The motion of molecules on solid surfaces is of interest for technologicalapplications such as catalysis and lubrication, but it is also a theoreticalchallenge at a more fundamental level. The concept of activation barriers isvery convenient for the interpretation of experiments and as input for MonteCarlo simulations but may become inadequate when mismatch with the substrateand molecular vibrations are considered. We study the simplest objectsdiffusing on a substrate at finite temperature $T$, namely an adatom and adiatomic molecule (dimer), using the Langevin approach. In the driven case, weanalyse the characteristic curves, comparing the motion for different values ofthe intramolecular spacing, both for T=0 and $T\ne 0$. The mobility of thedimer is higher than that of the monomer when the drift velocity is less thanthe natural stretching frequency. The role of intramolecular excitations iscrucial in this respect. In the undriven case, the diffusive dynamics isconsidered as a function of temperature. Contrary to atomic diffusion, for thedimer it is not possible to define a single, temperature independent,activation barrier. Our results suggest that vibrations can account for drasticvariations of the activation barrier. This reveals a complex behaviourdetermined by the interplay between vibrations and a temperature dependentintramolecular equilibrium length.
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机译：分子在固体表面上的运动对于诸如催化和润滑之类的技术应用来说是令人感兴趣的，但在更基本的水平上也是理论上的挑战。激活势垒的概念对于解释实验和作为蒙特卡洛模拟的输入非常方便，但是当考虑与底物的不匹配和分子振动时，可能变得不足。我们使用Langevin方法研究了在有限温度$ T $上扩散到基板上的最简单的物体，即原子和原子分子（二聚体）。在驱动情况下，我们分析特性曲线，比较不同分子间间距值（T = 0和$ T \ ne 0 $）的运动。当漂移速度小于自然拉伸频率时，二聚体的迁移率高于单体。在这方面，分子内激发的作用至关重要。在非驱动情况下，扩散动力学被认为是温度的函数。与原子扩散相反，对于二聚体，不可能定义单一的，与温度无关的活化势垒。我们的结果表明，振动可以解释激活屏障的剧烈变化。这揭示了由振动和与温度有关的分子间平衡长度之间的相互作用决定的复杂行为。
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