We demonstrate a technique for frequency measurements of UV transitions with sub-MHz precision. The frequency is measured using a ring-cavity resonator whose length is calibrated against a reference laser locked to the $D_2$ line of $^{ 87}$Rb. We have used this to measure the 398.8 nm ${ ^1S}_0 \leftrightarrow { ^1P}_1$ line of atomic Yb. We report isotope shifts of all the seven stable isotopes, including the rarest isotope $^{ 168}$Yb. We have been able to resolve the overlapping $^{ 173}$Yb($F = 3/2$) and $^{ 172}$Yb transitions for the first time. We also obtain high-precision measurements of excited-state hyperfine structure in the odd isotopes, $^{ 171}$Yb and $^{ 173}$Yb. The measurements resolve several discrepancies among earlier measurements.
Categories: physics.atom-ph physics.optics