Showing papers in "Review of Scientific Instruments in 1989"

Soft x-ray emission spectroscopy using monochromatized synchrotron radiation (invited)

Abstract: Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flippe...

A high‐intensity multi‐purpose piezoelectric pulsed molecular beam source

Abstract: A piezoelectric valve is described which delivers intense gas pulses limited by the nozzle for diameters of up to 1.0 mm. Minimum pulse lengths between 0.15 and 0.25 ms (fwhm) are obtained at full opening, depending on the nozzle size. The upper limit of the repetition rate is near 1 kHz. The valve and its modifications meet almost all experimental requirements. In particular, ways of operating in a wide temperature range are outlined. Reliable operation with a remote nozzle at 77 K was recently demonstrated. Nozzle temperatures of more than 1000 K should be possible with a similar setup.

Preparation and characterization of STM tips for electrochemical studies

Abstract: We report on a fabrication technique for scanning tunneling microscopy (STM) tips for in situ electrochemical investigations Unwanted Faradaic currents were minimized by insulating the STM tips with Apiezon wax Cyclic voltammetry showed Apiezon wax to be inert in various electrolytes

Performance of the Dragon soft x‐ray beamline (invited)

Abstract: We report on the performance of a newly constructed synchrotron radiation soft x‐ray beamline. This beamline, dubbed Dragon, is a spherical version of the cylindrical element monochromator design proposed previously. By measuring the K‐edge absorption spectra of condensed nitrogen, it is determined that this monochromator has achieved resolving power 104 at 400‐eV photon energy, using its full 15 by 1 mrad angular acceptance. The ideas and advantages contained in the CEM design have also been experimentally confirmed.

Imaging radio frequency electron‐spin‐resonance spectrometer with high resolution and sensitivity for in vivo measurements

Abstract: We report the development of a novel radio frequency electron‐spin‐resonance spectrometer designed to provide measurements with high molar sensitivity and resolution in vivo. Radio frequency (250 MHz) is chosen to obtain good penetration in animal tissue and large aqueous samples with modest sacrifice of sensitivity. The spectrometer has a lumped component resonator and operates in continuous‐wave mode. The spectrometer is capable of two‐dimensional imaging, and with a modest addition should be capable of three‐dimensional imaging. We demonstrate 3‐mm spatial resolution for DPPH samples. For 10‐ml samples of aqueous nitroxide, we demonstrate sensitivity (normalized to spectral width of 1 G) to 3×10−8‐M concentrations and spectral resolution of 0.1 G. Spectra from nitroxide spin label injected into a live mouse are shown.

A differential interferometer for force microscopy

Abstract: We present a polarizing optical interferometer especially developed for force microscopy. The deflections of the force‐sensing cantilever are measured by means of the phase shift of two orthogonally polarized light beams, both reflected off the cantilever. This arrangement minimizes perturbations arising from fluctuations of the optical path length. Since the measured quantity is normalized versus the reflected intensity, the system is less sensitive to intensity fluctuations of the light source. The device is especially well suited to static force measurements. The total rms noise measured is ≲0.01 A in a frequency range from 1 Hz to 20 kHz.

Photon scanning tunneling microscopy.

Abstract: A method and apparatus for photon scanning tunneling microscopy (10) for examining a sample (20) in a sample area (21) on a surface (16) at a subwavelength resolution is disclosed. The method comprises generating a near-field (18) of photons at a surface (16) containing a sample area (21). The near-field (18) has an area that is larger than the sample area (21), and the near-field (18) has an exponentially increasing intensity in a direction perpendicular to and towards the surface. The near-field (18) is sampled with a probe (26) which receives photons from that near-field (18) that tunnel from the surface (16) to the probe (26). The received photons are detected and a detector (30) produces an output signal that is proportional to the number of photons received by the probe (26). The sample area (21) is scanned with the probe (26) in at least one direction parallel to the surface (16). While scanning, the probe (26) may be maintained either at a position of a constant near-field intensity or at a position of constant distance from the surface (16). A photon scanning tunneling microscope for using the method is also disclosed.

Scanning tunneling microscope instrumentation

Abstract: A review is presented of the basic operating principles of scanning tunneling microscopy and spectroscopy. The physical and electronic design of the scanning tunneling microscope is discussed, and other new microscopes using similar concepts are described. Some examples of the past accomplishments of scanning tunneling microscopy are given, and prospects for future applications are assessed.

Coarse tip distance adjustment and positioner for a scanning tunneling microscope

Abstract: We report on a new coarse tip distance adjustment and positioner for a scanning tunneling microscope, which was designed with ease of sample manipulation and UHV compatibility in mind. It has no mechanical connections in UHV and has been successfully used for measurements of microfaceted platinum surfaces in air and cleaved Si(111) surfaces under liquids.

Microwave ion source

Abstract: A microwave ion source suitable for an apparatus which requires ions of an element of high reactivity such as oxygen, fluorine, etc., the microwave ion source being arranged to transmit microwaves between outer and inner conductors of a coaxial line. An ion extraction electrode is formed at least partly of a low magnetic permeability material while an acceleration electrode is formed of a high magnetic permeability material. The acceleration electrode is formed so as to have a structure in which a low magnetic permeability material of a certain thickness is stacked on the high magnetic permeability material at a plasma chamber side and openings of ion exit holes are formed in the portion of the low magnetic permeability material. A permanent magnet constituting a magnetic field generating means is provided to surround the microwave lead-in coaxial line. The direction of magnetization of the permanent magnet is made to coincide with the axial direction of the coaxial line. The end surface of the permanent magnet at the microwave lead-in side is coupled with the periphery of the high magnetic permeability material of the acceleration electrode through another high magnetic permeability material to form a magnetic path. The plasma chamber is formed of a dielectric insulator which transmits microwaves well. It is possible to realize an ion source in which ions can be extracted with a high electric field, and in which a high current ion beam can be extracted for a long time.

Device for measuring the force and separation between two surfaces down to molecular separations

Abstract: We present an apparatus for measuring the force as a function of distance between surfaces at separations down to the order of molecular dimensions. The device is a simplified yet improved version of the surface force apparatus first developed by Israelachvili and Adams [J. Chem. Soc. Faraday Trans. 1, 74, 975 (1978)]. It gives the same measurement resolution and has the same possibilities of studying various phenomena in thin films such as friction, viscosity, adhesion, and phase transitions. It offers improved performance with regard to control of surface separation and increased versatility by virtue of variable chamber dimensions and geometry.

New method for time dependent x-ray absorption studies

Abstract: An absorption spectrum containing both the XANES and the EXAFS region can be recorded in several seconds if x‐ray monochromators at synchrotron beamlines are scanned continuously. The experimental setups are described in detail. The accuracy of the method up to photon energies of 19 keV is demonstrated. Due to its advantages, the proposed approach may be used as an alternative to the energy dispersive EXAFS method, especially in the case of XANES spectroscopy.

20-ps timing resolution with single-photon avalanche diodes

Abstract: Single photon avalanche diodes (SPADs) are avalanche photodiodes specifically designed for reverse bias operation above the breakdown voltage and used for detecting single optical photons. A new silicon epitaxial device structure was designed to give improved timing performance with respect to previous SPADs. Extensive tests were carried out in order to establish the timing resolution of the device in time correlated photon counting (TCPC). The timing resolution of the SPAD in terms of its full‐width at half‐maximum (FWHM) contribution to the overall instrumental response width is 20 ps with the detector cooled to −65 °C, and 28 ps at room temperature. This is the highest resolution so far reported for solid‐state single‐photon detectors. In vacuum tubes, comparable results are obtained only with special microchannel‐plate photomultipliers (MCP‐PMT). Results from time‐resolved photoluminescence measurements in GaAs demonstrate the power of the TCPC technique when used with the new SPAD detector. With the ...

