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

Abstract

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 ${}^{23}\mathrm{Na}$ atoms. Large exchange relaxation coefficients for ${}^{23}\mathrm{Na}$ as compared to ${}^{87}\mathrm{Rb} F=2$ suggest that evaporative cooling of (2,2) Na will be more difficult than for the corresponding state of Rb.