Fellesseminar: Hvordan likevekt oppstår mellom en gass og veggene omkring

J. R. Manson,  Department of Physics and Astronomy, Clemson University,  South Carolina, USA

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How a Gas Comes into Equilibrium with the Surface that Contains it

Sammendrag:

James Clerk Maxwell worked for many years on the problem of explaining how a gas of ideal particles could ever come into thermal equilibrium with the walls of its container.  His solution was to make the simple assumption that when an impinging gas beam is scattered from a surface it can be divided into a part that instantly reflects and exchanges no energy and the remaining part that adsorbs on the walls.  This second part is then assumed to equilibrate or accommodate completely at the wall surface and subsequently desorbs with an equilibrium distribution.  Modern experiments using well-defined beams of neutral atomic particles colliding with surfaces have essentially confirmed the basic tenants of the Maxwell hypothesis.  The actual situation is more complex, but when gas atoms collide with clean surfaces the scattering spectra often exhibits two clearly distinct features, the first due to direct scattering and the second due to trapping in the physisorption well with subsequent desorption.  In this talk a scattering theory is presented that uses an iterative algorithm and classical mechanics for the collision in order to describe both direct scattering and trapping-desorption of the incident beam.  Those particles that are initially trapped can be followed as they continue to make further interactions with the surface until they are all eventually promoted back into the positive energy continuum and leave the surface region.  Consequently, this theory allows a rigorous test of the Maxwell assumption and determines the conditions under which it is valid.  The theory also gives good quantitative explanations of recent experimental measurements [S. J. Sibener et al., J. Chem. Phys. 119, 13083 (2003) ] which clearly exhibit both a direct scattering contribution and a trapping-desorption fraction in the energy-resolved spectra.

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