What determines the energy resolution of the RPD method?

The energy distribution of the primary electron beam effective when using the RPD method for cross section measurements is calculated from the difference of the electronic velocity distribution functions. Due to the difference method the shape of the distribution function changes from that of an accelerated Maxwellian to that of a true exponential function. Thus the energy halfwidth decreases from 2.47 kT to 0.69 kT (T, electronic temperature), when the difference voltage approaches zero, i.e. the energy width is not proportional to the difference voltage but to the temperature of the cathode.