JUSIFA—A new user‐dedicated ASAXS beamline for materials science

Abstract: The fully computer‐controlled user‐dedicated small‐angle scattering facility JUSIFA was set up for general use at the DORIS Synchrotron Radiation Source in DESY (Hamburg). This beamline is especially designed for anomalous scattering studies of anisotropic samples in materials science and enables ASAXS studies to be undertaken on the length scale 0.5 to 100 nm in an energy range of 4.5 to 35 keV.

A high magnetic field EPR spectrometer

Abstract: We describe a tunable electron paramagnetic resonance (EPR) spectrometer designed to operate at frequencies between 160 and 525 GHz and magnetic fields of up to 20 T. To operate in such a broad frequency range we use a very stable optically pumped far infrared laser. The performance of the spectrometer has been measured with solid and liquid samples. This allows us to outline the potential uses of the spectrometer.

Surface thermometry by laser-induced fluorescence

Abstract: A novel laser‐induced fluorescence technique has been developed for measuring the surface temperature of reacting and nonreacting materials. The technique involves seeding the material to be examined with a temperature‐sensitive phosphor (Dy:YAG) and monitoring the laser‐induced fluorescence of the phosphor to determine the temperature. The Dy:YAG phosphor displays a temperature sensitivity in the range 300–1700 K. The technique has been applied to both reacting and nonreacting surfaces under laser excitation, allowing temperature and temporal‐history profiles to be determined.

Time‐resolved x‐ray diffraction station: X‐ray optics, detectors, and data acquisition

Abstract: A new x‐ray beamline has recently been built, and is now operational, on dipole magnet 2 of the Synchrotron Radiation Source (SRS) at Daresbury. This beamline takes 32 mrad of horizontal aperture from a central tangent point. The time‐resolved x‐ray diffraction (TRXD) Station 2.1, takes 17 mrad of horizontal aperture and is the central point of this paper. Beamline 2 has been realized as part of a SERC–MRC agreement.

Digital parallel acquisition in frequency domain fluorimetry

Abstract: The analog electronics commonly found in frequency domain fluorometers are limited to collecting only a single frequency at a time, and can be a source of systematic errors. We have developed an instrument in which most of the analog electronics are replaced with a computer‐controlled digital‐acquisition system. The computer is used for the direct collection of multifrequency data; filtering and calculation of the phase and modulation ratio are done by the software. From these values, fluorescence lifetimes can be determined. This new approach reduces most of the systematic errors due to the analog electronics’ hardware and allows for reconfiguration of the instrument with only minor changes of the software. This digital‐acquisition system is not a simple substitution of an analog element, but it provides a new function and new capabilities for frequency domain fluorometers. We have used this digital approach to build a ‘‘parallel’’ phase fluorometer which simultaneously collects and processes several modulation frequencies.

Technique for measuring the frequency‐dependent complex dielectric constants of liquids up to 20 GHz

Abstract: A technique for the measurement of the frequency‐dependent complex dielectric constant e=e’−je‘ of liquids applicable to frequencies up to at least 20 GHz is described. The technique utilizes a coaxial probe dipped into the liquid. The reflection coefficient of the probe was measured using a network analyzer between 45 MHz and 20 GHz. A new de‐imbedding procedure for eliminating the connector and coax mismatches is described, during which the impedance Z(e) of the probe end was determined. The dielectric constant e was determined from Z(e) by modeling the coax–liquid interface as a capacitance. The de‐imbedding procedure, which utilizes three calibrations, directly eliminates the (unknown) fringe‐field impedance. Radiation effects were minimized by using narrow (0.047‐in.) semirigid coax. The technique yields accurate results for e’ and e‘ of liquids such as methanol and water over the entire frequency range up to 20 GHz, and can be used to determine the relaxation spectra of liquid and liquidlike samples.

