Molecular chemistry coincidence experiments

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

The value of g and hence deviation (Ag) in its value from the free-electron value is measured in the ESR experiment. The small differences arising from location of the pareunagnetic ion in differing molecular environments are readily detectable, enabling the technique to be used as a probe in crystal chemistry and bonding studies. Also, given the above, Ag may assume different values for each molecular or crystal axis if the symmetry is lower than cubic. Hence, the g value may be anisotropic and represented by a second-rank tensor with principal axes that may, or may not, coincide with the molecular axes. In axially symmetric molecules, two values g, and g are given for lower symmetry cases, three values are determined (g, gyy, g ). 

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