Study On Dynamics Of Molecular Alignment And Orientation By Laser Pulses

Alignment and orientation controlled by the laser pulse is an important subject of molecular stereoscopic reaction dynamics at present.It has an important application value in the fields of ultra-cool atom-molecular collision and photoassociation,photoelectron angle distribution,ultra-fast molecular switch,high-order harmonic generation,surface treatment and so on.From different sides,many researchers have used intense non-resonant femtosecond pulse,the terahertz pulses?THz pulse?,slow turn-on and rapid turn-off?STRT?shaped pulse and phase-shaped pulse,the combined pulse of same pulse,and the combination of different pulse to align and orientate diatomic or triatomic molecules.However,the intense femtosecond non-resonant pulses are almost all based on the traditional Gaussian envelope,and the study of THz pulses in controlling the molecular spatial direction mainly focuses on the molecular orientation.Although various shaped and combined pulses are applied,there is still room for further improvement in the molecular orientation.Using the density matrix theory?DMT?in quantum dynamics calculation,molecular alignment and orientation are discussed and studied,and the main content is divided into three parts:?1?Based on the traditional Gaussian envelope,a laser pulse of super-Gaussian envelope is constructed.The spatial alignment of CO molecule of in-pulse and post-pulse by the super Gaussian pulse at the rotational temperature T=0 K is tuned.?2?The half-cycle pulse?HCP?in the terahertz range is selected and the spatial alignment of triatomic molecule FCN with the system temperature T=50 K is studied by modulating the pulse parameters.?3?A scheme of field-free molecular orientation induced by combining the super-Gaussian and half-cycle terahertz laser pulse is proposed,and a specific method to greatly improve the positive and negative orientation of CO molecules is found,which lays a certain research foundation for accurate orientation control.Compared with the traditional Gaussian pulse,the on and off ramps of the super-Gaussian pulse are steeper,so that the energy mainly focuses on the upper part of the single pulse,which is beneficial to the improvement of the molecular spatial alignment.At present,the research on the alignment of the field-free molecules controlled by super-Gaussian laser pulses has not been reported in detail.Here,the adiabatic and non-adiabatic spatial alignment of CO molecules induced by super-Gaussian laser pulses is studied by using the time-dependent density matrix theory.The results show that under the same pulse parameters,the super-Gaussian pulse has an advantage over the standard Gaussian pulse in controlling the molecular alignment.When the system is in intermediate transition and non-adiabatic state,the maximum degree of molecular alignment during and after the super-Gaussian laser pulse is slightly higher than that of the conventional Gaussian laser pulse.During the pulse,the molecular alignment induced by super-Gaussian pulse has the characteristic of irregular oscillation,and the corresponding maximum degree of molecular alignmentáco s²q?_{m ax}=9.46.We analyze the population of the rotational particles in the pulse duration and find the possible reason for the random oscillation of the molecular orientation in the field.By changing the half-duration?,electric field amplitude E₀,shape parameter N and laser energy E,the effects of these parameters on the maximum degree of molecular alignment during and after the super-Gaussian pulse are discussed.It is found that the half-duration?=3 ps is the transition point where the alignment values of in-pulse and post-field begin to turn.Compared with the variation of spatial orientation in field,the variation of laser intensity has more influence on the orientation after the field.The laser shape parameter N affects the steepness of the on and off ramp of the super-Gaussian laser pulse,and the change of its value can improve the field-free orientation value by 0.4,that is?35?ácos²q?_{m ax}=0.4.At all pulse energy points,the maximum degree of the super-Gaussian pulse for the alignment is significantly higher than that of the standard Gaussian pulse and N=1 pulse,and with the increase of pulse energy,the alignment differences among the three become larger and larger.For the THz HCPs,they are characterized by strong interaction with the permanent dipole moment of molecules,short pulse duration,and small damage to the electron and vibration modes of molecules.Therefore,they have been widely used in the orientation study of field-free molecules.In theory,by solving the density matrix equation basing on the rigid rotor approximation,the spatial alignment of FCN molecules is calculated and analyzed at the rotational temperature T=50 K.The change of the field-free molecular alignment with time is given at the carrier envelope phase?