Efficiently field-free orientation of CO molecules by a femtosecond pulse and a THz pulse train

Abstract

We theoretically study the field-free orientation dynamics of CO molecules in three cases, with the orientation steered (I) by a single-cycle THz pulse, (II) by a single-cycle THz pulse train L1, and (III) by the combination of an off-resonant femtosecond pulse L0 with L1. In case (I), the strong intensity of the single-cycle THz pulse is required to realize efficient molecular orientation. In case (II), by using a pulse train L1 with experimentally accessible intensity 1 MV cm−1, the maximum orientation degree |〈cosθ〉|max = 0.906 is achieved when pulse number N = 25. In case (III), by introducing the Raman excitations of L0, the pulse train L1 with only N = 3 can realize a high degree of orientation, |〈cosθ〉|max = 0.896. Moreover, we investigate the dependence of the thermally averaged orientation on the initial rotational temperature in cases (II) and (III). The maximum orientation degree |〈〈cosθ〉〉|max decreases with the increase of rotational temperature in the two cases. However, case (II) is more robust against the rotational temperature: |〈〈cosθ〉〉|max > 0.5 can still be achieved when the rotational temperature is as high as T = 30 K.