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For the hydrogen atom in combined magnetic and electric fields we investigatethe dependence of the quantum spectra, classical dynamics, and statisticaldistributions of energy levels on the mutual orientation of the two externalfields. Resonance energies and oscillator strengths are obtained by exactdiagonalization of the Hamiltonian in a complete basis set, even far above theionization threshold. At high excitation energies around the Stark saddle pointthe eigenenergies exhibit strong level repulsions when the angle between thefields is varied. The large avoided crossings occur between states with thesame approximately conserved principal quantum number, n, and thisintramanifold mixing of states cannot be explained, not even qualitatively, byconventional perturbation theory. However, it is well reproduced by an extendedperturbation theory which takes into account all couplings between the angularmomentum and Runge-Lenz vector. The large avoided crossings are interpreted asa quantum manifestation of classical intramanifold chaos. This interpretationis supported by both classical Poincar\'e surfaces of section, which reveal amixed regular-chaotic intramanifold dynamics, and the statistical analysis ofnearest-neighbor-spacingComment: two-column version, 10 pages, REVTeX, 10 figures, uuencoded,  submitted to Rhys. Rev.

Hydrogen atom in combined electric and magnetic fields with arbitrary mutual orientations