Reversibility of bound-to-continuum transitions induced by a strong short laser pulse and the semiclassical uniform approximation

We show that the problem of the net absorption of one photon from a strong laser pulse in a bound-to-continuum transition can be recast as a single integral equation. For a certain class of absorption spectra, this integral equation can be converted to a second-order Schr\"odinger-like differential equation, which can be accurately solved in an essentially closed form using the semiclassical uniform approximation. With the aid of the integral equation and the uniform solutions we find that for strong short pulses, irreversible transitions to a perfectly absorbing continuum in the weak-field regime become reversible. In particular, continuum levels may execute Rabi oscillations with the precursor bound state. These oscillations occur at different frequencies, depending on the continuum energy. As a result, spectral migrations and formations of transparent lines may occur. Field-induced interferences between neighboring lines is also investigated. {} 1996 The American Physical Society.