Photoassociative spectroscopy at long range in ultracold strontium

We report photoassociative spectroscopy of ⁸⁸Sr₂ in a magneto-optical trap operating on the ¹S₀ → ³P₁ intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 6002400 MHz from the ¹S₀ → ¹P₁ atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the ¹P₁ atomic lifetime (τ = 5.22±0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.