  We experimentally demonstrate a one-color two-photon transition from the
$5\mathit{S}_{1/2}$ ground state to the $6\mathit{S}_{1/2}$ excited state in
rubidium (Rb) vapor using a continuous wave laser at 993 nm. The Rb vapor
contains both isotopes ($^{85}$Rb and $^{87}$Rb) in their natural abundances.
The electric dipole allowed transitions are characterized by varying the power
and polarization of the excitation laser. Since the optical setup is relatively
simple, and the energies of the allowed levels are impervious to stray magnetic
fields, this is an attractive choice for a frequency reference at 993 nm, with
possible applications in precision measurements and quantum information
processing.
