#### Date of Award

8-2008

#### Document Type

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Legacy Department

Physics

#### Advisor

Manson, Joseph R.

#### Committee Member

Daw , Murray S.

#### Committee Member

Sosolik , Chad E.

#### Committee Member

Marinescu , D. Catalina

#### Abstract

In this work a scattering theory is developed to simulate the gas-surface scattering procedure in three dimensions, using an iterative algorithm and classical mechanics for the collision process, that describes both direct scattering and trapping-desorption of an incident beam of atomic particles. The initially trapped fraction of particles can be followed as they continue to make further collisions with the surface until they are all eventually promoted back into the positive energy continuum and leave the surface region.

With two different models, the discrete model and the smooth-surface model of the differential reflection coefficient, a series of calculation are made to give some quantitative descriptions and to interpret recent experimental measurements. Two types of incident conditions are considered, one is a well defined beam impinging on a surface and the other is special case, for which the incident gas is in an equilibrium distribution.

Under many circumstances, when gas atoms or molecules collide with clean and ordered surfaces, the energy-resolved scattering spectra exhibit two clearly distinct features due to direct scattering and to trapping in the desorption well with subsequent desorption. James Clerk Maxwell

is credited with being the first to describe this situation by invoking the simple assumption that when an impinging gas beam is scattered from a surface it can be divided into a part that exchanges no energy and specularly reflects and another part that equilibrates or accommodates completely

and then desorbs with an equilibrium distribution. The discrete model is used to deal with this issue, it allows a rigorous test of the Maxwell assumption and determines the conditions under which it is valid. The theory also gives quantitative explanations of recent experimental measurements which exhibit both a direct contribution and a trapping-desorption fraction in the energy-resolved spectra.

With the smooth-surface model, a number of theoretical calculations are carried out to compare with recently published experimental data for rare gases colliding with molten metal surfaces. For the Ga surface, an effective mass which implies a collective effects from the surface is discussed.

The calculated results match the data reasonably. Some theoretical simulations are given to show

how the final state of these distributions evolve as a function of desorption time.

The case of equilibrium incident gas particles is treated with the smooth-surface model. The work for this case is mainly used to obtain the energy accommodation coefficient. A set of energy accommodation coefficients are calculated and compared with the available experimental data.

#### Recommended Citation

Fan, Guoqing, "3-D Calculation of Atom-Surface Collisions with Direct Scattering, Trapping and Desorption" (2008). *All Dissertations*. 243.

https://tigerprints.clemson.edu/all_dissertations/243