  We establish a theoretical model to analyze the photoassociative (PA)
spectroscopy of $(3)^3\Sigma^+$ state of $^{85}$Rb$^{133}$Cs molecule. The term
energy, spin-spin coupling constants and hyperfine interaction constant of nine
vibrational levels in the $(3)^3\Sigma^+$ state are determined. Based on the
fitted term energy and rotation constant, the potential energy curve of
$(3)^3\Sigma^+$ state is obtained by using the RKR method which is compared
with the ab initial potential curve.
