Hydrogen adsorption and diffusion on amorphous solid water ice
Hydrogen adsorption and diffusion on amorphous solid water ice
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DOI：
10.1111/j.1365-2966.2007.12415.x
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发表时间：
2007-12-21
影响因子：
4.8
通讯作者：
Van Dishoeck, E. F.
中科院分区：
文献类型：
--
作者：
Al-Halabi, A.;Van Dishoeck, E. F.
本文给出了表面温度T-S为10K时H原子在无定形固体水冰上吸附的经典轨迹计算结果。计算的入射能量E-I范围为10~1000K，随机入射。吸附几率P-S符合一个简单的衰减函数：P-S=1.0E(-Ei(K)/300)。我们的计算预测了H原子在结晶冰和ASW冰上的吸附几率相似，尽管在650+/-10K的ASW条件下计算的俘获H原子的平均结合能E-b高于在400+/-5K的结晶冰的计算结果。对于结晶冰和非晶冰表面，结合能分布符合高斯函数，全宽半峰分别为111 K和195 K。与结晶冰相比，ASW表面H原子结合位的变化导致E-b的分布变宽。我们还计算了入射原子在受热前在表面上的“热扩散”距离，在E-I=100K时约为30埃，并随E-I的增大而增大。在T-S=10K时，热束缚H原子的扩散系数D=1.09+/-0.04×10(-5)cm(2)S(-1)。对于相同的冰T-S，H原子在ASW上的停留时间比H原子在结晶冰上的停留时间长一个数量级，这表明H-2在结晶冰和无孔冰上的生成比在多孔冰上的生成要有限。这与在多孔和非多孔ASW表面上形成H-2的实验结果是一致的。在较低的T-S温度下，较长的tau值、较高的D值以及捕获入射H原子之前在ASW表面的较大热距离确保了H-2形成的朗缪尔-辛舍伍德机制和热原子机制是有效的。这里提供的数据将是描述H-2在星际冰上的形成以及H原子与冰表面其他物种反应的模型的重要组成部分。
Results of classical trajectory calculations on the adsorption of H atoms to amorphous solid water (ASW) ice, at a surface temperature T-s of 10 K are presented. The calculations were performed for incidence energies E-i ranging from 10 to 1000 K, at random incidence. The adsorption probability P-s can be fitted to a simple decay function: P-s = 1.0e(-Ei(K)/300). Our calculations predict similar adsorption probabilities for H atoms to crystalline and ASW ice, although the average binding energy E-b of the trapped H atoms calculated for ASW of 650 +/- 10 K is higher than that found for crystalline ice of 400 +/- 5 K. The binding energy distributions were fitted to Gaussian functions with full width half-maximum of 111 and 195 K for crystalline and amorphous ice surfaces, respectively. The variation of the H atom binding sites in the case of the ASW surface leads to broadening of the distribution of E-b compared to that of crystalline ice. We have also calculated the 'hot-diffusion' distance travelled by the impinging atom over the surface before being thermalized, which is found to be about 30 angstrom long at E-i = 100 K and increases with E-i. The diffusion coefficient D of thermally trapped H atoms is calculated to be 1.09 +/- 0.04 x 10(-5) cm(2) s(-1) at T-s = 10 K. The residence time tau of H atoms adsorbed on ASW is orders of magnitude longer than that of H atoms adsorbed on crystalline ice for the same ice T-s, suggesting that H-2 formation on crystalline and non-porous ice is quite limited compared to that on porous ice. This is in good agreement with the results of experiments on H-2 formation on porous and non-porous ASW surfaces. At low T-s, the long values of tau, the high values of D and the large hot distance travelled on the ASW surface before trapping the impinging H atom ensure that Langmuir-Hinshelwood and hot-atom mechanisms for H-2 formation will be effective. The data presented here will be important ingredients for models to describe the formation of H-2 on interstellar ices and reactions of H atoms with other species at the ice surface.