Photoassociative spectroscopy of a halo molecule in 86Sr

We present two-photon photoassociation to the least-bound vibrational level of the X-1 Sigma(+)(g) electronic ground state of the Sr-86(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 Sr-86 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(3)(9) 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 S-t(0)-P-3(1) potential. Large sensitivity of the dimer binding energy to light near resonant with the bound-bound transition to the intermediate state suggests that Sr-86 has great promise for manipulating atom interactions optically and probing naturally occurring Efimov states.