Driving the 5D$_{5/2}$ to 6P$_{1/2}$ transition of Barium 138 ions

We present our efforts to drive the 5D$_{5/2}$ to 6P$_{1/2}$ transition of Barium-138 ions. Ytterbium is the only element in which a similarly forbidden transition has been observed: an electric octupole transition with a subhertz linewidth connects the ground state to a metastable state [1]. In Barium, the transition connects the metastable 5D$_{5/2}$ state to the short-lived 6P$_{1/2}$ state, so the natural linewidth is much broader. Measurements were performed on a single Doppler-cooled ion in a linear Paul trap. We drive the transition with a frequency stabilized external cavity diode laser of nominal wavelength 686nm, with accuracy better than 20MHz. Detection is facilitated by a laser at 1762nm which takes the ion from the ground state to the 5D$_{5/2}$ level. To date, we have not found significant evidence of a driven transition, suggesting that it is too weak to be excited by laser intensities on the order of 10W/cm$^{2}$ over an integration times of 10s. \\[4pt] [1] M. Roberts \textit{et al}., Phys. Rev. Lett. \textbf{78,} 1876 (1997).