Laser-induced resonance trapping in atoms

The coupling of two autoionizing states or of a discrete and an autoionizin g state by a strong laser field is studied analytically as well as numerica lly. The motion of the complex energies is traced as a function of the fiel d strength for different field frequencies and atomic parameters. Most inte resting is the critical region where a crossing (or an avoided crossing) of the trajectories occurs. At this critical field intensity, level repulsion in the complex plane occurs. With further increasing intensity, the comple x energies move differently. When the resonances are coupled mainly via one common continuum, resonance trapping dominates, i.e. a short- and a long-l ived resonance state are formed (level repulsion along the imaginary axis). When, however, the direct coupling dominates, level repulsion along the re al axis takes place. Population trapping (defined by a vanishing decay widt h of one of the states at finite intensity) results from the interplay of t he direct coupling of the states and their coupling via the continuum. We a lso studied the corresponding variation of the cross section for ionization of a laser-driven atom by the probe field.