Prominent out-of-plane diffraction in helium scattering from a methyl-terminated Si(111) surface

Due to their electrochemical and oxidative stability, organic-terminated semiconductor surfaces are well suited to applications in, for example, photoelectrodes and electrochemical cells, which explains the lively interest in their detailed characterization. Helium atom scattering (HAS) is a useful tool to carry out such characterization. Here, we have simulated HAS in He/CH3-Si(111) based on density functional theory (DFT) potential energy surfaces (PESs) and multi-configuration time-dependent Hartree (MCTDH) dynamics. Our analysis of HAS shows that most diffraction taking place in this system corresponds to high-order out-of-plane peaks. This is a general trend that does not depend on the specific features of the simulations, such as the inclusion or not of the van der Waals long-range effects. This is the first and only He-surface system for which such huge out-of-plane diffraction has been described. This striking theoretical finding should encourage new experimental developments to confirm this previously unreported effect

Nonadiabatic effects in gas-surface dynamics

In this chapter, we will provide the theoretical foundations on which the local-density friction approximation (LDFA) is based and examples of its application to gas–surface interaction problems. With this aim, first in Sect. 28.2 we will review the derivation of the stopping power (energy lost per unit path length) for an atom or molecule traveling through a uniform electron gas in the strong coupling limit, i. e., when its velocity is lower than the Fermi velocity of the system. Real systems present electronic density nonlinearities. For this reason in Sect. 28.3, we show how this method for calculating the stopping power has been successful to reproduce and explain experimental energy-loss measurements for ions/atoms traveling through real solids and interacting with metal surfaces. The last part of Sect. 28.3 is devoted to the description of the LDFA method that accounts for the effect of energy losses in the dynamics of thermal and hyperthermal gas particles interacting with metal surfaces. Its implementation both in molecular dynamics performed in precalculated potential energy surfaces (PESs) and in ab-initio molecular dynamics is also discussed. Finally, an overview of the knowledge acquired during last years by the application of the LDFA will be presented in Sect. 28.4. In particular, we will analyze the importance of electron–hole (e–h) pair excitations in different gas–surface elementary processes that involve atoms and molecules of practical interest, such as H, N, H2, N2, and H2O. The analysis will mostly review the results obtained for the dissociative and nondissociative adsorption, for the Eley–Rideal and hot-atom recombinations and for the scattering of these gas species on different metal surfaces. As a general conclusion, it will be shown that e–h pair excitations are typically relevant for long-lasting processes. The last part of this section will review recent applications of the LDFA to model the desorption dynamics of atoms or molecules induced by femtosecond laser pulses. A summary of this chapter is provided in Sect. 28.5.Peer reviewe