Open-boundary cluster model for calculation of adsorbate-surface electronic states

We have developed a simple embedded-cluster model approach to investigate adsorbate-surface systems. In our approach, the physically relevant subsystem is described as an open-quantum system by considering a model cluster subject to an outgoing-wave boundary condition at the edge. This open-boundary cluster model !OCM" is free from artificial waves reflected at the cluster edge, and thus the adsorbate properties computed with the OCM are almost independent of the model cluster size. The exact continuous density of states !DOS"of a one-dimensional periodic potential model is shown to be precisely reproduced with the OCM. The accurate DOS leads to an appropriate description of adsorbate-surface chemical bonding. Moreover, the openboundary treatment of the OCM allows us to evaluate the electron-transfer rate from the adsorbate to the surface, whereas the conventional cluster model does not give any information about such a dynamical process

Open-boundary cluster model implemented in first-principles calculations for electronic excited states of an adsorbate-surface system

Our recently developed open-boundary cluster model (OCM), which allows us to calculate electronic states of a semi-infinite adsorbate-surface system with a finite-small cluster, has been implemented in first-principles calculations to investigate excited states of a real system of a low-coverage Cs/Cu(111). The first-principles calculations are based on a real-space density functional theory (DFT) approach, and the Cs/Cu(111) system is reasonably represented in terms of a cluster of CsCu13 within the OCM approach. An absorption spectrum and the lifetime of excited states of the system are calculated successfully within the linear-response approximation, and the computed results qualitatively agree with experimental observations. Such excited properties are difficult to calculate by using a conventional cluster model (CCM) approach. Despite these advantages, the OCM-DFT approach requires a computational cost almost identical to the cost of CCM

Photoinduced coherent adsorbate dynamics on a metal surface: Nuclear wave-packet simulation with quasi-diabatic potential energy curves using an open-boundary cluster model approach

We present a nuclear wave-packet simulation of photoinduced coherent adsorbate dynamics on a metal surface with quasi-diabatic potential energy curves obtained from our recently developed open-boundary cluster model approach. Photoexcitation to the resonant adsorbate state and the subsequent ultrafast decay to the electronically excited substrate states were found to cause a coherent vibration of the adsorbate on the metal surface. This process competes with a Raman scattering process, which is generally believed to explain the coherent adsorbate vibration. These two mechanisms induce vibrations with a common frequency, and therefore cannot be distinguished from each other in a frequency-domain experiment. However, they can be distinguished by determining the initial vibrational phase through a time domain experiment such as ultrafast pump-probe spectroscopy. We further demonstrate that for near-resonant excitation the oscillation amplitude induced by our proposed mechanism largely exceeds the amplitude due to the Raman mechanism

Collectivity of plasmonic excitations in small sodium clusters with ring and linear structures

The collectivity of the electronic motion in finite systems is studied by using both the linear response density functional theory (LRDFT) and the collectivity index defined by the transition density matrix. We demonstrate a collectivity analysis on the size-dependent peaks of electronic excitations of small sodium clusters (rings and linear chains). We find the excitation-mode dependence of the collectivity and large collectivities for the higher-energy plasmonic excitations. The collectivity analysis also clarifies the existence of the nondipolar collective motion at the energies very close to the higher-energy plasmonic excitations. The importance of the nondipolar motion is pointed out in light of nano-optics

Angstrom-scale flatness using selective nanoscale etching

The realization of flat surfaces on the angstrom scale is required in advanced devices to avoid loss due to carrier (electron and/or photon) scattering. In this work, we have developed a new surface flattening method that involves near-field etching, where optical near-fields (ONFs) act to dissociate the molecules. ONFs selectively generated at the apex of protrusions on the surface selectively etch the protrusions. To confirm the selective etching of the nanoscale structure, we compared near-field etching using both gas molecules and ions in liquid phase. Using two-dimensional Fourier analysis, we found that near-field etching is an effective way to etch on the scale of less than 10 nm for both wet and dry etching techniques. In addition, near-field dry etching may be effective for the selective etching of nanoscale structures with large mean free path values