Non-Maxwellian anisotropic velocity distribution of metastable argon atoms in a thin discharge cell

For an argon glow discharge in a 1 mm thick cell at room temperature, we have observed the absorption spectrum of the 1s3–2p2 (Paschen notation) line by using a diode laser. It is found that, with a decrease in the gas pressure below 0.1 Torr, the linewidth drastically decreases; at 0.02 Torr the velocity distribution of the metastable atoms in the surface normal direction has an apparent temperature of 144 K. The velocity distribution is also found to be anisotropic: the spectrum profile depends on the direction of observation with respect to the surface normal. These phenomena are explained by a model in which the velocity distribution of the metastable atoms is controlled by collisional quenching at the wall surface and velocity-changing collisions with ground state atoms.