Miyoshi Nobuya, Osuka Kenichi, Kinefuchi Ikuya, Takagi Shu, Matsumoto Yoichiro
Department of Mechanical Engineering, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
J Phys Chem A. 2014 Jul 3;118(26):4611-9. doi: 10.1021/jp500884p. Epub 2014 Jun 19.
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.
纳米空间中的气体流动受气体分子在固体表面散射行为的极大影响，从而产生独特的质量输运特性。本文进行了水分子在石墨表面的分子束散射实验，以了解它们在与室温下热速度分布相对应的入射能量范围（35 - 130毫电子伏特）内的散射动力学。由于吸附能较大（约100毫电子伏特），即使在这个狭窄的能量范围内，散射行为对入射能量也相当敏感。对于相对较大的入射能量，直接非弹性和俘获 - 解吸通道对300K表面的散射过程贡献相当。相反，当入射能量降至远低于表面吸附能时，俘获 - 解吸通道占主导，使散射模式从定向散射变为漫散射。结果，对纳米空间中的质量输运有显著影响的切向动量调节系数（TMAC）在很大程度上取决于入射能量。入射能量从130毫电子伏特降至35毫电子伏特会使TMAC翻倍（从0.42变为0.86）。除了入射能量外，TMAC还强烈依赖于表面温度。随着表面温度从300K升高到500K，由于直接非弹性通道的贡献增加，散射变得更具方向性，这使得35毫电子伏特入射束能量的TMAC降至0.48。
