Volume 72, Issue 3, March 2001
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
72(2001); http://dx.doi.org/10.1063/1.1347965View Description Hide Description
The spatial resolution for infrared microspectroscopy is investigated to determine the practical limits imposed by diffraction or optical aberrations. Quantitative results are obtained using high brightnesssynchrotron radiation, which serves as a diffraction-limited infrared “point source” for the microscope. The measured resolving power is in good agreement with diffraction theory, including a ∼ 30% improvement for a confocal optical arrangement. The diffraction calculation also shows how the confocal setup leads to better image contrast. The full width at half maximum of the instrument’s resolution pattern is approximately λ/2 for this arrangement. One achieves this diffraction limit when the instrument’s apertures define a region having dimensions equal to the wavelength of interest. While commercial microspectrometers are well corrected for optical aberrations (allowing diffraction-limited results), the standard substrates used for supporting specimens introduce chromatic aberrations. An analysis of this aberration is also presented, and correction methods described.
72(2001); http://dx.doi.org/10.1063/1.1347976View Description Hide Description
The optical design of a brilliant synchrotron source of infrared radiation for structural and time resolved studies in the biological and material sciences is described. With this infrared source, BESSY will open its doors to a new community of scientists and to new research possibilities.
72(2001); http://dx.doi.org/10.1063/1.1338486View Description Hide Description
A new instrumentation variation on laser induced breakdown spectroscopy(LIBS) is presented that allows simultaneous determination of all detectable elements using a multiple spectrograph and synchronous, multiple charge coupled device spectral acquisition system. The device has been designed to reduce the cost penalties often associated with the application of LIBS while allowing accurate and precise determination of the elemental composition of bulk materials. The system is particularly suited to the analysis of heterogeneous materials such as coal and mineral ores. In coalanalysis detectable elements include the key inorganic components of coal—such as Al, Si, Mg, Ca, Fe, Na, and K—in addition to C and H. Detection limits vary depending on the particular element, but are typically of the order of 0.01% for as received, moist materials. Measurement repeatability and accuracy are typically within ±10% absolute, which is similar to results from standard analysis procedures for heterogeneous materials. Beta versions of the device have been produced and are currently being used routinely in two commercial coal-fired power stations.
Phase modulation at 125 kHz in a Michelson interferometer using an inexpensive piezoelectric stack driven at resonance72(2001); http://dx.doi.org/10.1063/1.1347375View Description Hide Description
Fast phase modulation has been achieved in a Michelson interferometer by attaching a lightweight reference mirror to a piezoelectric stack and driving the stack at a resonance frequency of about 125 kHz. The electrical behavior of the piezo stack and the mechanical properties of the piezo-mirror arrangement are described. A displacement amplitude at resonance of about 350 nm was achieved using a standard function generator. Phase drift in the interferometer and piezo wobble were readily circumvented. This approach to phase modulation is less expensive by a factor of roughly 50 than one based on an electro-opticeffect.
72(2001); http://dx.doi.org/10.1063/1.1344601View Description Hide Description
Fast particle detectors with subnanosecond pulse widths are key elements in modern time-of-flightmass spectrometers. Typically, an impedance matched transmission line from the extended anode to the coaxial cable is necessary to obtain fast pulses. We present an approach using a planar geometry for the transmission line. Thereby, the impedance match is realized with a transition from a 50 Ω suspended substrate microstrip line to a coaxial 50 Ω line. A prototype with an active area of 1.8 cm diameter including a strategy to reduce peak ringing was built and tested. The mean pulse width measured with a 1 GHz analog oscilloscope was 520 ps (full width at half maximum) with a rise time of 380 ps. The robust, compact, and low-weight design of the detector is well suited for an application in space, where weight, space, and power consumption are very limited resources.
