Rydberg states of atoms in parallel electric and magnetic fields

We present theoretical results for the photoabsorption spectrum of an atom in parallel electric and magnetic fields, using the R-matrix method combine d with quantum-defect theory. We introduce a radial basis set which is comp lete and orthonormal over a semi-infinite interval [r(0),infinity), to allo w calculations to be performed for high Rydberg states in nonhydrogenic ato ms without encountering problems due to linear dependence of the basis set. The nonhydrogenic character of the spectra is analyzed for Li and Rb, and a comparison is made with previous high-precision experiments which shows t hat the theoretical results agree very well with experiment. [S1050-2947(99 )06002-3].