PHOTOIONIZATION OF THE HYDROGEN 4S STATE 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 f ield-dependent time-dependent wave functions numerically as a function of r and theta. We calculate the probabilities of bound states that a re dipole connected to the 4s state both directly and by two-photon Ra man coupling through the continuum. We find that for intensities in th e neighborhood of 10(15) W/cm2 the bound states exchange population re latively slowly in time while ionizing. Space-time plots of the electr onic charge cloud show a complex but very stable extended microstructu re 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 behavio r although the extent of the charge cloud is much greater than the fre e-electron oscillation amplitude.