Photoassociation spectra and the validity of the dipole approximation for weakly bound dimers

Photoassociation (PA) of ultracold metastable helium to the 2s2p manifold is theoretically investigated using a nonperturbative close-coupled treatment in which the laser coupling is evaluated without assuming the dipole approximation. The results are compared with our previous study [D.G. Cocks and I. B. Whittingham, Phys. Rev. A 80, 023417 (2009)], which makes use of the dipole approximation. The approximation is found to strongly affect the PA spectra because the photoassociated levels are weakly bound, and a similar impact is predicted to occur in other systems of a weakly bound nature. The inclusion of the approximation does not affect the resonance positions or widths; however, significant differences are observed in the background of the spectra and the maximum laser intensity at which resonances are discernible. Couplings not satisfying the dipole selection rule vertical bar J-1 vertical bar <= J' <= vertical bar J + 1 vertical bar do not lead to observable resonances.