Field-free molecular orientation induced by single-cycle THz pulses: the role of resonance and quantum interference

We consider the rotational excitation of molecules induced by asymmetric (“half-cycle”) as well as symmetric single-cycle THz pulses, leading to field-free time-dependent orientation. We show that the basic excitation mechanisms are very similar for the two types of pulses; i.e., the frequency distributions of the pulses at the rotational resonance frequencies play an important role.

Furthermore, we investigate the interference between multiple rotational excitation pathways following prealignment with a nonresonant 800-nm femtosecond pulse. It is shown that such interference can lead to an enhancement of the orientation of the linear HCN molecule by a factor close to 2.