We investigate the physical origin of the phase lag in the modulation of photoionization products under two-color phase-controlled fields. We show that the phase lag comes from two physical mechanisms: the resonance phase shift, which is already incorporated in the simplest model, i.e., a single discrete state embedded in two continua, and the coupling between continua due to intrasystem interactions. Three specific examples using, respectively, a simple model, multichannel quantum defect theory, and an $L^2$ basis set including configuration interaction are also provided to support the argument.

• Received 7 May 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2266