Formation on grain surfaces
Formation on grain surfaces
复制标题
DOI：
10.1086/381775
复制
发表时间：
2004-03-20
影响因子：
4.9
通讯作者：
Tielens, AGGM
中科院分区：
文献类型：
--
作者：
Cazaux, S;Tielens, AGGM
最丰富的星际分子H-2通常被认为是通过H原子在颗粒表面的复合形成的。在表面上，氢原子可以被物理吸附和化学吸附，它们的迁移率可以通过量子力学隧道效应或热跳跃来控制。我们发展了一个分子氢在表面形成的模型。该模型求解了原子和分子氢及其同位素的含时动力学速率方程，考虑了物理吸附和化学吸附位置的存在，以及量子力学扩散和热跳跃的存在。结果表明，该体系的时间演化主要受吸附物种的结合能和迁移势垒的控制。我们将我们的模型结果与在低温和高温下H和D原子束辐照下硅酸盐和碳质表面上形成HD的实验结果进行了比较。这种比较表明，包括两种同位素，包括物理吸附和化学吸附的势垒，以及量子力学隧道和热跳跃，对于正确解释实验是必不可少的。这种比较使我们能够得出这些表面的特征。对于我们考虑的两个表面，我们确定了H原子和H-2分子的结合能，以及H原子从一个位置移动到另一个位置的扩散势垒。我们的结论是，在相当高的温度(类似于500K)之前，分子氢的形成是有效的。在低温下，可移动的物理吸附原子和捕获的化学吸附原子之间的复合占主导地位。在更高的温度下，化学吸附的原子变得可移动，这就驱动了分子氢的形成。我们已将我们的模型扩展到与天体物理相关的条件。结果表明，在较低温度下(T小于或等于20K)，分子氢的生成效率接近单位效率。虽然效率下降，但ISM中的分子氢形成即使在高温下也可以非常有效，这取决于表面的物理特性。
The most abundant interstellar molecule, H-2, is generally thought to form by recombination of H atoms on grain surfaces. On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping. We have developed a model for molecular hydrogen formation on surfaces. This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account. The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species. We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures. This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments. This comparison allows us to derive the characteristics of these surfaces. For the two surfaces we consider, we determine the binding energy of H atoms and H-2 molecules, as well as the barrier against diffusion for the H atoms to move from one site to another. We conclude that molecular hydrogen formation is efficient until quite high (similar to 500 K) temperatures. At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates. At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation. We have extended our model to astrophysically relevant conditions. The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures ( T less than or equal to 20 K). While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface.