Atomic and molecular dynamics in ultrashort intense laser fields

The interaction of ultrashort, intense laser pulses with atomic and molecular systems is studied theoretically. The strong field approximation is applied to describe atomic dynamic processes. We first show the characterization of attosecond pulses from photoionization of Argon atoms assisted by a circularly polarized laser pulse. By studying the angle-resolved photoelectron spectra, we show how to retrieve the electric field of the attosecond pulses by fitting the measured electron spectra using a genetic algorithm.; A quantum theory is further developed to describe the dynamics of an autoionizing state generated by an attosecond XUV pulse in the presence of a femtosecond IR laser pulse. The lifetime of the autoionizing state is determined directly in the time domain from the time-resolved photoelectron spectra. By coherently treating different transition channels, interference effects can be observed. This work extends Fano's theory covering an autoionizing state to the time domain in a time-dependent field.; Compared to atoms, molecules have more degrees of freedom when interacting with laser fields. Because of the different time scales, these motions are generally weakly coupled. We studied the dynamic aligning of molecules by a laser pulse. The experimental observations of ionization suppression of certain molecules suggests the ionization of molecules depends on molecular structure, and thus the alignment of the molecules. We demonstrated the possibility of determining this alignment dependence in a double-pulse configuration. Distinct time dependence of the total ionization yield has been predicted, and the results have been verified by recent experiments.