Raman transitions between discrete and continuum atomic states
in an operating magneto-optical trap

Tomasz M. Brzozowski & Maria Brzozowska

Department of Photonics, Instiute of Physics, Jagiellonian University Reymonta 4, 30-059 Cracow, Poland

We report on pump-probe measurements of absorption and four-wave-mixing spectra in a 85Rb magneto-optical trap taken while the trap is in operation. The trapping beams are used as pump light. We concentrate on the central feature of the spectra at small pump-probe detuning and attribute its narrow resonant structures to the superposition of Raman transitions between light-shifted sublevels of the ground atomic state and to atomic recoil processes. These two contributions have different dependencies on trap parameters. The strong dependence of the spectra on the probe-beam polarization indicates the existence of large optical anisotropy of the cold-atom sample, which is attributed to the recoil effects. We point out that the recoil-induced resonances can be isolated from other contributions, making pump-probe spectroscopy a highly sensitive diagnostic tool for atoms in a working MOT.