We report the measurements of the temperature dependence of the cross section σ 4D for the energy pooling process Na(3P) + Na(3P) → Na(4D) + Na(3S). The latest two, as yet undisputed, results for σ 4D obtained by different authors at T = 597 K and T = 483 K suggest that this cross section decreases with increasing T , which contradicts the theory and other experiments on similar processes. To resolve this controversy and to examine the temperature trend of the cross section, we have measured

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... σ 4D in the temperature range 567-705 K, covering the hightemperature region that has not yet been investigated experimentally. To determine σ 4D we have excited sodium atoms in the quasistatic wing of the D1 line using a cw dye laser and measured the fluorescence intensity for the 4D → 3P 3/2 transition, relative to the intensity of the optically thin quasistatic wing of the D2 line. The spatial distribution of the number density of the sodium atoms in the 3P 3/2 state and the sodium ground-state number density were measured too. The method used for the determination of the cross section is advantageous since it entirely circumvents the need to account for the radiation trapping of 3P level radiation, which was substantial under experimental conditions of the ground-state densities being 10 14 -10 16 cm −3 . The measurements of the cross section σ 4D in the investigated temperature range have shown that it increases as ∼ exp(− E/kT ). From the experiment we obtained E = (608 ± 95) cm −1 , which is in excellent agreement with the energy defect (613 cm −1 ) for the considered process, and in fair agreement with the values which follow from recent theoretical calculations.