This chapter provides a simple explanation of post-field ionization and the probability of its occurrence.
In practice, the lower limit of the integral is determined by the critical distance xjcr at which the topmost occupied orbital in the j+ ion rises above the emitter Fermi level, thereby allowing PFI to take place into an empty emitter electron state above the Fermi level. Details of the various approximations used by Kingham are given in [82K].
For a large number of elements, Kingham [82K] plotted curves showing charge-state abundances as a function of field, for all relevant charge states. Parameters of interest are the local field values Fj,j+1 at which the probable abundances of both j+ and (j+1)+ ions are 50%. These equal-abundance fields have been extracted from the Kingham diagrams and are shown above in Table 159.2.
There is some reason to believe [92L, 14F] that Kingham’s approximations are less satisfactory than he thought, but it is currently not clear what effect any corrections would have. However, there is no doubt that PFI occurs, and there is no empirical evidence that his PFI predictions are seriously incorrect.