Molecular beam study of the scattering behavior of water molecules from a graphite surface

Gas flow in nanospaces is greatly affected by the scattering behavior of gas molecules on solid surfaces, resulting in unique mass transport properties. In this paper, the molecular beam scattering experiment of water molecules on a graphite surface was conducted to understand their scattering dynamics in an incident energy range that corresponds to their thermal velocity distribution at room temperature (35-130 meV). Because of the large adsorption energy (∼100 meV), the scattering behavior is quite sensitive to the incident energy even within this narrow energy range. For relatively large incident energies, the direct-inelastic and trapping-desorption channels have comparable contributions to the scattering process on the surface at 300 K. In contrast, when the incident energy decreases well below the adsorption energy on the surface, the trapping-desorption channel becomes dominant, changing the scattering pattern from directional to diffusive scattering. As a result, the tangential momentum accommodation coefficient (TMAC), which significantly impacts the mass transport in nanospaces, largely depends on the incident energy. A decrease in the incident energy from 130 to 35 meV doubles the TMAC (0.42 to 0.86). In addition to the incident energy, the TMAC shows a strong dependence on the surface temperature. With increasing the surface temperature from 300 to 500 K, the scattering becomes more directional because of the increasing contribution of the direct-inelastic channel, which reduces the TMAC for the incident beam energy of 35 meV to 0.48.