Field-free orientation of LiH molecule induced by two-color femtosecond and terahertz laser pulses

Abstract

We theoretically study the field-free molecular orientation by the combination of a two-color femtosecond laser pulse and a time-delayed terahertz (THz) laser pulse, with LiH as the prototype molecule. It is shown that the molecular hyperpolarizability is a vital parameter in achieving molecular orientation according to the pre-excitation created by an intense twocolor femtosecond laser pulse. The relative amplitude of the two-color femtosecond pulse is discussed. In addition, the center frequency of the THz laser pulse is chosen on the basis of the pre-excited population distributions. The carrier-envelope phase of the THz laser pulse as well as the delay time between the femtosecond laser pulse and THz laser pulse are adjusted to achieve a high orientation degree. In addition, the effect of temperature on molecular orientation is also discussed. Finally, according to the population distributions at room temperature, the parameters of the THz laser pulse are re-optimized, and the achieved maximal orientation degree is 0.221 at 300 K.

Controlling molecular orientation by laser pulses with two different envelope shapes

“…This is consistent with the conclusion that a uniform population distribution of rotational states would lead to a large molecular orientation degree [60]. The molecular orientation also depends on the phase difference between two adjacent rotational states and can be further enhanced by optimizing the decreasing the reduction of orientation resulting from the offset phase [43,60]. Figure 1C shows the phase differences Δφ j,jÀ1 in the scheme (I), the Δφ j,jÀ1 changes from flatly to steeply when 0.65 Â 10 7 V/m ≤ E 01 ≤ 1.00 Â 10 7 V/m.…”

“…Shu et al provided an important insight into the THz-laser-induced molecular orientation and achieved a large orientation degree by the interaction of a SCP with the electric-dipole moment [33]. To further optimize the molecular orientation, researchers proposed many laser field schemes including pulse train [35][36][37][38][39][40][41] and the combination of laser pulses [42][43][44][45]. The combination of two-color femtosecond and delayed terahertz laser pulses has been proven theoretically and experimentally as a versatile method to obtain a high orientation degree.…”

Quantum control of field-free molecular orientation

“…76 Subsequent works have further improved upon this enhancement by incorporating additional pre-alignment pulses or optimizing the parameters of the pulses. 122–124 Experimental demonstrations of this hybrid scheme have demonstrated its capability to induce transient field-free orientation of OCS molecules at low temperatures around 2 K, as well as the rotational dynamics of asymmetric molecules at ambient temperatures. 74 Moreover, a recent experiment has revealed that this scheme can significantly increase the orientation of iodomethane molecules at room temperature, surpassing the levels achieved by employing pure terahertz pulses alone.…”

Influence of molecular orientation on the photodissociation process of LiH molecules

“…LiH as a prototype diatomic molecule in molecular reaction dynamics has attracted considerable attention, leading to widespread investigations in recent years [1][2][3][4][5][6]. The ground state structure and potential energy function of LiH have been provided [7][8][9], and dedicated efforts have been devoted to manipulate both orientation and photodissociation process [10][11][12].…”