Photoassociative Spectroscopy of a Halo Molecule in ${}^{86}$Sr

Abstract

We present two-photon photoassociation to the least-bound vibrational level of the X${}^1{\Sigma }_g^{+}$ electronic ground state of the ${}^{86}$Sr${}_2$ 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 $a=810.6(12)$\,$a_0$, which is consistent with, but substantially more accurate than the previously determined $a=798(12)a_0$ found from mass-scaling and precision spectroscopy of other Sr isotopes. For the intermediate state, we use a bound level on the metastable ${}^1{S}_0-{}^3{P}_1$ 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.

10 pages, 9 figures