Observation of the 4f¹⁴6s² ¹S₀-4f¹³5d6s²(J=2) transition at 431 nm in ytterbium

The 4f¹⁴6s^{2 1}S₀-4f¹³5d6s²(J=2) transition in ytterbium (Yb) at 431 nm is theoretically predicted as a narrow-linewidth transition that has high sensitivity to the variation of the fine structure constant, and experimental searches for this transition were ongoing. We observed this 431 nm transition in ¹⁷¹Yb atoms laser-cooled by a magneto-optical trap formed with the 6s^{2 1}S⁰-6s6p³P₁ intercombination transition at 556 nm. Some initial characterizations of the transition were performed. Presicion spectroscopy of this transition opens ways to new physics searches with isotope shift measurements and a search for the time-variation of the fine structure constant.