Two-Photon Rotational Action Spectroscopy of Cold OH- at 1 ppb Accuracy

The fundamental rotational transition J = 1 <- 0 of the anion OH- has been measured by cooling mass-selected OH- ions to 10 K in a 22-pole ion trap and applying a novel rotational-rovibrational two-photon scheme. A transition frequency of (1 123 101.0410 +/- 0.0014) MHz was obtained with so far unprecedented accuracy. The general application of the presented action-spectroscopy scheme to other anions and cations is discussed.