We study in detail the coherent interference mechanism leading to the intermediate-field two-photon absorption enhancement recently found for shaped femtosecond pulses with spectral phases that are antisymmetric around one-half of the transition frequency. We particularly investigate the tolerance of the phenomenon to the phase deviation from perfect antisymmetry. We theoretically and experimentally find that this tolerance increases as the field strength increases. For the present Na excitation, the enhancement occurs even when $~$30% of the phase pattern is not antisymmetric. Our findings are of particular importance for multichannel coherent control scenarios.

• Received 28 October 2008

DOI:https://doi.org/10.1103/PhysRevA.81.045401