Improved IE Accuracy from Data Post-Processing

2 Improved IE Accuracy from Data Post-Processing 

The slightly diffuse energy of the electrons effects that the ionization efficiency curves do not approach zero in a straight line they rather bend close to the ionization threshold and exponentially approximate zero. Even if the electron energy scale of the instrument is properly calibrated against lEs of established standards, the accuracy of the IE data obtained ftom direct readout is only 0.3 eV (Fig. 2.19a). 

To overcome the unpredictability of the actual onset of ionization, the critical slope method has been developed [24,84], among several other methods [83]. It makes use of the fact that, according to theory, reahstic values of IE are expected at the position of the ionization efficiency curve where the slope of a semilog plot of the curve is 

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). 

The absorption of UV light by a neutral can result in electronically excited states that undergo relaxation either by emission of light or by emission of an electron. Thus, the photon in photoionization (PI) serves the same purpose as the energetic 

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