Steady-state excitation of a sodium beam using a cw laser

We demonstrate a method for maintaining a large fraction (≳17%) of an atomic sodium beam in an excited state. The method uses the Doppler shift to produce, from a single-frequency laser source, a frequency spectrum which prevents optical pumping of the atoms into either of the hyperfine levels of the ground state. A small magnetic field is used to prevent pumping into a magnetic sublevel of the ground state. The method may be used for both the P1/2 and P3/2 fine-structure levels.