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Na atoms in a buffer gas are compressed to a steep front by a laser beam tuned into the red Doppler wing of the Na D transition. The Na densities on either side of the front differ by a factor of 500. The front formation is due to a laser-induced drift which is a consequence of the state dependence of the diffusion cross sections involved. The Na drift velocity is deduced to be 4 m/s.
Werij et al. (Mon,) studied this question.