Scattering of image-potential-state electrons by steps on Cu(001)

Scanning tunneling spectroscopy (STS) reveals a distinct asymmetry in the scattering properties of an isolated step for the n=1 image-potential state on Cu(001). The elastic scattering probability for an electron traveling downstairs is determined from the strength of density oscillations in front of a step edge and is found to be approximately two times higher than for the opposite upstairs direction. A one-dimensional scattering model is extended to the case of asymmetric transmission and reflection coefficients. The calculations using the asymmetry measured by STS explain the dispersion and the decay rate of the n=1 band on Cu(119) measured by two-photon photoemission. In particular, the asymmetry of the decay rate can be described quantitatively with a minimum of adjustable parameters. While the results can also be transferred successfully to the Cu(1115) surface, the limit of applicability is reached for Cu(117) with a step separation of 3.5 nearest-neighbor distances. PACS 73.20.At; 79.60.Bm; 68.37.Ef; 72.10.Fk     

Studies of alkali adsorption on Rh(111) using optical second-harmonic generation

Dramatic changes were observed in the optical second-harmonic generation from the Rh(111) crystal surface as the surface was covered with alkali atoms. For low alkali coverage, these changes are explained qualitatively in terms of optical transitions between alkali atomic-derived electronic states and, for high coverage, in terms of the plasmon resonance of the alkali layer. The results establish the surface specificity of second-harmonic generation to the topmost 1 or 2 atomic layers of alkali metals.     

Adsorption of oxygen and hydrogen on Pt(111) studied with second-harmonic generation

The adsorption of oxygen and hydrogen on a Pt(111) surface has been studied with phase-sensitive Second-Harmonic Generation (SHG) signal detection. The SHG-signal change measured with p-in, p-out polarization during the adsorption of oxygen and hydrogen was found to be different in amplitude and phase for the two adsorbates. At the wavelength used (1064 nm), only a localized interaction between adsorbate and substrate is seen, leading to a linear dependence of the susceptibility on the coverage. Sticking coefficients,s, and their coverage dependence were determined. For hydrogen, a linear decrease ins with coverage was found; the initial sticking coefficient beings ₀=0.06 at a temperature ofT=130 K. For oxygen,s whows a quadratic decrease with coverage, strongly dependent on temperature, withs ₀=0.05 atT=350 K.A method based on these results is proposed, which would allow the determination of adsorbate coverages of coadsorption systems with SHG using phase-sensitive signal detection.Presented at the 129th WE-Heraues-Seminar on Surface Studies by Nonliner Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Reversible shape transition of pb islands on Cu(111)

Using low-energy electron microscopy, we have observed a reversible transition in the shape of Pb adatom and vacancy islands on Cu(111). With increasing temperature, circular islands become elongated in one direction. In previous work we have shown that surface stress domain patterns are observed in this system with a characteristic feature size which decreases with increasing temperature. We show that the island shape transition occurs when the ratio of the island size to this characteristic feature size reaches a particular value. The observed critical ratio matches the value expected from stress domains.     

Resonant and nonresonant second-harmonic generation in a single cadmium sulfide nanowire

We experimentally investigate the resonant and nonresonant second-harmonic generation in a single cadmium sulfide(Cd S) nanowire. The second-order susceptibility tensor is determined by analyzing the forward secondharmonic signals of the Cd S nanowire. Our results show that(1) d33∕d31=-2.5 at a nonresonant input wavelength of 1050 nm;(2) d33∕d31=-1.9 at a resonant wavelength of 740 nm. The difference can be attributed to the polarization-dependent resonance.     

Thin noble metal films on Si (111) investigated by optical second-harmonic generation and photoemission

Thin noble metal films (Ag, Au and Cu) on Si (111) have been investigated by optical second-harmonic generation (SHG) in combination with synchrotron radiation photoemission spectroscopy. The valence band spectra of Ag films show a quantization of the sp-band in the 4-eV energy range from the Fermi level down to the onset of the d-bands. For Cu and Au the corresponding energy range is much narrower and quantization effects are less visible. Quantization effects in SHG are observed as oscillations in the signal as a function of film thickness. The oscillations are strongest for Ag and less pronounced for Cu, in agreement with valence band photoemission spectra. In the case of Au, a reacted layer floating on top of the Au film masks the observation of quantum well levels by photoemission. However, SHG shows a well-developed quantization of levels in the Au film below the reacted layer. For Ag films, the relation between film thickness and photon energy of the SHG resonances indicates different types of resonances, some of which involve both quantum well and substrate states. Received: 16 October 2001 / Revised version: 14 March 2002 / Published online: 29 May 2002     

Investigation of native-oxide growth on HF-treated Si(111) surfaces by measuring the surface-state distribution

The evaluation of the surface state distribution of differently HF-treated Si(111) surfaces during the native-oxide growth in air is investigated by the large-signal field-modulated photovoltage technique. The surface state distribution consisting of intrinsic and extrinsic Si dangling bond defects is directly related to the state of oxidation of the Si surface. It is shown that the kind of HF treatment strongly influences the concentration of extrinsic defects with a lower state of oxidation. Special HF preparations for H termination of the Si(111) surface result in a nearly intrinsic surface state distribution. During the oxidation process three typical phases can be distinguished each characterized by specific defect structures. It was found that native-oxide growth is highly sensitive to the concentration of extrinsic defects directly after HF treatment.     

Wavelength-dependent anisotropy of surface second-harmonic generation from Si(111) in the vicinity of bulk absorption

A wavelength-dependence study of surface second-harmonic generation from Si(111) is presented. Femtosecond laser pulses tuned from 430 nm to 710 nm were used to probe the dispersion characteristics of the nonlinear response from the surface in the vicinity of bulk absorption. Changes in the rotational symmetry and in the phase of the generated second-harmonic signal were observed. An explanation of the observed changes is given based on a simple anharmonic oscillator model [1]     

局域功函数图像及其在Cu(111)-Au/Pd表面的应用

用扫描隧道显微镜（STM）对Cu(111)-Au和Cu(111)-Pd表面的局域功函数进行了研究.通过 测量隧道电流对针尖样品间距的响应，得到了与STM形貌图一一对应的表面局域功函数图像. 实验发现，Au/Pd覆盖层和Cu衬底间的功函数有明显的不同.Pd薄膜的功函数甚至超过了其体 本征值，且功函数在台阶处变小.用偶极子的形成解释了台阶处功函数的降低.这一工作表明 ，用测量局域功函数的方法容易区分表面上不同的元素，并具有纳米尺度的空间分辨率. 关键词： 扫描隧道显微镜 局域功函数 台阶