The flux from foil diffusion sources has previously been shown to contain atoms, ions and clusters which are electronically excited and can be field ionized by weak fields. Pulsed field ionization of the flux from a diffusion source at temperatures between 1200 and 1500 K is studied here at field strengths below 500 V/cm. At field strengths below approximately 60 V/cm, no field ionization is observed for any choice of the parameters. These values would indicate principal quantum numbers of n=28-50 for Rydberg atoms Cs*. The minimum emitter voltage where a time-of-flight peak due to field ionization is observed is recorded under varying conditions, and the flight time of the corresponding ions is observed to be as expected for Cs⁺ ions which have passed the pulsing grid after it has returned to its high voltage. Theoretical expressions are derived which describe the field ionization giving ions with a total energy just enough to pass this high voltage. The formulae involve characteristic life times of the excited states, that are some average over the distribution of states. The average lifetimes determined from the fit to the experiments vary between 0.7 and 0.3 mu s when the field strength varies from 67 to 450 V/cm, and from 0.7 to 8 mu s when the emitter temperature increases from 1200 to 1500 K.