Volume 63, Issue 12, December 1992
Index of content:
63(1992); http://dx.doi.org/10.1063/1.1143377View Description Hide Description
Instrumental aspects of low‐energy electron microscopy are reviewed with a view toward the future evolution of this reemergent technology. Both elastically scattered and inelastically excited electrons in the 0–1000‐eV range may be used to form direct rather than scanned images of surfaces in the 3–10‐nm resolution range. Different instrumental setups may be used to form images that selectively contain information about the topography, crystalline structure,chemical composition, or magnetic orientation of the first few monolayers. Frequently, parallel imaging allows observation of dynamic processes occurring during observation. Key electron optical elements and their systemic relationships are described in the context of a still hypothetical generalized instrument that would allow complementary exploitation of many contrast modes. Image quality issues such as resolution, sensitivity, statistics, and contrast selectivity are considered with a view toward their optimization, in many cases by drawing ideas and technologies from other fields of microscopy.
63(1992); http://dx.doi.org/10.1063/1.1143841View Description Hide Description
A novel experimental technique to observe the Bragg scattering of evanescent neutronwaves is presented. The so‐called EVA diffractometer, installed at the high‐flux reactor of the Institut Laue‐Langevin, allows the identification of neutron scattering from single crystalsurfaces. We discuss the experimental setup, intensity, and resolution considerations and some first Bragg scattering signals from evanescent neutronwaves excited at CaF2, InP, and MnF2single crystalsurfaces. The experimental results are discussed within the framework of the so‐called distorted wave Born approximation and within a dynamical scattering theory. The limitations of both theoretical approaches to describe the Bragg scattering of evanescent neutronwaves are indicated.
63(1992); http://dx.doi.org/10.1063/1.1143378View Description Hide Description
Long pulse operation of a multijoule transverse‐electric‐CO2 laser using a magnetic‐spiker sustainer excitation in conjunction with a pulse forming network is described. The laser has produced 4.5 J in a 5 μs long pulse at a high efficiency of ∼10% from an active discharge volume of 416 cm3. The overall system is simple and reliable.
63(1992); http://dx.doi.org/10.1063/1.1143379View Description Hide Description
A high‐resolution photoacoustic Ti:sapphire/dye ring laserspectrometer with a high level of automation is described. It produces high quality Doppler‐limited overtone spectra. The performance of the spectrometer has been demonstrated by measuring both low‐ and high‐resolution spectra of acetylene in the wave number regions 10850–12900 cm−1 and 11500–11650 cm−1, respectively. The spectra show large signal‐to‐noise ratios (≳7000) for strong absorption peaks. Some strong absorption lines have been compared with published FTIRspectral data.
63(1992); http://dx.doi.org/10.1063/1.1143380View Description Hide Description
In order to make a microscopic observation of the surface elastic wave on inhomogeneous opaque materials, an apparatus for Brillouin scatteringspectroscopy was constructed utilizing a commercial microscope. An optical system which serves both as an inlet of a laser beam and as an outlet of the scattered light was made and incorporated in the microscope. The apparatus was proved to be particularly useful to make microscopic‐scale observations of surface elastic waves on metal films.
63(1992); http://dx.doi.org/10.1063/1.1143381View Description Hide Description
A broadband coherent anti‐Stokes Raman spectroscopy(CARS) system optimized for single shot measurements in turbulent flames is described. The CARS spectrometer incorporates an intensified photodiode array (IPDA) with a P46‐phosphor based intensifier that is shown to have an order of magnitude less image persistence than P20‐phosphor based IPDAs, and thus largely eliminates the temperature errors that can result from this image persistence. The low and high signal nonlinearity of IPDAs incorporating both P20 and P46 phosphor based intensifiers is described. The CARS signal is dispersed with a single element concave holographicdiffraction grating, which is fiber‐optically coupled. To provide the necessary dynamic range (approximately 1000:1) to handle CARS spectra whose temperatures can vary from 300 K to adiabatic flame temperatures a fiber‐optic splitter was employed and the nonlinearity of the IPDA detectors was characterized. A method of determining the nonlinearity was developed that was convenient and the fiber‐optic link was shown to provide a stable calibration of this nonlinearity. The problem of dye laser spectral drift, which is particularly severe in a long duration experiment, was addressed by constructing a compact high resolution spectrometer to continuously monitor the dye laser. The dye laser center frequency was maintained constant by manually adjusting an intracavity tuning filter. The accuracy of this technique and its effect on CARS derived temperatures was evaluated. The standard method of determining the instrument function by fitting to 295 K air spectra was compared to other methods and its use was shown to result in an uncertainty of less than 0.5% in CARS derived temperatures. The operation of this CARS instrument was demonstrated in a turbulent diffusion flame. The varying composition in these flames was accounted for by fitting C, the ratio of the nitrogen mole fraction to the total third order nonlinear susceptibility.
