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 600--2400 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.