Atomic resonances in parallel electric and magnetic fields

In a quantum mechanical treatment, an atom in external parallel electric and magnetic fields ionizes at energies above the classical ionization threshold and, at energies below tunnels through a potential barrier, in each case forming quasi-bound states (resonances). Such resonances can be calculated for hydrogen using the complex-coordinate method. The combination of this method with the R-matrix method allows the corresponding resonances to be calculated for non-hydrogenic Rydberg atoms. We employ such a combination of methods in an investigation of these resonances in sodium for various relative strengths of parallel, laboratory strength, external fields. Results are also presented for hydrogen in strong fields. (Author)