1 Illustration of adiabatic and vertical ionization potentials. Adiabatic I.P. refers to the energy difference between the lowest quantum states of the molecule and its positive ion. Often, Franck-Condon (vertical) transitions lead to a higher value, the vertical ionization potential. 

The evaluation of ionization potentials by spectroscopic methods (adiabatic IA), yields very accurate values. This amounts to identifying an appropriate Rydberg series in the absorption spectroscopy of complex atoms or molecules. The excitations leading to Rydberg-like series may be expressed as 

The appearance potential (AP) of an ion refers to the experimentally determined minimum energy required to produce that specific ion from the ground state of the neutral atom. The appearance potentials of ions of specific types, such as the fragment ions, have been listed by Franklin et al. (1969). Some common appearance potentials of ions are given in Table 4.2. 

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The combination of the flash vacuum pyrolysis (FVP) technique169 with mass spectrometry proved to be particularly useful in identification and characterization of both the fragmentation/rearrangement patterns, intermediates and/or final products formed (see Section IV.E.l). Usually, no structures are indicated in the mass spectra, although ionization and appearance potential can, in principle, provide structural information. 

The mass spectra of a series of ring system were examined by the DADI technique. The primary fragmentation produced a [M-28]+ species in the case of 309 and 311 (Me enol), which resulted from the loss of N2, whereas in the case of 308, it resulted from loss of CO (79KGS1270). Ionization and appearance potentials were also determined. 

Excess energy terms such as the value E in equation (8) must be either negligible or cancel for thermochemical arguments to lead to accurate bond dissociation energies. There is little evidence that excess energy terms and other errors of interpretation are not more important in more complex cases than the reaction (8). Further, the empirical assumptions used to determine ionization and appearance potentials by electron-impact methods work best for very simple molecules. 

Ionization and appearance potential measurements by mass spectrometry. Steudel and Schenk (3 ) estimated the enthalpy of formation... 

K) (1 ) using the spectroscopic value of IP(W) = 64400 1.20 kJ mol" ) from Moore (2). The ionization limit is converted from cm" to kJ mol" using the factor, 1 cm", which is derived from the 1973 CODATA fundamental constants (3) additional ionization and appearance potential data. 

However, the deviations may be larger than 0.2 eV due to low lying electronic states or the energy spread of the ionizing electrons [53], An account of ionization and appearance potentials for different ions is given in Refs. 54-56. 

For the molecular ion XY+, the energy required to break the X-Y bond (Dx.y) can be calculated from the ionization and appearance potentials it is simply the difference between the two (Equation 4.3) ... 

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