Out-diffusion in solids

Jan 2015
I am having trouble understanding the physical mechanism of out-diffusion from a solid.

My system is a crystalline solid which has ions implanted in it. I understand how the ion diffuses through the solid, and the mechanisms involved (vacancy, interstitial etc.), but where I am struggling is what happens once the ion reaches the solid-gas interface. I assume that some binding energy must be broken, but I don't know how or what.

Any help (preferably with some references or reading suggestions specific to out-diffusion) would be great!