COLLISIONAL AND ELECTRIC-FIELD EFFECTS IN THE DELAYED PULSED-FIELD IONIZATION ZERO-KINETIC-ENERGY PHOTOELECTRON-SPECTRUM OF ARGON

The pulsed field ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectrum of argon has been recorded in the region of the transition from the ground neutral state (S-1(0)) to the first two ionization limits corresponding to the two spin-orbit levels (P-2(3/2) and P-2(1/2)) of the ground state of the ion. The high-n Rydberg states (85 < n < 200) belonging to the series converging to the upper spin-orbit state have a lifetime which is more than 50 times longer than expected for the optically accessible ns' and nd' series. A series of experiments with pulsed and continuous electric fields of different magnitude shows that the nature-and the lifetimes of the high-n Rydberg states probed by ZEKE spectroscopy depend critically on the experimental conditions, in particular on electric field and collisional effects. New experimental results are presented which contribute to a better understanding of the mechanisms which lead to the formation of the unexpectedly long-lived states which are observed in ZEKE spectroscopy.