Measurement of holmium Rydberg series through magneto-optical trap depletion spectroscopy

We report measurements of the absolute excitation frequencies of Ho-165 4f(11)6sns and 4f(11)6snd odd-parity Rydberg series. The states are detected through depletion of a magneto-optical trap via a two-photon excitation scheme. Measurements of 162 Rydberg levels in the range n = 40-101 yield quantum defects well described by the Rydberg-Ritz formula. We observe a strong perturbation in the ns series around n = 51 due to an unidentified interloper at 48515.47(4) cm(-1). From the series convergence, we determine the first ionization potential E-IP = 48565.910(3) cm(-1), which is three orders of magnitude more accurate than previous work. This work is an important step towards using Ho atoms for collective encoding of a quantum register.