Spontaneous Raman Processes in Multiphoton Excitation of Sodium

We have observed spontaneous Raman processes in a sodium atomic beam. When one sodium $3p{}_{}{}^2{}_{}{}^{}P$ fine-structure level is strongly excited by a dye laser, the other level, is weakly populated by spontaneous Raman emission and the absorption of another laser photon. This effect is detected by further stepwise excitation plus photoionization. The observed magnitude agrees well with theory. This technique is much more sensitive than methods involving direct observation of the Raman emission.