Millimeter Wave Spectroscopy of cold Rb Rydberg atoms

By using the cold $^{85}$Rb Rydberg atoms in a magneto optical trap, we have measured the single photon $^{85}$Rb \textit{nd}-($n$-2)$f$ millimeter wave transitions for 32$\le n\le $39. The measurements were carried out at densities of 10$^{9}$ atoms/cm$^{-3}$, roughly five orders of magnitude lower than those used in optical measurements. Since the 10 G/cm gradient of the trap magnetic field would result in 5 MHz wide resonances, we switched off the field during the measurements. The observed narrow resonances will be used to improve the accuracy of the $f$ quantum defect of $^{85}$Rb.