Strong-field photoionization and emission of light in the wave-packet-spreading regime.

Barrier suppression ionization and wave-packet-spreading models are used to describe to first order in a perturbative expansion the quantum-mechanical interaction between a photodetached electron wave packet and its parent nucleus in the presence of a very strong laser field. The attraction between the wave packet and the nucleus (the first-order approximation to the dipole acceleration) is interpreted in terms of a force arising from an effective potential, where the effective potential is defined to attract the electron treated as a point particle by the same amount that the true Coulomb potential attracts the spreading wave packet. Thus the effective potential has no Coulomb singularity at the origin. The obtained expression for dipole acceleration is used to calculate the emission spectrum of the detached wave packet interacting in the strong laser field with the nucleus. In the case of detachment of the hydrogen ground-state wave packet, the spectrum is shown to be broad and almost completely devoid of definite harmonics of the laser frequency. The lack of any pronounced structure in the emission spectrum is explained by rapid spreading of the electron wave packet and by the resulting strong smoothing of the effective interaction potential and force. The limitations on the applicability of the presented theory are discussed and are shown to be rather stringent: the laser pulse must be very short (a few optical cycles) and very strong (of the order of the atomic field).