Sakong Sung, Kratzer Peter, Han Xu, Balgar Thorsten, Hasselbrink Eckart
Fakultat für Physik and Center for Nanointegration (CeNIDE), Universität Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany.
J Chem Phys. 2009 Sep 28;131(12):124502. doi: 10.1063/1.3224121.
Combining first-principles calculations and sum frequency generation spectroscopy, we elucidate the microscopic details in the relaxation of the stretching vibration of hydrogen adsorbed on Ge(100). The dominant decay channels involve energy transfer from the stretching to the hydrogen bending modes, with the remaining energy difference being transferred to or from substrate phonons. The coupling between stretching and bending modes is treated from first principles using the calculated multidimensional adiabatic potential energy surface, while the coupling to phonons is treated in perturbation theory. For a surface solely saturated with light hydrogen, we calculate a vibrational lifetime of 1.56 ns at 400 K, in good agreement with experiment, and find a similar temperature dependence of the lifetime in both experiment and theory. The calculations show that the stretching energy dissipates to a vibrational state involving four bending quanta of hydrogen, concurrently absorbing a thermally excited surface phonon related to the Ge dimer rocking mode. For a Ge surface saturated with a mixture of H and D, our experiments find that the relaxation rate of the H stretching vibration is markedly increased when compared to a surface saturated with H only. Experimentally, a single decay is observed although H and D atoms will statistically pair on the surface dimers. The vibrational lifetime of the Ge-H stretching mode is up to six times shorter in the presence of adsorbed D atoms. The calculated relaxation rates are consistent with the experimentally observed trend. The theoretical analysis shows that the breaking of symmetry within the Ge surface dimer due to coadsorption of D opens up further relaxation channels that involve absorption or emission of a substrate phonon at various energies. Moreover, the calculations predict an even shorter vibrational lifetime of the Ge-D stretch mode due to efficient coupling to the Ge dimer rocking mode.
结合第一性原理计算和和频产生光谱，我们阐明了吸附在Ge(100)上的氢的拉伸振动弛豫过程中的微观细节。主要的衰减通道涉及从拉伸模式到氢弯曲模式的能量转移，剩余的能量差则在与衬底声子之间进行转移。利用计算得到的多维绝热势能面从第一性原理出发处理拉伸模式与弯曲模式之间的耦合，而与声子的耦合则在微扰理论中进行处理。对于仅被轻氢饱和的表面，我们计算出在400 K时的振动寿命为1.56 ns，与实验结果吻合良好，并且发现在实验和理论中寿命都有类似的温度依赖性。计算结果表明，拉伸能量耗散到一个涉及四个氢弯曲量子的振动态，同时吸收一个与Ge二聚体摇摆模式相关的热激发表面声子。对于被H和D混合物饱和的Ge表面，我们的实验发现，与仅被H饱和的表面相比，H拉伸振动的弛豫速率显著增加。实验上，尽管H和D原子在表面二聚体上会进行统计配对，但只观察到单一的衰减。在存在吸附的D原子时，Ge-H拉伸模式的振动寿命缩短至原来的六分之一。计算得到的弛豫速率与实验观察到的趋势一致。理论分析表明，由于D的共吸附导致Ge表面二聚体内对称性的破坏，开辟了更多的弛豫通道，这些通道涉及在不同能量下吸收或发射衬底声子。此外，计算预测由于与Ge二聚体摇摆模式的有效耦合，Ge-D拉伸模式的振动寿命会更短。
