Spectroscopy of a narrow-line optical pumping transition in dysprosium

We present measurements of the hyperfine coefficients and isotope shifts of the Dy I $683.731$nm transition, using saturated absorption spectroscopy on an atomic beam. A King Plot is drawn resulting in an updated value for the specific mass shift $\delta \nu_$684,\mathrm{s}\mathrm{m}\mathrm{s}$^$164-162$=-534 \pm 17 MHz$\mathrm{.}Usingfluorescencespectroscopywemeasuretheexcitedstatelifetime$\tau_{684}=1.68(5) \mu$s,yieldingalinewidthof$\gamma_$684$ = 95 \pm 3 kHz$. We give an upper limit to the branching ratio between the two decay channels from the excited state showing that this transition is useable for optical pumping into a dark state and demagnetization cooling.