The effect of thermal vibrations on extended x-ray absorption fine structure. I

The form of the Debye–Waller factor in EXAFS is discussed in detail, and an expression is obtained for this factor in a general three‐atom system of C_s symmetry. The normal modes which contribute to the Debye–Waller factors for each scattering path are dependent on the symmetry of the system. A series of model three‐atom systems with C_(2v) symmetry are studied and the Debye–Waller factors as a function of the bridging angle are discussed for each of these systems. The temperature dependence of the Debye‐Waller factor for each scattering path is also studied. In a system of C_(2v) symmetry, approximating the double and triple scattering Debye–Waller factors by the second shell single scattering factor is shown to be strictly valid only for a system close to linearity. The error due to this approximation is dependent upon the amplitude of the individual scattering paths and is shown to increase with temperature. When the single scattering contribution is unimportant, there is shown to exist a temperature where the above approximation is exact

Data analysis in extended x-ray-absorption fine structure: Determination of the background absorption and the threshold energy

Two approaches for the determination of the background absorption (μ_0) in the extended x-ray-absorption fine structure (EXAFS) are presented. Both methods, experimental and computational, take advantage of the damping of the EXAFS amplitude resulting from the convolution with Gaussian functions of different widths. In the experimental method two or more spectra are collected with the use of different spectrometer slit widths, resulting in spectra of different resolutions for the same sample. In the computational approach the convolution is accomplished via a convolution algorithm. The intersection points of the resulting spectra are used to generate μ_0. At the absorption edge, the spectra intersect at a unique point, which is shown to be a measure of the threshold energy, E_0. Illustration of the two methods for background removal is given for a copper-foil sample. The computational approach is superior to the experimental method of damping the EXAFS spectra to give μ_0

"Oxide-free" tip for scanning tunneling microscopy

We report a new tip for scanning tunneling microscopy and a tip repair procedure that allows one to reproducibly obtain atomic images of highly oriented pyrolytic graphite with previously inoperable tips. The tips are shown to be relatively oxide-free and highly resistant to oxidation. The tips are fabricated with graphite by two distinct methods

Scanning tunneling microscopy investigation of 2H-MoS_2: A layered semiconducting transition‐metal dichalcogenide

Scanning tunneling microscopy (STM) has been enormously successful in solving several important problems in the geometric and electronic structure of homogeneous metallic and semiconducting surfaces. A central question which remains to be answered with respect to the study of compound surfaces, however, is the extent to which the chemical identity of constituent atoms may be established. Recently, progress in this area was made by Feenstra et al. who succeeded in selectively imaging either Ga or As atoms on the GaAs (110) surface. So far this is the only case where such selectivity has been achieved. In an effort to add to our understanding of compound surface imaging we have undertaken a vacuum STM study of 2H-MoS_2, a material which has two structurally and electronically different atomic species at its surface

Gamble mode: Resonance contact mode in atomic force microscopy

Active noise reduction has been accomplished in atomic force microscopy by applying a high frequency, low amplitude vibration to the cantilever while it is in contact with a surface. The applied excitation (>~ 200 kHz; ~ 1 nm) is acoustically coupled to the tip and dampens the resonance Q factors of the system. The applied frequency is well above the bandwidth of the acquisition system (50 kHz). We call this mode "gamble mode" or "resonance contact.

Ion-Cyclotron Double Resonance

A charged particle in a uniform moving magnetic field H describes a circular orbit in a plance perpendicular to H with an angular frequency or "cyclotron frequency" omagae. When an alternating electric field E(t) is applied normal to H at omegae, the ions absorb energy from the alternating electric field, and are accelerated to larger velocities and orbital radii. [1] The absorption of energy from E(t) at the cyclotron resonance frequency can be conveniently detected using a marginal oscillator detector. When the ions accelerated by E(t) collide with other particles, they lose some of their excess energy. A mixture of ions and neutral molecules in the presence of H and E(t) then reaches a steady-state condition in which the energy gained by the ions from E(t) between collisions is lost to the neutral molecules in collisions

Noise reduction in atomic force microscopy: Resonance contact mode

Noise reduction has been accomplished in atomic force microscopy by applying a high frequency, low amplitude vibration to the cantilever while it is in contact with a surface. The applied excitation (>~200 kHz; ~1 nm) is acoustically coupled to the tip and dampens the resonance Q factors of the system. The applied frequency is well above the bandwidth of the acquisition system (50 kHz). We call this mode "resonance contact" mode. The nonlinear behavior of the tip–sample interaction allows the high frequency excitation to effectively broaden the frequency response of the system resonances

Hardware for digitally controlled scanned probe microscopes

The design and implementation of a flexible and modular digital control and data acquisition system for scanned probe microscopes (SPMs) is presented. The measured performance of the system shows it to be capable of 14-bit data acquisition at a 100-kHz rate and a full 18-bit output resolution resulting in less than 0.02-Å rms position noise while maintaining a scan range in excess of 1 µm in both the X and Y dimensions. This level of performance achieves the goal of making the noise of the microscope control system an insignificant factor for most experiments. The adaptation of the system to various types of SPM experiments is discussed. Advances in audio electronics and digital signal processors have made the construction of such high performance systems possible at low cost

Remotely controlled mirror of variable geometry for small angle x-ray diffraction with synchrotron radiation

A total-reflecting mirror of 120-cm length was designed and built to focus synchrotron radiation emanating from the electron-positron storage ring at the Stanford Linear Accelerator Center (SPEAR). The reflecting surface is of unpolished float glass. The bending and tilt mechanism allows very fine control of the curvature and selectability of the critical angle for wavelengths ranging from 0.5 to 3.0 Å. Elliptical curvature is used to minimize aberrations. The mirror is placed asymmetrically onto the ellipse so as to achieve a tenfold demagnification of the source. The bending mechanism reduces nonelastic deformation (flow) and minimizes strains and stresses in the glass despite its length. Special design features assure stability of the focused image. The mirror reduces the intensity of shorter wavelength harmonics by a factor of approximately 100