Effects of external electric fields on high Rydberg states formed in foil and gas interactions of 85-MeVNe6+ions

High Rydberg states in neon ions are formed in interactions of 85-MeV ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}_{}^{6+}{}_{}{}^{}$ ions with a thin carbon-foil target and with a helium-gas target. The effect of a weak, transverse, static electric field on the Rydberg states is investigated by measuring the electron yield due to field ionization in the strong electric field of an electron spectrometer. For both targets, marked oscillatory structure is observed in the dependence of the electron yield on the strength of the weak field. The Fourier analysis of the data reveals the dominance of a narrow band of fundamental frequencies in the oscillations and indicates the presence of second-harmonic contributions. The oscillations are interpreted in terms of quantum interferences among Stark states coherently developing in the weak electric field. The relevance of experiments of the present type to the understanding of the mechanisms of Rydberg state formation is discussed.