Single-atom detection by SONRES

The new method of atomic fluorescence detection, called saturated optical nonresonant emission spectroscopy (SONRES), has been modeled for a three-level atom, and experiments on sodium have been conducted that support the model. A rate equation analysis yielded expressions for excited-state atomic populations and saturation intensity. The detection of sodium in buffer gases that promote collisional transfer of excitation between 3²P1/2→ 3²p3/2both without quenching the fluorescence emission and with quenching was considered. Experimental results are presented for sodium in argon. At -25°C, approximately 180 atoms/cm³were monitored with a S/N of ∼ 15 representing detection at the level of one part in 10¹⁷. The signal at this temperature was generated by less than a single atom in the laser beam.