Measurement of collisions between laser-cooled cesium atoms and trapped cesium ions

We report the measurement of collision rate coefficient for collisions between ultracold Cs atoms and low-energy C s⁺ ions. The experiments are performed in a hybrid trap consisting of a magneto-optical trap (MOT) for Cs atoms and a Paul trap for C s⁺ ions. The ion-atom collisions impart kinetic energy to the ultracold Cs atoms, resulting in their escape from the shallow MOT and, therefore, in a reduction in the number of Cs atoms in the MOT. By monitoring, using fluorescence measurements, the Cs atom number, and the MOT loading dynamics and then fitting the data to a rate equation model, the ion-atom collision rate is derived. The Cs -C s⁺ collision rate coefficient 9.3 (±0.4 ) (±1.2 ) (±3.5 ) ×10^-14m³s^-1 , measured for an ion distribution with most probable collision energy of 95 meV (≈k_B1100 K ) , is in fair agreement with theoretical calculations. As an intermediate step, we also determine the photoionization cross section of Cs 6 P_{3 /2} atoms at 473 nm wavelength to be 2.28 (±0.33 ) ×10^-21m² .