We discuss the geometric phase accumulated by the wave function of an atom ionizing in the presence of a bichromatic field as physical parameters are varied adiabatically around a closed circuit. As an illustration we calculate the geometric phase for a hydrogen atom in the presence of 355-nm light and its third harmonic when the phase and intensity of the two components are varied. The wave function need not be single valued after one complete circuit—two circuits may be necessary to map the original eigenray onto itself. Furthermore, the geometric phase may be complex, and may therefore modify the ionization yield calculated from the width of the instantaneous quasienergy.

• Received 17 January 1992

DOI:https://doi.org/10.1103/PhysRevA.46.555