Abstract

The collisional interactions between the atoms in an cold, dense atomic gas play a crucial role in determining the dynamics and stability of the gas and the approach to Bose-Einstein condensation. The evaporative cooling rate, the rate of collisional loss from a trap, and the sign of the scattering length are all important consequences of such collisional interactions. The magnitudes of the elastic and inelastic collisional rate coefficients are strongly affected by the quantum properties of the scattering wave function near zero collision energy, where the de Broglie wavelength greatly exceeds the typical range of the interatomic interaction potential. Although the basic theory of these quantum threshold properties was known as early as the 1930s, the magnitudes of the rate coefficients needs to be worked out for the specific alkali systems currently being studied. This talk will give simple interpretive physical models of threshold scattering, backed up by full quantum scattering calculations for collisions between extremely cold Na atoms or Kb atoms. The ground state scattering wave function near zero energy is extremely sensitive to small uncertainties in the interatomic interaction potential. Fortunately, the ground state scattering wave function and collision rates are strongly constrained by the detailed information provided by experimental high-resolution photoassociation spectroscopy for these species.^1-3 Our calculations, based on a quantum mechanical description of the ground and excited molecular states of the diatom system, including all electron and nuclear spin interactions, give synthetic photoassociation spectra in excellent agreement with recent experimental measurements for trapped Na. We discuss the constraints provided on ground state scattering properties by current data, and make suggestions where additional work is needed. Knowledge of the ground state wave function also permits simple estimation of the rate of atom loss from the trap resulting from light scattering during binary collisions if laser light is incident on the gas. For atomic densities typical of current BEC experiments, the collisional loss resulting from binary collisions will generally exceed that resulting from off-resonant atomic light scattering for wide ranges of detuning to the red or blue of resonance.

© 1996 Optical Society of America