Velocity distribution of atoms releasing from a surface

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Main Question or Discussion Point

I have a surface (paraffin) and a vapor of atoms (alkali). When atom get the wall, it is sticked for some time, exchange it energy and momentum with wall and then release. I want to know the atomic velocity distribution (both in directions orthogonal and parallel to the surface) after atom releases.

I guess the velocity distribution in direction parallel to the surface should be Maxwellian. But what about the orthogonal direction? More common, will atoms go out preferably in direction orthogonal to the surface, or preferably along the surface, with small angles between their velocity vectors and surface?

If somebody know something about this topic (ideas, references et.c) --- could you help me?
 

Answers and Replies

  • #2
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Simplest model I can think of: treat the surface as requiring some definite energy to escape, i.e. a work function W, analogously with electron emission. Assume that the initial velocity whilst at the bottom of the well is Maxwell-Boltzmann. As you said, the parallel directions are not effected, i.e. we assume the potential is translationally symmetric along the surface. You must lose W energy, and this has to come from the perp. direction, i.e. mv^2 = mv_0^2 - W. Plug in the M-B distribution for v_0, and see what it looks like for v.
 
  • #3
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Yes, this is a good thing, and it was also in my head too.

By the way, if we have a closed volume with atoms inside, and the system is in thermal equilibrium, the atomic velocities inside should be distributed according Maxwell-Boltsmann distribution... therefore it looks like after releasing atoms should have the Maxwell-Boltsmann velocity distribution in any direction. The question is how to harmonize it?
 

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