Active control scheme and mechanism in the two-pulse molecular alignment

The maximal field-free molecular alignment achievable by two ultrafast laser pulses can be actively controlled by predetermining the timing of the second pulse according to the molecular alignment by the first pulse. We demonstrate that the active control scheme is manifested by the probability of Raman transition back to the original states. When the time delay between two aligning pulses varies, constructive or destructive interference occurs among different quantum paths, leading to the modulation of the transition probability back to the initial rotational states, therefore can control the molecular alignment and also the contribution from even/odd states.