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

TL;DR

This study investigates the validity of the dipole approximation in photoassociation spectra of ultracold helium dimers, revealing significant effects on spectral background and intensity thresholds but not on resonance positions or widths.

Contribution

It provides a non-perturbative analysis without the dipole approximation, highlighting its impact on weakly bound systems and challenging previous assumptions.

Findings

Dipole approximation significantly affects spectral background.

Resonance positions and widths are unaffected by the approximation.

Non-dipole couplings outside the selection rule do not produce observable resonances.

Abstract

Photoassociation (PA) of ultracold metastable helium to the 2s2p manifold is theoretically investigated using a non-perturbative close-coupled treatment in which the laser coupling is evaluated without assuming the dipole approximation. The results are compared with our previous study [Cocks and Whittingham, Phys. Rev. A 80, 023417 (2009)] that 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 or not 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 discernable. Couplings not satisfying the dipole selection rule |J-1| <= J' <= |J+1| do not lead to observable resonances.