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We have used the resonance radiation pressure from 40 mW of cw dye laser light propagating axially down a tube filled with sodium vapor to increase the sodium pressure (density) up to 50% over a length of 20 cm. The magnitude of the effect agrees well with measurements of the absorbed power. These observations were made with the laser tuned to the low-frequency side of the sodium D2 resonance line; for tuning to the high-frequency side, an additional unexplained effect leading to density decreases was observed.
Bjorkholm et al. (Sat,) studied this question.