Effects of local field variations on the contrast of a field-ion microscope

Both the gas concentration and the electron transition rate constant (for ionisation) of the imaging gas may be important factors determining the contrast in the field-ion microscope. It is demonstrated that the field dependent variation in the electron transition rate constant may be sufficient to explain the observed contrast, and that simple application of Boltzmann statistics gives implausibly large differences in gas concentration particularly at temperatures <20K.