Ion density effects in the pulsed field ionization of high Rydberg states

Two-photon nonresonant photoexcitation is used to record pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectra of the X-2 Pi(1/2)(upsilon = 0)-->X-1 Sigma(+)(upsilon(+) = 0) transition of the NO molecule. By varying the laser power and modifying the focussing conditions, the ion density in the photoexcitation volume is varied over more than two orders of magnitude. A direct correlation is established between ion densities and the pulsed field ionization behaviour of high Rydberg states. Sequences of identical pulses lead to recurrent PFI signals at high ion concentrations. These recurrent signals can be suppressed by maintaining a weak de field to prevent the electric field from returning to zero between successive pulses. The magnitude of the minimum de field required to suppress recurrent field ionization signals increases with increasing ion densities. The spectral resolution that can be achieved by PFI-ZEKE spectroscopy improves when the fluence of the exciting radiation is decreased.