Study Of The Photoassociation And Orientation Dynamics Of Diatomic Systems By Using The Random-phase Wavepacket Method

A variety of important photophysical and photochemical phenomena may occur due to the interactions of atoms and molecules with lasers.For instances,two colliding atoms can combine and form molecules,i.e.,photoassociation(PA),and some featured vectors of the molecules can point towards a space-fixed direction,i.e.,molecular orientation.PA plays an important role in the preparation of cold molecules,the coherent control of bond making,etc.Laser-induced molecular orientation plays an important role in molecular quantum control,strong field physics,etc.Focusing on the influence of the initial temperature of the systems on these two photophysical and photochemical processes,with the self-developed full-dimensional random-phase wavepacket method,we investigate the PA processes of the ultracold(～μK)Cs atoms and the hot(～1000 K)Mg atoms,and the laser-induced molecular orientation of NaI in a large temperature range(0～1000 K),respectively.The research content mainly includes the following four parts:(1)The thermal-average effect of the initial energy eigenstates on the PA process of the ultracold Cs atoms is investigated.In previous theoretical treatments for PA under the～μK condition,the suppression of PA from the thermal-average effect(The PA probability with consideration of the thermal-average effect is smaller than the one considering only a single initial state.)was usually neglected.By taking the PA of the Cs atoms at 54 μK as an example,we find that the thermal-average effect is non-ignorable,and that the accurate PA probability can not be obtained until the initial transitional energy distribution and the rotational distribution including the high-order partial waves are considered.It is also found that the thermal-average effect is dependent on the time profile of the pulse.Due to the broad bandwidth and the asymmetric time profile,the slowly-turned-on and rapidly-turnedoff(STRT)pulse can induce high efficient off-resonant and near-resonant transitions at the same time,which can decrease the thermal-average effect and result in a high PA probability.(2)To accurately describe the PA process at high temperatures,we develop the fulldimensional random-phase wavepacket method,in which the random phases of the randomphase wavepackets are expanded in both the vibrational and rotational degrees of freedom to model the thermal ensemble of the initial eigenstates.The numerical accuracy and the computational efficiency of this method are both satisfying.With this method,we investigate the PA process where the Mg2 molecules of the excited states are created by the interaction of the two ground-state Mg atoms with a femtosecond laser pulse.Due to the weak van der Waals attractive well of the ground electronic state,the initial thermal ensemble of the rovibrational eigenfunctions includes both the continuum states and the bound levels.It is found that at the lower temperatures(1,10,100 K),the contribution from the bound levels is so large that the influence of the continuum states can even be neglected,while at 1000 K,the phase space of the thermally active initial states increases dramatically and the contribution from continuum states dominates the PA process.(3)The multi-path population transfer mechanism in the PA process of Mg atoms at 1000 K is elucidated.Induced by the 840 nm femtosecond laser pulse,the population can be transferred from the ground state X1∑g+ to the first excited state(1)1Πg and to the other three excited states |i>(|i>=(1)1Πu,(2)1Πu,(2)1∑u+).There can be various population transfer paths constructed by the two-photon transition from X1Σg+ to(1)1Γg,the threephoton transition from X1Σg+ to |i>,and the one-photon transition between |i>and(1)1Πg.By comparing the population transfer probabilities from given paths,it is found that the major transition path is X1Σg+(?)|i>(?)(1)1Πg.(4)Based on the full-dimensional random-phase wavepacket method,the influence of the thermal average of initial rovibrational states on the orientation of the NaI molecule is investigated.At different temperatures(T=0,100,200,300,500,1000 K),the maximum orientation,|<cosθ>|max,as a function of the central frequency of the single-cycle THz laser pulse is obtained.Firstly,although the orientation is suppressed at high temperatures,a distinguishable periodic field-free orientation is still observable at T=1000 K.Secondly,the variation of |<cosθ>|max with the central frequency is strongly dependent on the temperature,which can be ascribed to the different thermal distributions of the initial rovibrational states and to the consequently different final rotational distributions at different temperatures.In brief,we combine the full-dimensional random-phase wavepacket method with the time-dependent quantum wavepacket theory,and elucidate the thermal-average effect of the initial states on the PA and orientation dynamics at different(ultracold and high)temperatures,which could provide theoretical bases for the coherent preparation and control of molecular quantum states in related physical and chemical processes.