Narrow-line photoassociation spectroscopy and mass-scaling of bosonic strontium

Abstract: Using new experimental measurements of photoassociation resonances near the $^1\mathrm{S}_0 \rightarrow \phantom{ }^3\mathrm{P}_1$ intercombination transition in $^{84}$Sr and $^{86}$Sr, we present an updated study into the mass-scaling behavior of bosonic strontium dimers. A previous mass-scaling model [Borkowski et al., Phys. Rev. A 90, 032713 (2014)] was able to incorporate a large number of photoassociation resonances for $^{88}$Sr, but at the time only a handful of resonances close to the dissociation limit were known for $^{84}$Sr and $^{86}$Sr. In this work, we perform a more thorough measurement of $^{84}$Sr and $^{86}$Sr bound states, identifying multiple new resonances at deeper binding energies out to $E/h=-5$ GHz. We also identify several previously measured resonances that cannot be experimentally reproduced and provide alternative binding energies instead. With this improved spectrum, we develop a mass-scaled model that reproduces the observed binding energies of $^{86}$Sr and $^{88}$Sr to within 1 MHz. In order to accurately reproduce the deeper bound states, our model includes a second $1_u$ channel and more faithfully reproduces the depth of the potential. As determined by the previous mass-scaling study, $^{84}$Sr $0_u^+$ levels are strongly perturbed by the avoided crossing between the $^1\mathrm{S}_0 + \phantom{ }^3\mathrm{P}_1$ $0_u^+$ $(^3\Pi_u)$ and $^1\mathrm{S}_0 + \phantom{ }^1\mathrm{D}_2$ $0_u^+$ $(^1\Sigma_u^+)$ potential curves and therefore are not included in this mass-scaled model, but are accurately reproduced using an isotope-specific model with slightly different quantum defect parameters. In addition, the optical lengths of the $^{84}$Sr $0_u^+,\ \nu=-2$ to $\nu=-5$ states are measured and compared to numerical estimates to characterize their use as optical Feshbach resonances.