Day B Scott, Morris John R, Alexander William A, Troya Diego
Department of Chemistry, Virginia Tech, 107 Davidson Hall, Blacksburg, Virginia 24061-0212, USA.
J Phys Chem A. 2006 Feb 2;110(4):1319-26. doi: 10.1021/jp054043j.
We present a classical-trajectory study of energy transfer in collisions of Ar atoms with alkanethiolate self-assembled monolayers (SAMs) of different densities. The density of the SAMs is varied by changing the distance between the alkanethiolate chains in the organic monolayers. Our calculations indicate that SAMs with smaller packing densities absorb more energy from the impinging Ar atoms, in agreement with recent molecular-beam scattering experiments. We find that energy transfer is enhanced by a decrease in the SAM density because (1) less dense SAMs increase the probability of multiple encounters between Ar and the SAM, (2) the vibrational frequencies of large-amplitude motions of the SAM chains decrease for less dense SAMs, which makes energy transfer more efficient in single-encounter collisions, and (3) increases in the distance between chains promote surface penetration of the Ar atom. Analysis of angular distributions reveals that the polar-angle distributions do not have a cosine shape in trapping-desorption processes involving penetration of the Ar atom into the alkanethiolate self-assembled monolayers. Instead, there is a preference for Ar atoms that penetrate the surface to desorb along the chain-tilt direction.
我们展示了一项关于氩原子与不同密度的链烷硫醇盐自组装单分子层（SAMs）碰撞时能量转移的经典轨迹研究。通过改变有机单分子层中链烷硫醇盐链之间的距离来改变SAMs的密度。我们的计算表明，堆积密度较小的SAMs从撞击的氩原子吸收更多能量，这与最近的分子束散射实验结果一致。我们发现，SAM密度降低会增强能量转移，原因如下：（1）密度较小的SAMs增加了氩原子与SAM多次碰撞的概率；（2）对于密度较小的SAMs，SAM链大幅振动的频率降低，这使得单次碰撞中的能量转移更高效；（3）链间距离增加促进了氩原子的表面渗透。角分布分析表明，在涉及氩原子穿透链烷硫醇盐自组装单分子层的俘获 - 解吸过程中，极角分布不呈余弦形状。相反，穿透表面的氩原子更倾向于沿链倾斜方向解吸。
