(Phys.org)—As long as the temperature is above absolute zero,gas molecules are always in fixed random motion. They may diffuse—or spread out—through three-dimensional space or, in a process called "surface diffusion, or " along the two-dimensional surface of a solid. The most well-known mechanism to explain surface diffusion is a classical mechanism called thermal hopping,in which gas molecules jump from one adsorption site (the position where the gas molecules attach to the surface) to another at a temperature-dependent rate. So far, thermal hopping is the only mechanism known to explain the surface diffusion of hydrogen atoms on the surface of ice.
Source: phys.org