The energy relaxation of fast atoms moving in a thermal bath gas is explored theoretically. We found that two time scales characterize the equilibration, one a short time, in which the isotropic energy distribution profile relaxes to a Maxwellian shape at some intermediate effective temperature, and the second, a longer time in which the relaxation preserves a Maxwellian distribution and its effective temperature decreases continuously to the bath gas temperature. It is shown that the formation and preservation of a Maxwellian distribution does not depend on the projectile to bath gas atom mass ratio, contrary to predictions of the hard-sphere model. This two-stage behavior is universal. It arises due to the dominance of small angle scattering and small energy transfer in the collisions of neutral particles and reflects a fundamental property of long-range atomic forces. The Boltzmann equation is solved numerically for nitrogen in He and in Ar. The solutions are in close agreement with the experimental measurements of the evolving Doppler profiles of emission from excited initially energetic N atoms traversing bath gases of helium and argon. Our investigation provides the first experimental and theoretical evidence of the formation and preservation of hot Maxwell distributions with a time-dependent effective temperature in actual atomic gases.
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