Line shapes of singlet-state photoassociation spectra of trapped strontium atoms near the photon-recoil temperature

We report studies of spectral line shapes of ultracold (T = 1 muK) Sr-88 in the presence of a laser beam tuned to the red of the S-1-P-1 transition, as functions of laser intensity and polarization, using the relevant currently available potential curves. These studies predict that, at high enough intensities of the probe laser, for temperatures near the recoil limit, an extra, in principle unexpected, peak appears on the red side of each line in the photoassociation spectrum. We show that the appearance of this extra peak is due to a near-zero-energy resonance of the dressed ground state. We also emphasize that the correct relative position of this peak can only be calculated if multichannel effects are taken into account.