Evolution of beam-foil-excited Rydberg states on femtosecond time scales

Traditional beam-foil methods for measuring lifetimes of low-lying atomic levels are generally impractical beyond $Z\sim 35$ because of the correspondingly shorter flight times when lifetimes become less than a few tens of picoseconds. We recently reported the results of an experiment using a novel two-foil method to measure the 9.5-psec lifetime of the $2{}^3{P}_2$ level in heliumlike krypton. We describe here the results of an extension of that work in which we measured the time evolution of the $N>~2$ excited state population between the foils by observing the dependence upon foil separation of the charge-state distributions emerging from the second foil. The results are consistent with the well-known power-law dependence of Rydberg-fed atomic transitions observed in beam-foil studies, but probe a much shorter time scale than has hitherto been accessible.