Photoionization of atomic hydrogen in an electric field

The photoionization cross section of atomic hydrogen in an electric field can be understood as the sum of resonance contributions. Each contribution is proportional to the imaginary part of the square of a complex transition-dipole matrix element divided by an energy denominator and has a naturally asymmetric line shape. The main features of experiments, including asymmetry of the lines, blue shift of maxima with respect to calculated resonances near zero energy, field-induced'' structure above zero energy, and increasing background'' that begins above the classical saddle-point energy, are reproduced via variational calculations of the resonance energies and wave functions in parabolic coordinates.