=0 and 0.5?of the THz HCP,respectively.Under the same laser parameters and rotational temperature,we respectively give the population of different rotational quantum numbers J when the carrier envelope phase?=0 and?=0.5,and find that both rotational quantum populations focus on the quantum state J<50 and present a positively skewed normal distribution.As the electric field intensity increases,the post-field alignment of FCN molecule appears saturation threshold.The reason for the saturation or decrease of alignment may be due to the destruction of a positively skewed normal distribution of the rotational quantum states.It is only determined by the interaction between the laser pulse and the molecular permanent dipole and almost unaffected by the polarizability and hyperpolarizability interactions.Define the frequency width at half alignment peak value as the bandwidth??of the THz HCP,we obtain that the sensitive area of the center frequency is around 22⁸1 cm^-1.In this frequency region,the molecular spatial alignment be significantly improved.Under different carrier envelope phases,the difference of maximum degree of molecular alignment is obvious when the matching number G of THz HCP is adjusted in the range of 1/2 to 2.However,as the matching number G continues to increase?when G>2?,the maximum degrees of molecular alignment at all carrier envelope phases tend to be basically the same.The studies shows that the FCN molecular system induced by a single THz HCP has a higher degree of field-free alignment than that induced by a double laser pulse.The effect of the delay time t_d of two pulses on the molecular spatial alignment is studied with the rotational temperature T unchanged.We find that when the delay time t_d is close to the rotational period of the molecule T_rot,the delay time t_d has a great influence on the field-free alignment.Next,a scheme of field-free molecular spatial orientation is designed by the super-Gaussian linearly polarized laser pulse together with the THz HCP.After the non-resonant excitation of the super-Gaussian pulse,adding the THz HCP,the maximal orientation of the moleculeác osq?_{m ax}is increased from 0.23 to 0.5.This enhancement of maximal orientation is much larger than that by the combined pulse of STRT and THz.The post-pulse orientation of the molecule in the negative direction is studied and it is found that the negative orientation is better than the positive orientation under the specific intensity amplitude of the super-Gaussian laser pulse.Compared with the positive orientation of the molecule,an optimal result of negative orientation can be obtained with the smaller amplitude of the THz HCP,which plays an important role in preventing the occurrence of nonlinear effect.By using 0.1?step modulation method,we study the influence of carrier envelope phase on the positive and negative orientation of molecules.The maximal degrees of positive and negative orientation are obtained when the carrier envelope phase?=1.7?+2n??n=0,1,2…?and 0.7?+2n??n=0,1,2…?,respectively.This is also different from the previous results of molecular orientation obtained at the carrier envelope phase?=?.When the delay time t_d between the super-Gaussian pulse and the THz HCP increases from 0.05T_rotot to 2T_rot,the maximum degrees of positive and negative orientation induced by the combined laser pulse is increased by 0.35 and 0.39,respectively,compared with that induced by a super-Gaussian laser pulse only.Next,when the laser shape parameter N increases from 1 to 21,the maximum degrees of positive orientation increases gradually and then then became saturated.The maximum degrees of negative orientation first increases rapidly and then gradually decreases to the saturation value,and the optimal orientation of the molecule is obtained at the laser shape parameter N=3.Modulating the pulse half-duration within a certain range,the degree of positive and negative orientation have a large change,especially the maximum variation of positive orientation?35?ác osq?_maxis close to 0.61 when the variation of pulse half-duration??is only 500 fs.,By adjusting the pulse half-duration,we can get a variation of maximum orientation degree?35?ác osq?_max=0.72 for the negative orientation.When the temperature T increases from 0 to 5 K,an obvious transition from the rotational states J=1 to J=4,5,and 6 occurs,resulting in a decrease in molecular spatial orientation.