On the paradigm of coherent control: the phase-dependent light–matter interaction in the shaping window

Coherent control is unquestioningly a powerful method to investigate the interaction of femtosecond light pulses with ultrafast molecular processes. In order to exploit to a great extent the capabilities of coherent control, a judicious analysis of experimental control results is required, which is especially true for the manipulation of matter waves with phase-locked femtosecond pulses. Due to its importance for the optical control of chemical reactions, we discuss the tailoring of wavepackets and the interpretation of such coherent control results in the region where the pump and probe pulses temporally overlap. In this region, called the shaping window, the observed dynamics are phase dependent and may misleadingly resemble oscillatory molecular dynamics. In a transient absorption experiment, part of these oscillations is successfully identified as intra-multipulse interferences and the phenomenon is qualitatively and quantitatively explained by the spectral appearance of the unshaped laser pulse. Furthermore a strategy is presented based on phase-cycling for easy differentiation between interference effects and true wavepacket dynamics. These new findings have important consequences for the interpretation of adaptive coherent control results.