Controlling The Quantum States Of The Photoassociated Molecules By Using A Frequency Cut-off Laser Pulse

It is an interesting task to realize the quantum control of ultracold molecules with laser. It has been applied for researching collision dynamics, molecule spectrum and laser chemistry, etc. Especially in the ultralow temperature region, photoassociation is one of the most efficient ways to produce ultracold molecules. However, in most cases, photoassociated molecules are in their weakly bound states of the long-range potential, which are not stable with respect to subsequent collisions and vibrational relaxations. Therefore, it is important to find a good scheme to control the photoassociation process and enhance associated molecules in the lower vibrational states.The photoassociation dynamics of ultracold lithium atoms is investigated theoretically by solving numerically the time-dependent Schrodinger equation using time-dependent wavepacket method together with mapped Fourier grid (MFG) method. Based on the Born-Oppenheimer approximation, we use the MFG method to calculate initial wave packet, and cut-off laser pulse is applied to suppress the weakly bound states.In this paper, we firstly Fourier-transform Gaussian pulse from temporal to frequency domain, then, based on our need, parts of frequency components are cut off. After that, another Fourier-transform is conducted from frequency to temporal domain. To highlight its advantages, we use two traditional Gaussian pulses for comparison. When the laser pulses are over, we can see that using Gaussian pulse, most population assembles at v=100-110, which is4.42×10-4in total and population in lower states(v=53-69) comes to2.63×10-10. However, using shaped pulse can make most population gather at v=53-69, which is3.84×10-8in total and population in vibrational states(v=100-110) falls to7.21×10-10. Population in the weakly bound states decreases by almost six orders of magnitude and that in the lower vibrational states increases by approximately two orders of magnitude. At the end of this paper, the dependence of photoassociation probability on the cut-off position of the laser pulse is explored.