Theoretical Investigation Of Magnetically Tuned Ultracold Two-and Three-atom Quantum Scatterings And Thermal Atom Photoassociation

The collision reaction of atoms and molecules controlled by external electromagnetic fields is a hot topic in the field of modern atom and molecule physics.This dissertation theoretically inves-tigates the magnetically tuned ultracold two-and three-atom quantum scatterings and thermal atom photoassociation,which are summarized as follows.Based on the multi-channel quantum defect theory,a simple theoretical model is presented for describing ultracold atomic collision dynamics and Feshbach resonances without onerous numer-ical computation.An application example of 6Li-40K collision shows that the result given by the simple model agrees quite well with both the relevant experimental data and the coupled channel calculation.The high-rank separable potential method is presented for investigating the magnetically tuned Feshbach resonance.We discuss the relationship and difference between the high-rank separable potential method and the asymptotic-bound-state method.By using the high-rank separable poten-tial method,we calculate the weakly bound state energy and magnetically tuned scattering length of the 6Li-40K,7Li2 and 6Li2 systems,and the calculated results are in good agreement with those calculated by using the coupled channel method.Compared to the coupled channel method,the high-rank separable potential method reduces the computation amount by a fact of 10-6.A simple model is constructed to calculate the trimer bound state energy and three-body re-combination rate of identical bosons without three-body parameter.Using this simple model,we investigate the influence of the van der Waals finite-range effect on the three-atom quantum scatter-ing.The result of our simple model agrees within a few percents with other theoretical works with van der Waals interaction and also the experimental data.A high-rank separable potential formula is derived to solve numerically the two-body Lippmann-Schwinger equation and three-body Faddeev equation.We analyze the convenience and stability of the numerical equations,and calculate the dimer bound state energy,two-body scattering phase shift,two-body t-matrix,trimer bound state energy,atom-dimer scattering length and three-body recombination rate by using the high-rank separable potentials,taking the identical 4He atoms as an application example.All the calculations converge quickly for the rank number N? 3 with a high accuracy.A multi-channel finite-range three-body scattering theoretical model is presented for investi-gating the magnetically tuned alkali-metal atom-dimer collision reaction.By using 6Li-6Li2 system as an example,we find that the two-body p-wave interaction has a significant influence on the atom-dimer collision reaction and reveal a new university in the atom-dimer collision reaction,i.e.,the university of the deep dimer product.The university of the deep dimer product in the 6Li-6Li2 collision reaction is more robust at magnetic field B>620 G than that at B<620 G.A pump-dump photoassociation scheme is proposed to prepare NaH molecules in the lowest rovibrational state(the target state)of the ground electronic state.In short-range region,the Na atom and H atom are efficiently associated into the target state via the intermediately rovibrational|10.1)state of the excited electronic state.The photoassociation probability reaches 0.623.The changes of populations with the electric field amplitudes,frequency detunings,dump pulse dura-tion and delay time between two laser pulses are calculated and discussed.