Solitary resonance in the velocity-selective magnetic-optical activity spectrum of the ^87Rb D_2 line

We report the velocity-selective magnetic-optical activity of a (87)Rb vapor when the counterpropagating pump and probe beams are linearly cross polarized. A semiconductor laser tuned to the D(2) line (780 nm) is used to perform the experiment. When a weak axial magnetic field is applied to the vapor, the magnetic-optical activity spectrum reveals a narrow dominant peak, the solitary resonance, corresponding to the cycling transition 5S((1/2)), F = 2 ?5P(3/2), F = 3. The light-shift origin of this spectrum is discussed.