Nonadiabatic dynamics in evaporative cooling of trapped atoms by a radio-frequency field

Magnetically trapped neutral atoms can be cooled with the evaporation technique. This is typically done by using a radio-frequency (rf) field that adiabatically couples trapped and untrapped internal atomic states for atoms with kinetic energies above a value set by the field frequency. The rf field can also induce nonadiabatic changes of internal atomic spin states (F,M) that lead to heating and enhanced loss of atoms. In this paper we use wave-packet simulations to show that the evaporation process can induce these nonadiabatic transitions, which change the internal spin state of doubly spin-polarized (2,2) trapped atoms. We also verify the validity of a multistate Landau-Zener model in describing the nonadiabatic dynamics. In addition, we calculate exchange relaxation rate coefficients for collisions between atoms in the (2,M) states of Na-23 atoms. Large exchange relaxation coefficients for Na-23 as compared to Rb-87 F=2 suggest that evaporative cooling of (2,2) Na will be more difficult than for the corresponding state of Rb. [S1050-2947(98)07811-1].