Temperature effects in the hyperfine spectrum of the D1 transition of sodium

The velocity selective optical pumping technique has been used to obtain th e Doppler-free spectrum of the D1 transition of sodium in the temperature r ange 110-169 degrees C. The pump-induced changes in probe transmission acro ss the Lamb dip and crossover resonances first increase and then decrease w ith temperature. We also observe a reversal in the relative magnitudes of t hese changes among some of the narrow resonances. These and other temperatu re-dependent features of the Doppler-free spectrum of the D1 transition of sodium are successfully explained within the framework of the composite fiv e-level model. The degeneracies of the ground-state composite levels and th e beam depletion effects at higher temperatures play a crucial role in thes e investigations. [S1050-2947(98)05011-2].