Laser cooling and trapping prepares atomic samples at ultralow temperatures (< 1 mKelvin) and high densities (10^10 - 10^12 atoms/cm^3). Under such ultracold conditions, the kinetic energy distribution of free atoms is as narrow as < 20 MHz, and only the lowest partial waves (s, p and d waves) contribute to the atomic scattering process. When cold colliding atomic pairs meet narrow band laser light, excited diatomic molecules can be formed through free-bound transitions at large internuclear distances. At T < 1 mK, such a free-bound photoassociation spectrum has the same high resolution as a bound-bound spectrum. This newly developed photoassociative spectroscopy of ultracold atoms has proven to be very powerful in precision measurements as well as in studies of cold collision physics.
In this talk, I will discuss two exciting applications of this new spectroscopic technique: (1) Observation of pure long-range molecules and precise determination of atomic radiative lifetimes (with an accuracy of 0.2%); and (2) Molecular fine-structure predissociation and atomic fine-structure changing collisions. Through the above two examples, I would like to demonstrate the physics of long-range interactions and the physical connections between molecular and atomic properties. These experiments are performed using the cold K-39 atomic gas ( 350 mK, >10^11 atoms/cm^3) prepared in our "dark spot" magneto-optical trap.