Abstract

Ultracold temperatures in the 10⁻⁵–10⁻³-K range are now possible using optical cooling methods, and collision processes, such as the associative ionization of two excited Na atoms, have been reported to occur in atom traps.¹ Such collisions are uniquely sensitive to the influence of radiative interactions, since the atom–field interaction energy, E d, can be very large compared to kT and since the collision time can be long compared to an excited-state spontaneous emission lifetime. Ultracold collisions offer the prospect of manipulating collision rate coefficients by varying laser parameters. Some of the novel qualitative features of ultracold excited-state collisions can be explained using a simple model which accounts simultaneously for the interaction of each atom with the radiation field and for the resonant dipole–dipole interaction between the two atoms. The nature of the collision dynamics changes between two zones of interatomic distance R. In the outer zone, R ≫ R_t, where R_t, is the zonal boundary, the atoms are far apart and field dressed by their strong interaction with the radiation field. In the inner zone, R ≪ R_t, the atoms are strongly detuned from the radiation field by the R-dependent molecular interactions and are best described as a bare molecule. The effective long-range potential between the two atoms in the outer zone is modified by field dressing. In addition, the absorption and emission of radiation by the pair of atoms is strongly correlated as long as R < c/ω. Only for R ≫ c/ω does the effect of retardation ensure that the two atoms behave independently. It is only in the outer zone that the radiation field can produce an excited atom. In the inner molecular zone, the atoms can no longer be excited by the radiation field, and spontaneous emission causes a decrease in the effective rate coefficients for any process which requires having an excited-state atom. A consequence of this picture is that the effective associative ionization cross section for a collision of two excited-state sodium atoms is predicted to be much larger in an intense laser optical trap than in the weak laser conditions of optical molasses. It may be possible to observe the transition between normal and ultracold collision dynamics as a function of collision energy by using cold atomic beams instead of atom traps. The nature of the quantum threshold behavior of an ultracold collision cross section is also considered.

© 1989 Optical Society of America