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The first nonperturbative calculation of the amplitude for the excitation of a discrete atomic state by a classical electrornagnetic field is described. Specifically, the excitation amplitude for the H(ls) → H(2p) transition involving the net absorption of only one photon is shown to exhibit a nonlinear dependence on the field amplitude beyond an intensity I ≈ 1013 W cm-2 In this region, the rate of change of the transition amplitude decreases with increasing field strength, finally becoming negative after I ≈ 4 X 1014 W cm-2. Degeneracy of the excited state is shown to be largely responsible for the early onset of such behaviour, thereby picking out atomic hydrogen as the most suitable target for experimental observations.

Nonlinear Effects in Single-photon Resonant Excitation of Atomic Hydrogen