Subthermal linewidths in photoassociation spectra of cold alkaline earth atoms

Narrow s-wave features with subthermal widths are predicted for the ^1Pi_g photoassociation spectra of cold alkaline earth atoms. The phenomenon is explained by numerical and analytical calculations. These show that only a small subthermal range of collision energies near threshold contributes to the s-wave features that are excited when the atoms are very far apart. The resonances survive thermal averaging, and may be detectable for Ca cooled near the Doppler cooling temperature of the 4^1P <- 4^1S laser cooling transition.