Level-crossing under nearly monochromatic excitation by a narrow-band cw dye laser

The shape of level-crossing signals under excitation by a narrow-band tunable dye laser has been investigated. The light of the laser was scattered on a highly collimated atomic beam of Na. The laser radiation (bandwidth about 3 MHz) was fairly monochromatic compared with the natural line-width of the 3 ²P_{$\text{3}\text{2}$} state (10 MHz). The form of the zero-field level-crossing signal of this state depends strongly on the frequency of the exciting radiation. The experimental results agree well with calculated signal shapes.