Laser Intensity Dependence of Photoassociation in Ultracold Metastable Helium

TL;DR

This study investigates how laser intensity affects photoassociation in ultracold metastable helium, using both perturbative and non-perturbative methods to analyze resonance profiles and line shifts.

Contribution

It introduces a comprehensive non-perturbative approach for arbitrary laser intensities and compares it with perturbative results, revealing the intensity dependence of photoassociation resonances.

Findings

Line shifts depend nearly linearly and quadratically on laser intensity for the two lowest levels.

Resonance profiles are superimposed on a significant background loss due to the shallow excited state potential.

Perturbative and non-perturbative results agree closely at low laser intensities.

Abstract

Photoassociation of spin-polarized metastable helium to the three lowest rovibrational levels of the J=1, $0_u^+$ state asymptoting to 2$s {}^{3}$S$_{1}+2p {}^{3}$P$_{0}$ is studied using a second-order perturbative treatment of the line shifts valid for low laser intensities, and two variants of a non-perturbative close-coupled treatment, one based upon dressed states of the matter plus laser system, and the other on a modified radiative coupling which vanishes asymptotically, thus simulating experimental conditions. These non-perturbative treatments are valid for arbitrary laser intensities and yield the complete photoassociation resonance profile. Both variants give nearly identical results for the line shifts and widths of the resonances and show that their dependence upon laser intensity is very close to linear and quadratic respectively for the two lowest levels. The resonance profiles are superimposed upon a significant background loss, a feature for this metastable helium system not present in studies of photoassociation in other systems, which is due to the very shallow nature of the excited state $0_u^+$ potential. The results for the line shifts from the close-coupled and perturbative calculations agree very closely at low laser intensities.