  We observe the formation of long-range Cs$_2$ Rydberg molecules consisting of
a Rydberg and a ground-state atom by photoassociation spectroscopy in an
ultracold Cs gas near 6s$_{1/2}$($F$=3,4)$\rightarrow$np$_{3/2}$ resonances
(n=26-34). The spectra reveal two types of molecular states recently predicted
by D. A. Anderson, S. A. Miller, and G. Raithel [Phys. Rev. A 90, 062518
(2014)]: states bound purely by triplet s-wave scattering with binding energies
ranging from 400 MHz at n=26 to 80 MHz at n=34, and states bound by mixed
singlet-triplet s-wave scattering with smaller and F-dependent binding
energies. The experimental observations are accounted for by an effective
Hamiltonian including s-wave scattering pseudopotentials, the hyperfine
interaction of the ground-state atom, and the spin-orbit interaction of the
Rydberg atom. The analysis enabled the characterization of the role of singlet
scattering in the formation of long-range Rydberg molecules and the
determination of an effective singlet s-wave scattering length for low-energy
electron-Cs collisions.
