Optical Pumping of Samarium Atoms in a Bichromatic Laser Field in the Presence of Velocity-Changing Collisions

Dark resonances in the ¹⁵⁴Sm -system 4f ⁶6s ²(⁷ F ₀) 4f ⁶6s6p(⁹ F ₁ ⁰) 4f ⁶ s ²(⁷ F ₁) are observed alongside the velocity selective optical pumping. The shape of the resulting spectra strongly depended on the buffer gas (He, Ar) pressure due to velocity-changing collisions (VCC): the sign of the effect could be reversed from the dark to the bright resonance. The observed spectra are interpreted within the framework of the hard-sphere collision model. The role of VCC in the formation of the dark state in the -system is discussed.