Effects of the phase of a laser field on autoionization

We present a formal theory and detailed calculations for phase-dependent laser-atom interactions involving autoionizing states. First, through simple models, we demonstrate that the simultaneous one- and three-photon excitation of one or two neighboring autoionizing states can exhibit profound changes of the line shape, as the relative phase of the two fields is varied from 0 to {π}. Through a proper choice of the field intensities and the phase, we obtain analytical results showing that one can cancel the transition to the discrete or the continuum part of the wave function, thereby leading to a flat or a completely symmetric line shape, respectively. At higher intensities, additional effects come into play, providing additional coupling between the discrete and continuum parts, which also exhibits a phase dependence. Finally, our theory is applied to a much more complex situation in Xe, involving many channels, not amenable to simple analytical expressions, but exhibiting nevertheless equally profound effects, including a modification of the branching ratio of two different products. The theory, which is here developed for atomic autoionization, is in fact fairly general and should pertain to related problems in any system involving discrete states embedded in continua.