Improvements of phase‐modulated ellipsometry

Abstract: This article gives several new insights on ellipsometers using photoelastic modulators. The assumption that the modulation has the form δ=δ0+A sin ωt is ruled out. In contrast, it is shown that the presence of higher harmonics in the modulation affects the measured signal. A new formalism is proposed to take this effect into account, and experimental evidences of its consistency and relevance are exposed. Using this multiple‐harmonic model, ellipsometric measurements showed a dispersion four to five times less than using the conventional model. A new method is proposed to adjust the modulation amplitude during the measurements, by measuring the third harmonic of the signal. It is proven experimentally that this method actually improves the precision of phase‐modulated ellipsometry. Other sources of errors are reviewed, such as multiple reflections in coherent light. A practical procedure to test whether a modulator is well adapted to ellipsometry is given.

JET polari-interferometer

Abstract: A multichannel far‐infrared interferometer used on the Joint European Torus (JET) is described. The light source is a 195‐μm DCN laser. The instrument is of the Mach–Zehnder type, with a heterodyne detection system. The modulation frequency (100 kHz) is produced by diffraction from a rotating grating. There are six vertical and two oblique channels. The latter rely on retroreflection from mirrors mounted on the vessel wall. Their vibration is compensated by a second wavelength interferometer at 118.8 μm. The various subsystems are described, with emphasis on features necessitated by (a) large path lengths, (b) remote handling requirements, (c) fluctuations in atmospheric humidity, and (d) unmanned automatic operation. Typical measurements, along with real‐time and off‐line data analysis, are presented. The phase‐shift measurement is made with an accuracy of (1)/(20) of a fringe, corresponding to a line‐integrated electron density of 5×1017 m−2. Comparison with other electron density diagnostics are shown. The introduction of additional optics allows measurements of the Faraday effect and a determination of the poloidal magnetic field distribution. The signal processing and data analysis are described. Errors introduced by the calibration procedure, birefringence of the probing beams, toroidal field pickup, the flux geometry, and the density profile are considered. The Faraday angle is measured with an accuracy of 5% and a time resolution of 1–10 ms. The poloidal magnetic field is deduced with an accuracy of ±15%.

Variable‐heating‐rate wire‐mesh pyrolysis apparatus

Abstract: An electrically heated wire‐mesh apparatus for pyrolysis studies has been developed which uses computer‐driven feedback control for the heating system and thus can apply virtually any time‐temperature history to the sample. Internal components are water cooled to prevent heat buildup during long runs. Using this system, coal pyrolysis has been studied at heating rates from 0.1 to about 5000 K/s and temperatures up to 1000 °C. Alternating current is used for heating; this allows the thermocouples to be attached directly to the sample holder and also makes power regulation relatively simple. For atmospheric‐pressure experiments, a gas sweep can be forced through the sample holder to remove products from the heated zone and also to concentrate them in a trap which can be removed from the apparatus and weighed to establish tar yields directly. Although the design is optimized for atmospheric‐pressure operation, relatively simple modifications allow operation under vacuum or at pressures of up to 160 bars in inert gas or hydrogen. The apparatus has been used to investigate a number of phenomena in coal pyrolysis and, most significantly, has demonstrated the existence of a heating‐rate effect which is independent of reactor geometry.

An in situ cell for transmission EXAFS measurements on catalytic samples

Abstract: An in situ cell suitable for transmission EXAFS measurements on catalytic samples is described. The cell can be used for catalyst pretreatments in various atmospheres (including H2, H2S, O2 and CO) in a temperature range upto 700 K. The sample is heated by conducting heat from an external heater to the sample. During measurement the samples can be cooled down to 77 K by conducting heat from the sample to an external liquid nitrogen container. During the pretreatment and the measurement a waterflow through the body of the cell keeps certain crucial parts from overheating or icing up. To avoid radiation leaks in powdery samples these samples are pressed in a selfsupporting wafer and held in a disk‐shaped sampleholder. Tests by various catalytic groups have proven the suitability of the design.

