Ionization energy of rubidium Rydberg atoms in strong crossed electric and magnetic fields

This paper describes the determination of the ionization threshold of rubidium Rydberg atoms in crossed electric and magnetic fields. An experimental setup is used that allows one to count separately Rydberg atoms which ionize in the region of optical excitation, and those which leave the excitation region in a bound state. The obtained spectra allow one to determine the energy and the width of the ionization threshold. For different values of the electric-field strength E the influence of the magnetic field B on the ionization threshold was investigated. It was found that the scaled electric field ε=${\mathit{EB}}^{\mathrm{-}4/3}$ and the scaled energy ω=${\mathit{WB}}^{\mathrm{-}2/3}$ with excitation energy W are sufficient in order to describe the observed ionization threshold. This is remarkable since ε and ω are the classical parameters of the system. For ε≳1.5 (atomic units), i.e., a weak magnetic field, the ionization energy does not depend on B and can be explained classically by direct-path ionization. In the regime 1.5≥ε≥0.2, the energy and the width of the ionization threshold increase with increasing magnetic-field strength.