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We present two-photon photoassociation to the least-bound vibrational level of the X 1 Σ + g electronic ground state of the 86 Sr2 dimer and measure a binding energy of E b = −83.00(7)(20) kHz. Because of the very small binding energy, this is a halo state corresponding to the scattering resonance for two 86 Sr atoms at low temperature. The measured binding energy, combined with universal theory for a very weakly bound state on a potential that asymptotes to a van der Waals form, is used to determine an s-wave scattering length a = 810.6(12) a0, which is consistent with, but substantially more accurate than the previously determined a = 798(12) a0 found from mass-scaling and precision spectroscopy of other Sr isotopes. For the intermediate state, we use a bound level on the metastable 1 S0 − 3 P1 potential. Large sensitivity of the dimer binding energy to light near-resonant with the bound-bound transition to the intermediate state suggests that 86 Sr has great promise for manipulating atom interactions optically and probing naturally occurring Efimov states. arXiv:1809.09267v1 [physics.atom-ph]

1896The probability of spin exchange in the afterglow period may be somewhat greater than in an active discharge. However, the question is probably academic because there can be little doubt that the only electronproducing reaction in the afterglow (He +He He+He' +e ) leaves the typical electron already spin polarized.

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We investigate two-frequency photoassociation of a weakly bound molecular state, focusing on a regime where the AC Stark shift is comparable to the halo-state energy. In this "high-intensity" regime, we observe features absent in low-intensity two-frequency photoassociation. We experimentally measure the spectra of 86 Sr atoms coupled to the least bound state of the 86 Sr2 ground electronic channel through an intermediate electronically excited molecular state. We compare the spectra to a simple three-level model that includes a two-frequency drive on each leg of the transition. With numerical solution of the time-dependent Schrödinger equation, we show that this model accurately captures (1) the existence of experimentally observed satellite peaks that arise from nonlinear processes, (2) the locations of the two-photon peak in the spectrum, including AC Stark shifts, and (3) in some cases, spectral lineshapes. To better understand these numerical results, we develop an approximate treatment of this model, based on Floquet and perturbation theory, that gives simple formulas that accurately capture the halo-state energies. We expect these expressions to be valuable tools to analyze and guide future two-frequency photoassociation experiments. arXiv:1812.11682v2 [cond-mat.quant-gas]