Experimental investigation of the 6s2 1S0->5d6s 3D1,2 forbiddentransitions in atomic ytterbium

We have observed the Stark-induced ${6s}^{2}{}^{1}{S}_{0}\ensuremath{\rightarrow}5d6s{}^{3}{D}_{1}$ transition in Yb, and measured properties relevant to the study of atomic parity nonconservation in this transition. An atomic beam of Yb is excited by 408-nm light in the presence of an external dc electric field, resulting in cascade fluorescence at 556 nm as the excitation decays through the $6s6p{}^{3}{P}_{1}$ state. The density of the atomic beam and the fluorescence detection efficiency are calibrated by directly exciting the $6s6p{}^{3}{P}_{1}$ state with 556-nm light. Using an estimate of the ${}^{3}{D}_{1}{\ensuremath{-}}^{3}{P}_{1}$ branching fraction, we obtain a measurement of the vector transition polarizability, $|\ensuremath{\beta}|=2.18(33)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8} {\mathrm{ea}}_{0}/(\mathrm{V}/\mathrm{cm}).$ In addition, we have observed the ${6s}^{2}{}^{1}{S}_{0}\ensuremath{\rightarrow}5d6s{}^{3}{D}_{2}$ transition at 404 nm. The electric quadrupole transition amplitude and tensor transition polarizability have been measured. We have also measured the dc Stark shifts, hyperfine structure, and isotope shifts for both transitions.