  We investigate the excitation of the 5D_{5/2} level in Rb atoms using
counter-propagating laser beams, which are nearly resonant to the one-photon
5S_{1/2} - 5P_{3/2} and 5P_{3/2} - 5D_{5/2} transitions, ensuring that a sum of
the optical frequencies corresponds to the 5S_{1/2} - 5D_{5/2} transition. The
excitation produced by two-photon and step-wise processes is detected via
spontaneously emitted fluorescence at 420 nm arising from the 6P_{3/2} -
5S_{1/2} transition. The dependences of blue fluorescence intensity on atomic
density and laser detuning from the intermediate 5P_{3/2} level have been
investigated. The sensitivity of the frequency detuned bi-chromatic scheme for
atom detection has been estimated. A novel method for sum frequency
stabilization of two free-running lasers has been suggested and implemented
using two-photon Doppler-free fluorescence and polarization resonances.
