The Role of Electric Fields and Ion Concentrations in the Formation and Stabilization of High-n Rydberg States

ZEro Kinetic Energy (ZEKE) spectroscopy relies on electrons produced through delayed field ionization of the narrow band of high-n Rydberg states which exist just below the ionization limit of each ionic eigenstate. Using the unique properties of these weakly bound, stable, high-n Rydberg states (ZEKE states) below the ionization limit rather than the unbound states above the limit, as in PES, leads to an improvement in resolution of more than two orders of magnitude. Several different types of ZEKE experiments, each designed to probe the formation and stability of these states, are presented here. These experiments were performed with pulsed and static electric fields of different magnitude and duration at different ion concentrations. The results indicate an enhanced ZEKE state decay with increasing electric field strengths and an enhanced formation and stabilization with increasing ion concentrations. A strong interplay between field strength and ion concentration ZEKE state formation is demonstrated. The strong influence of electric fields and ion concentrations on the physical properties of the ZEKE state, above and below the classical ionization threshold, is also demonstrated through late time (tens of microseconds) decay rate measurements.