Direct Measurement of Excited State Fractions in a MOT

Accurate measurement of excited state fraction is critical in measurements of photo-ionization and electron impact cross sections, and the rapidly growing field of cold and ultra-cold collision cross section measurements. Many groups have reported that a large source of uncertainty in their results comes from the fluorescence-based measurement of the MOT excited state fraction. In this work we report the results of directly measured in-MOT excited state fractions as a function of trapping laser intensity and detuning. The experimental approach is based on the MOTRIMS methodology. Here instead of looking at the fluorescence, charge transfer to a beam of 7 keV Na$^{+}$ is used as a probe. The results are compared with theoretical models.