Coherent control of the efficient ladder-type population transfer by four-color harmonic laser pulses.

The ladder-type population transfer of the HF molecule steered by four-color harmonic laser pulses (HLPs) is investigated using the time-dependent quantum wave packet method. It is found that although there exist large background excitations and many (resonant) transition pathways during the driving, nearly 100% of the population could be transferred to the target state. In particular, such a process could be coherently controlled by changing the phases of the four HLPs, especially the phases of the fundamental and second HLPs, which can be accounted for in large part by the combined effects of the corresponding transition pathways and the maximal amplitude of the total electric field. However, for manipulating the phases of the third and fourth HLPs, both the changes in the maximal electric field amplitude and the asymmetry size fail to guide the variation of the target-state population because of the correlated effects of all these transition pathways, particularly the ones that do not contain the third and fourth HLP excitations. Importantly, our results also denote that the variation of the maximal electric field amplitude may give a well qualitative prediction about the phase-controlled population when the manipulated phase is directly related to all the transition pathways, which is the general case in the widely used two-color scheme. In addition, the maximal amplitude of the total electric field tends to play a more important role than its asymmetry size in the phase-controlled population transfer process.