Toward a cold hybrid-trap measurement of charge-exchange between Na and Ca$^+$: Na excited state fraction

We present progress towards the measurement of the charge-exchange collision rate coefficient between neutral sodium and ionic calcium. The rate constant for charge exchange between ground state sodium and calcium ion has been previously calculated and predicts a lifetime in our system of the order of days.\footnote{O.P. Makarov, et al. PRA 67, 042705 (2003).} Experiments by our group show a much larger charge exchange collision rate, probably from the excited 3P state of sodium.\footnote{W.W. Smith, et al., Applied Phys. B 114, 75 (2014).} Therefore, an accurate measurement of the charge exchange collision rate constant will require an accurate value for the excited state fraction of the Na MOT. We have developed a technique for making a model-independent measurement of the excited state fraction of a MOT inside a hybrid trap. We compare the measured excited state fraction using this technique with measurements assuming a two-level model of the atom. In addition, we review our recent measurement of the total elastic and resonant charge exchange collision rate between Na and Na$^+$.\footnote{D.S. Goodman, et al PRA 91, 012709 (2015).}