Laser-induced resonance trapping in atoms

The coupling of two autoionizing states or of a discrete and an autoionizing state by a strong laser field is studied analytically as well as numerically. The motion of the complex energies is traced as a function of the field strength for different field frequencies and atomic parameters. Most interesting 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 complex energies move differently. When the resonances are coupled mainly via one common continuum, resonance trapping dominates, i.e. a short- and a long-lived resonance state are formed (level repulsion along the imaginary axis). When, however, the direct coupling dominates, level repulsion along the real axis takes place. Population trapping (defined by a vanishing decay width of one of the states at finite intensity) results from the interplay of the direct coupling of the states and their coupling via the continuum. We also studied the corresponding variation of the cross section for ionization of a laser-driven atom by the probe field.