Laser spectroscopy of the 1001-nm ground-state transition in dysprosium

We present the first direct excitation of the presumably ultra-narrow $1001 \, \textrm{nm}$ ground state transition in atomic dysproium. By using resonance ionization spectroscopy with pulsed Ti:sapphire lasers at a hot cavity laser ion source, we were able to measure the isotopic shifts in the $1001 \, \textrm{nm}$ line between all seven stable isotopes. Furthermore, we determined the upper level energy from the atomic transition frequency of the $^{164} \textrm{Dy}$ isotope as $9991.004(1) \, \textrm{cm}^{-1}$ and confirm the level energy listed in the NIST database. Since a sufficiently narrow natural linewidth is an essential prerequisite for high precision spectroscopic investigations for fundamental questions we furthermore determined a lower limit of $2.9(1) \, \mu\textrm{s}$ for the lifetime of the excited state.