University of Waterloo
200 University Avenue West
Waterloo, Ontario, Canada N2L 3G1
Phone: (519) 888-4567 ext 32215
Fax: (519) 746-8115
Professor Brett DePaola,
Department of Physics and J. R. MacDonald
Kansas State University
In many experiments it is important to know the populations of particular states. For example, in multi-level coherent excitation experiments it would extremely interesting to be able to directly measure the temporal evolution of the populations of all the levels involved in the coherent excitation – and on a time scale that is fast compared with the radiative lifetimes of the states. Pump-probe experiments work well, but the results can be difficult to interpret, especially if many states are involved.
To address this problem, we have developed a technique known as MOTRIMS (magneto-optical trap, recoil ion spectroscopy) wherein an ion beam is made to be incident on the optically excited target. Following charge transfer between the target atoms and the incident ions, the momentum vectors of the ionized target atoms are measured, particle by particle. The momentum “kick” given to the target ions during the ionization process uniquely identifies both the state of the atom at the time of ionization, and the state of the projectile after it has captured an electron from the target. Because charge transfer is an extremely fast process, the temporal resolution can be quite high. Furthermore, MOTRIMS can be used with “slow” quasi-cw lasers or ultra-fast pulsed optical systems.In this talk we focus on a very simple application of this methodology: In many experiments involving magneto-optical traps (MOTs), it is imperative to know the fraction of atoms left in an excited state by the cooling and trapping lasers. In most cases, researchers have used formulas that were derived for simple 2-level systems interacting with a single beam of light having a well-defined polarization, and in the absence of magnetic or electric fields. In fact a MOT environment is much more complex than this. We describe the use of MOTRIMS to directly measure the excited fraction in a MOT of 85Rb atoms in a model-independent manner for a wide range of trapping conditions. We then fit our measured fractions to an ansatz based on a simple model. Knowing only the trapping laser's total intensity and detuning from resonance, one can then use this ansatz to accurately predict the excited fraction. While this is a very simple excitation process, it is illustrative of the sort of diagnostic power that the MOTRIMS methodology is capable of.