Sub-natural linewidth laser spectroscopy of Doppler-free two-photon resonances

Precision tuning of a narrow-band cw dye laser allows to determine effects that normally are masked by the natural width of short-lived atomic states. With the 15 kHz accuracy of our laser system we were able to detect small frequency shifts between the components of the 3 s → 4 d two-photon transition of sodium which are due to the 4d hyperfine splitting. The A -factors of the 23 Na 4d states were determined: A( 2 D 3 2 ) = + 215 (15) kHz , A( 2 D 5 2 ) = + 29 (6) kHz .