Spectroscopy of the 5s4d ¹D₂ → 5s10p ¹P₁ transition in a strontium magneto-optical trap

We perform spectroscopy of the 5s4d ¹D₂ → 5s10p ¹P₁ transition at 420 nm in a strontium magneto-optical trap (MOT). In a standard 461 nm MOT operating on the 5s^{2 1}S₀ → 5s5p ¹P₁ cooling transition, atoms gradually leak out of the nearly closed cycle via 5s5p ¹P₁ → 5s4d ¹D₂ → 5s5p ³P_1,2 decay path, limiting both MOT atom number and 461 nm fluorescence detection fidelity. Similar to recent work using the 5s4d ¹D₂ → 5s8p ¹P₁ transition at 448 nm, repumping on the 5s4d ¹D₂ → 5s10p ¹P₁ line offers a straightforward approach to address this loss channel. An additional practical advantage is the possibility of referencing the 420 nm light to the 5S_1/2 → 6P_3/2 transition in rubidium, potentially simplifying laser requirements in quantum technology applications.