Photoassociation, the light-assisted formation of molecules, has become a powerful tool for studying weakly bound molecules, and for extracting atomic properties with high resolution. However, few experiments have focused on the dynamics of these collisions. We have used short laser pulses to examine the process by which two ground state sodium atoms cooled to 0.5 mK in a magnetooptic trap absorb two photons tuned to the atomic 3S -> 3P transition (16973 cm-1) to produce a molecular ion:
The 10 ps duration pulses are split into two equal pulses with the second pulse delayed up to 12 ns relative to the first. The rate of molecule formation is dependent upon the time delay between the pump and probe pulses, T, and we can identify three time scales. For T ~ 10 ps, we see a slight autocorrelation of the pump and probe pulses. For T < 4 ns, we can enhance the molecular signal up to 400%, while for long time delays (T > 4 ns) the enhancement diminishes due to the molecular radiative lifetime (~ 10 ns).
We suggest a picture whereby the first laser pulse excites the colliding atom pair to the asymptote of the 3S + 3P potential, which varies with internuclear distance R as 1/R3 at long range. The second laser pulse promotes this system to the 3P + 3P potential, which is a much shorter range potential, varying as 1/R5. Long pump-probe delay times allow the atom pair to accelerate on the intermediate potential, so that they have enough speed on the 3P+3P potential to undergo associative ionization before radiatively decaying, producing an enhancement in the molecular ion signal. This behavior has been modeled with numerical simulations that present a conceptually simple yet accurate picture of the collision dynamics.
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