Collisions of Doubly Spin-Polarized, Ultracold85RbAtoms

We study the collisions of doubly spin-polarized ${}^{85}\mathrm{Rb}$ atoms at millikelvin temperatures using photoassociation spectroscopy. Because the atoms are spin polarized, only triplet collisional states are formed. This leads to photoassociation spectra of a particularly simple form, which provide a very direct probe of the ground state collision. These spectra are analyzed to yield the ground state triplet scattering length ${-1000a}_0<a_T<{-60a}_0$ for ${}^{85}\mathrm{Rb}$, ${+85a}_0<a_T<{+140a}_0$ for ${}^{87}\mathrm{Rb}$, and the product of the $D$-line dipole matrix elements $d\left(P_{1/2}\right)d\left(P_{3/2}\right)=8.75\mathrm{}\pm 0.25$ a.u.