63(1992); http://dx.doi.org/10.1063/1.1143382View Description Hide Description
The design of this linear ultraviolet (UV) flashlamp ensures an equilibrium between the mercurycathodeerosion and the return of mercury atoms from the discharge, making the cathode self‐replenishing. The lamp, which requires a simmer current of only 0.3 A, can operate in the region of high pulse peak currents and current densities where a conventional xenon flashlamp experiences a rapid deterioration. The lamp has a minimal length of 7.5 cm and can be loaded up to 100 W/cm. It has demonstrated about 30% output in the UV region and has proven efficient for dissociation of organic compounds and for disinfection, and has promise for other photochemical applications.
63(1992); http://dx.doi.org/10.1063/1.1143383View Description Hide Description
We have developed a new method of signal detection that permits the direct measurement of the wavelength derivative transmittance or reflectance spectra of any sample that can be studied by optical spectroscopy. This method uses multiple lock‐in amplifiers where the output of one lock‐in is, in turn, measured by another. In doing this we can achieve both high signal‐to‐noise ratios and eliminate variations in the background response of the spectrometer to approximately one part in 104, while keeping the spectrometer simple in design.
63(1992); http://dx.doi.org/10.1063/1.1143384View Description Hide Description
A shielded cylindrical space‐charge‐limited diode ionizationdetector with high sensitivity and high resolution has been constructed to detectionization in metal vapors, e.g., multiphoton ionization in the sodium atom and collisional associative ionization in the Rydberg states of the sodium dimer. The diode design and construction and measurements of its characteristics are described. Using this diode detector, a wide variety of high resolution spectroscopy can be obtained using techniques such as two‐photon excitation, optical–optical double resonance, perturbation‐facilitated optical–optical double resonance, and all‐optical triple resonance.
63(1992); http://dx.doi.org/10.1063/1.1143385View Description Hide Description
The sensitivity of an optical field sensor with a Pockels crystal of LiNbO3 for measuring a field distribution in a long‐gap discharge has been much improved by using an intensity difference method with an intensity‐modulated laser beam. The signal‐to‐noise ratio (S/N) is also improved by 37 dB from that without the above‐mentioned method. The resultant sensitivity of the new field measuring system is 100 times higher than that of the previous one. Finally, it is possible to measure an electric field down to 7.5 V/cm in the long‐gap discharge with high accuracy corresponding to the S/N ratio of 20 dB and 1.5 V/cm with the accuracy of 6.0 dB.
63(1992); http://dx.doi.org/10.1063/1.1143386View Description Hide Description
We describe the concept and performance of a new type of tunable Faradayoptical isolator for the near infrared. 60‐dB isolation is achieved with three dielectricpolarizers having an extinction of only 40 dB. The compact instrument does not cause an overall polarization rotation or beam displacement and gives access to all optical beams.
63(1992); http://dx.doi.org/10.1063/1.1143387View Description Hide Description
The conversion of a commercial double crystal x‐ray diffractometer to a flexible, high‐resolution triple crystal diffractometer is described. This conversion, which has been accomplished at low cost, greatly enhances the flexibility of a double crystal diffractometer for the characterization of both highly perfect and relatively imperfect bulk single crystals and epitaxial layers. The conversion is easily reversible, thus allowing a single double crystal diffractometer to operate in either a double crystal or triple crystal mode. Specific examples of the capabilities of this instrument are presented.
63(1992); http://dx.doi.org/10.1063/1.1143388View Description Hide Description
An undulator‐radiation‐excited x‐ray fluorescenceanalysis system for light elements has been developed and tested on the 26‐period multipole wiggler/undulator beamline at the Photon Factory. An entrance‐slitless optics with a variable‐spacing grating was devised for a fluorescent x‐ray dispersion system to achieve spectroscopic analysis with high efficiency and high resolution. Both soft x‐ray emission (SXE) spectra and x‐ray absorption near‐edge structure (XANES)spectra with fluorescent x‐ray detection were obtained by this system, taking advantage of the high brightness and quasimonochromatic character of the incident undulator beam. The analysis system was evaluated by measuring the B K α SXE and fluorescenceXANESspectra of B, BN, and B2O3.
63(1992); http://dx.doi.org/10.1063/1.1143389View Description Hide Description
An x‐ray diffractometer using synchrotron radiation (SR) has been constructed and its performance was evaluated. Its characteristics are (1) a fine and highly intense x‐ray beam from focusing SR x rays on a sample using two spherical mirrors which are arranged in the Kirkpatrick–Baez configuration and (2) a high detection efficiency of the cylindrical bent imaging plate (IP). A focused beam size of 10×11 μm2 and intensity of more than 107 photons/s were obtained at a wavelength of 1.38 Å. Accuracy of the lattice parameters, δa/a, was 2×10−4 estimated from measuring a superconductor YBa2Cu3O7−δ powder. The developed microdiffractometer was applied to measure diffraction images from a specific region on the NbTi metal superconducting wire which consisted of many NbTi fine wires buried in a Cu matrix. Diffraction peaks from a NbTi fine wire of 30 μm diameter were clearly observed and were distinct from the Cu matrix. The developed microdiffractometer can be applied to measurements of as‐received samples in the micro‐order specific region.