Statistics of ion‐induced electron emission from a clean metal surface

Abstract: With a novel combination of experimental approaches, ion‐induced electron emission from clean polycrystalline gold bombarded with (1–5 keV/amu) H+, H+2, and H+3 has been studied by measuring total electron yields, ejected‐electron energy distributions, and, in particular, electron emission statistics (ES). Evaluation of ES included corrections for backscattering of electrons from the surface of the applied solid‐state electron detector. By comparing ES for atomic and molecular ions of equal impact velocities, a negative molecular effect could be clearly demonstrated. This effect was further investigated by comparing measured ES for molecular ion impact with ES synthesized from measured ES for H+ and H+2 ion impact, respectively. The quality of approximating measured ES by Poisson and Polya statistical distributions was investigated in detail and found to be generally unsatisfactory.

A sensitive double beam laser interferometer for studying high-frequency piezoelectric and electrostrictive strains

Abstract: A double beam laser interferometer is built up to study high‐frequency piezoelectric and electrostrictive strains. The system is capable of resolving a displacement of 10−2 A using lock‐in detection and measuring the strain all the way to the piezoelectric resonance frequencies using a digital oscilloscope for detection. The interference of sample bending to the detected signal is effectively avoided.

High‐resolution time‐of‐flight mass spectrometers: Part I. Effects of field distortions in the vicinity of wire meshes

Abstract: Field distortions in the vicinity of wire meshes can change the direction of passing ions. Small deflection angles of ions passing through wire meshes in the reflector can reduce the mass resolution of time‐of‐flight mass spectrometers. We calculate these effects.

On the concentration, focusing, and collimation of x‐rays and neutrons using microchannel plates and configurations of holes

Abstract: A new class of focusing, condensing, and collimating devices for x rays, γ rays, and neutrons is described which is based on the use of microchannel plates (MCPs) or configurations of holes. As well as providing a simple collimating action due to the absorption of severely divergent rays in the channel walls, the microchannel‐plate‐type structures are capable of providing a focusing and collimating action arising from the reflection of near‐grazing‐incidence rays at the channel walls. The focusing action is in principle controllable by mechanical bending or slump forming of the channel plates or by appropriate shaping of the channel walls and is remarkably insensitive to source location, device alignment, and wavelength. Theoretical predictions for the performance of such devices are presented via both simple model calculations and Monte Carlo simulations. These suggest that some simple scaling relations hold between key parameters, and provide a guide to the optimum performance levels achievable with such devices for a wide range of geometries. Some experimental observations of the focusing action of MCPs for 0.071‐nm x rays are presented. A wide variety of x‐ray and neutron optical applications is possible.

High‐resolution time‐of‐flight mass spectrometer

Abstract: To perform experiments with clusters, a time‐of‐flight mass spectrometer with a mass resolution of 35 000 (m/Δm, 50% definition) has been constructed. The main design features of the instrument are presented.

Effect of contact size and placement, and of resistive inhomogeneities on van der Pauw measurements

Abstract: The measurement errors of resistivity and Hall coefficient in the van der Pauw technique due to contact lead size or lead placement errors is calculated for square and cloverleaf‐shaped samples, and compared to the results for a simple circular disk. Both the square and the cloverleaf greatly reduce these errors, compared to the circular sample shape. Another source of measurement error, sample inhomogeneity, is also analyzed, and it is shown that the error introduced by inhomogeneities is proportional to E2 in the region of the inhomogeneity. Consequences of these results are discussed.

Theory of the gasket in diamond anvil high‐pressure cells

Abstract: The operation of the gasket in the diamond anvil cell (DAC) is analyzed and some practical conclusions are given, relevant to users of DACs in the submegabar pressure range. Two distinct regimes are found, one where the gasket is at the maximum thickness and the DAC is intrinsically unstable, and the other where the gasket is thin and the cell is stable in operation, so that there are good reasons to use preindented gaskets, of thickness rather less than usually recommended. Force‐pressure plots are shown to give valuable information in routine use, and their interpretation is discussed.