Photoionization of the hydrogen 4sstate by a strong laser pulse: Bare-state dynamics and extended-charge-cloud oscillations

The response of a hydrogen atom initially prepared in the 4s state and subjected to a short, intense laser pulse is studied by calculating field-dependent time-dependent wave functions numerically as a function of r and θ. We calculate the probabilities of bound states that are dipole connected to the 4s state both directly and by two-photon Raman coupling through the continuum. We find that for intensities in the neighborhood of ${10}^{15}$ W/${\mathrm{cm}}^2$ the bound states exchange population relatively slowly in time while ionizing. Space-time plots of the electronic charge cloud show a complex but very stable extended microstructure like that of a stabilized polychotomous wave packet. A calculation of the expectation value of the charge cloud’s position along the axis of laser polarization shows behavior similar to free-electron behavior although the extent of the charge cloud is much greater than the free-electron oscillation amplitude.