63(1992); http://dx.doi.org/10.1063/1.1143390View Description Hide Description
A pyrolysis instrument operating with an infrared radiationsource in an optical focusing system has been developed to obtain volatile products from solid samples for gas chromatographic/mass spectrometric analysis. The device spatially separates heating source and heated material, avoiding severe contaminations. Solid materials like soil samples or polymeric compounds can be pyrolyzed in the reactor tube of the instrument. Gases formed during the pyrolysis are collected in a sorbent‐filled preconcentration tube. The analysis is performed after desorbing the collected gases by gas chromatography/mass spectrometry. Analysis results of the pyrolysis products are then correlated to the compounds in the solid samples.
63(1992); http://dx.doi.org/10.1063/1.1143391View Description Hide Description
A hybrid mass spectrometer consisting of a magnetic sector, two electric sectors, and a quadrupole mass filter (BEEQ) has been built for the study of polyatomic ion/surface collision phenomena over the energy range of a few electron volts to several keV. Primary ions are generated by electron ionization or by chemical ionization, and the first two sectors are used to deliver a monoenergetic beam of ions, of a selected mass‐to‐charge m/z ratio, to a decelerator which sets the desired collision energy. The target, which can be introduced into the system without breaking vacuum, is mounted on a goniometer and situated in an electrically shielded region in the center of a large scattering chamber which contains an electric sector and a quadrupole mass analyzer used for kinetic energy and mass measurements on the ejected ions. These analyzers rotate around the scattering center to allow selection of the scattering angle of ions leaving the surface.
Ultimate pressures attainable in the main scattering chamber are below 10−9 Torr allowing molecular targets, such as self‐assembled monolayers of alkyl thiols on gold, to be examined without surface contamination. Low‐energy (20–100 eV) collisions of polyatomic ions are reported, and examples are given of the effects of collision energy and scattering angle on surface induced dissociation mass spectra. The kinetic energy of the inelastically scattered ions is also measured, and in some cases, the internal energy can be estimated, the two measurements together providing information on energy partitioning associated with surface collisions. For example, it is shown that n‐butylbenzene molecular ions of 25 eV colliding with ferrocenyl‐terminated self‐assembled monolayersurfaces, rebound with 10 eV of recoil energy and 3 eV of internal energy.
The remainder of the energy goes into the surface. The capability of the BEEQ instrument to provide data on ion/surface reactive collisions is also illustrated with reactions such as alkyl group transfer at self‐assembled monolayersurfaces. In addition, data are given showing the ability of the system to provide information on the kinetic energy distributions of ions generated in the course of high‐energy collisions at the surface. Mass analysis of the sputtered products provides the instrument with secondary‐ion mass spectrometry capabilities.
63(1992); http://dx.doi.org/10.1063/1.1143392View Description Hide Description
The principles of operation of a force feedback surface force apparatus are described. A magnetic force transducer and a bimorph displacement sensor are connected in a servo loop. The dynamic behavior of the system is analyzed and the effect of eddy currents on the frequency response of the magnetic field is discussed. The apparatus has several advantages over more traditional force measuring techniques. Cantilever instability associated with the measurement of attractive forces is reduced and the apparatus may be used for hydrodynamic measurements. A resonance technique for measuring the spring constant is also described.
Variable‐temperature ultrahigh vacuum scanning tunneling microscope: Mechanical and electronic instrumentation63(1992); http://dx.doi.org/10.1063/1.1143393View Description Hide Description
A variable‐temperature ultrahigh vacuum scanning tunneling microscope is described which was specifically designed for the study of Ga surfaces close to the bulk melting point (T m =29.8 °C). Since the temperature must be controlled with great accuracy the sample stage and the rest of the microscope are in thermal equilibrium. The instrument also incorporates a novel approach to vibration isolation that is based on a lossy acoustic waveguide. Analog electronics form an integral part of the microscope since a dynamic range of 120 dB must be reached. Interconnection techniques are discussed in this context and a circuit of a high‐voltage amplifier is shown that provides a voltage swing of ±400 V with less than 1 mV pp noise over a bandwidth of 10 kHz. Results are presented which confirm the design concepts applied to this instrument.
In situ tip exchange for an ultrahigh vacuum scanning tunneling microscope using dual‐axes piezoelectric micropositioners63(1992); http://dx.doi.org/10.1063/1.1143394View Description Hide Description
We have constructed an in situ tip‐exchangeable ultrahigh vacuum scanning tunneling microscope (UHV–STM). The STM has dual‐axes piezoelectric micropositioners, and a damaged tip can be exchanged in situ for a new tip by moving the two micropositioners alternately. We have demonstrated that our tip‐exchange method has great advantages of reliability, accuracy, simplicity, and ease of operation. These advantages have been shown to result from extremely high mechanical resolutions of these micropositioners. From the measured STM image of a GaInAs/InP multiquantum well (110) cleaved surface it has been demonstrated that the constructed tip‐exchangeable STM works stably with an atomic resolution under UHV.