Field ionization and quantum interferences of Rydberg levels in fast heavy ions

Rydberg states in highly stripped fast oxygen and neon ions are formed in the interaction with thin carbon foil and gas targets. They are ionized in the field of an electrostatic parallel-plate spectrometer and produce pronounced structures in the electron energy spectra at apparent energies higher than the cusp peak. The measured electron yield shows marked oscillatory structures in the dependence of the strength of an additional weak electric field in the drift space between target and spectrometer. Fourier analysis of the data reveals the presence of an asymmetric band of fundamental frequencies and contributions from higher harmonics. The oscillations are interpreted in terms of quantum interferences among Stark states which coherently develop in the weak drift-space field. A predominant influence of the experimental arrangement on the shape and the position of the Rydberg structure in the electron spectrum is found through analysis of the data on the basis of a program which calculates the electron trajectories through the fringe fields of the detector. The analysis also reveals that the dependence of the oscillation periods on the detector field strength is influenced by fringe-field effects. The absolute values for the periods and the ionization process in the detector are explained in the frame of a simple model and conclusions are drawn about the contribution of experiments of the present type in understanding the mechanisms of Rydberg formation