Controlling the Branching Ratio of Photoionization Products under Two-Color Excitation: Competition between ac Stark Splitting and Two-Path Interference

We investigate the variation of the photoionization yields into different ionic channels by means of the one-photon-near-resonant two-photon ionization scheme under two-color excitation. We present a general formulation and the results of specific calculations pertaining to the Ca atom with realistic parameters. A significant change of the ionization into Ca{sup +} 4s , 3d , and 4p channels has been observed as a detuning is varied, which agrees qualitatively well with the observation by Wang, Chen, and Elliott [Phys.Rev.Lett.{bold 77}, 2416 (1996)] for the Ba atom. The importance of the laser intensity effects is also addressed. {copyright} {ital 1997} {ital The American Physical Society}