Millimeter-wave spectroscopy of cold Rb Rydberg atoms in a magneto-optical trap: Quantum defects of the ns, np, and nd series

By using a magneto-optical trap we have measured the Rb ns-(n+1)s and nd_j-(n+1)d_j two-photon millimeter-wave transitions for 32<~n<~37, observing 100-kHz-wide resonances, in spite of the trap’s 10 G/cm magnetic-field gradient, in which one might expect to observe resonances 5 MHz wide. This resolution is possible because of the similarity of the g_j factors in the initial and final states. Under the same conditions, the single-photon ns-np resonances are ∼5 MHz wide. To make useful measurements of these intervals, we turned off the trap field and used the 300-K atoms of the background Rb vapor. Together these measurements improve the accuracy of the s, p, and d quantum defects by an order of magnitude.