72(2001); http://dx.doi.org/10.1063/1.1331325View Description Hide Description
We report a simple technique for generation of tunable subnanosecond pulses (0.2–0.4 ns) based on single spike selection from the relaxation oscillations in a dye laser using preformed and overlapping in time excitation pulses. The pulses were obtained by temporal division of a 5–50 ns initial pulse using a Pockels cell with a 1–2 ns switching time and an optical delay time. The proposed technique widens the choice of suitable dye lasers in which the single spike selection technique may be applied, to include dye lasers excited by a Q-switched solid-state laser or a Cu-vapor laser. Other advantages to this technique are the shortened length of the generated subnanosecond pulse, the increased energetic output, higher reproducibility, and the possibility for synchronization of spike generation to an external signal accurate to within 2–3 ns.
- PARTICLE SOURCES, OPTICS and ACCELERATION
72(2001); http://dx.doi.org/10.1063/1.1344599View Description Hide Description
72(2001); http://dx.doi.org/10.1063/1.1344600View Description Hide Description
We report here on the construction and use of a pair of matched glass collimating slits which are suitable for molecular beam work. These glass slits transmit nearly all (>85%) of an incident laser beam. Additionally, these slits are matched to better than 5–10 μm at a gap size of 200 μm.
72(2001); http://dx.doi.org/10.1063/1.1340029View Description Hide Description
A third generation plasma immersion ion implanter dedicated to biomedical materials and research has been designed and constructed. The distinct improvement over first and second generation multipurpose plasma immersion ion implantation equipment is that hybrid and combination techniques utilizing metal and gas plasmas,sputter deposition, and ion beam enhanced deposition can be effectively conducted in the same machine. The machine consists of four sets of high-efficiency metal arc plasma sources with magnetic filters, a custom designed high voltage modulator for operation up to 60 kV, a separate high-frequency, low-voltage power supply for hybrid treatment processes, special rotating sample stage for samples with an irregular shape, and other advanced features. The machine has been installed at Southwest Jiaotong University and operated reliably for 6 months. This article describes the design principles and performances of the machine as well as pertinent biomedical applications.
Effect of discharge microwave frequency on electron temperature of electron cyclotron resonance plasma72(2001); http://dx.doi.org/10.1063/1.1347972View Description Hide Description
The characteristics of argon electron cyclotron resonanceplasmadischarged by 7.0, 8.0, and 9.4 GHz microwaves were investigated. Experiments were conducted in a fixed magnetic field. It was found that the electron temperatures exhibited a trend of (suffixes represent the dischargemicrowave frequencies).
- NUCLEAR PHYSICS, FUSION and PLASMAS
72(2001); http://dx.doi.org/10.1063/1.1344175View Description Hide Description
A toroidally viewing x-ray imaging system is used in the T-10 tokamak for reconstruction of the small-scale plasma perturbations (high-m modes). The imaging system is based on the matrix array of the silicon surface-barrier diodes with a two-dimensional Soller collimator and an automatic positioning system. An additional hard x-raydetector and magnetic probe are placed inside the tokamak vessel for measurements of the nonthermal emission and fast magnetic field perturbations. Initial experiments in T-10 indicated improved sensitivity of the system for identification of the small-scale plasma perturbations prior the density limit disruptions.
72(2001); http://dx.doi.org/10.1063/1.1347971View Description Hide Description
Standard single frequency, “fringe-counting,” microwaveinterferometers are of limited use for steady-state plasma experiments. We have constructed a swept frequency microwaveinterferometer, similar to a classic zebra-stripe interferometer, optimized for electron density measurements in steady-state plasma experiments. The key element in the system is a frequency doubled YIG oscillator capable of sweeping from 20 to 40 GHz. As the source frequency is swept, the sum of the reference and plasma leg signals exhibits a series of beats. Both the frequency shift and phase shift of the beat pattern due to the addition of plasma in one leg of the interferometer is used to determine the line-integrated electron density.
72(2001); http://dx.doi.org/10.1063/1.1344598View Description Hide Description
Traditional solid window materials used for x-ray synchrotron beamlines may introduce undesirably high attenuation, or are subject to failure under high heat loads. A plasma window can in principle obviate these problems over a wide range of energies. Experiments were performed at the Brookhaven National Laboratory National Synchrotron Light Source on beamline X6A to study the transmission characteristics of a plasma window using argon as the arc gas. Measurements were made around the Ar K edge and far from resonance. The “white-line” absorption at the K edge was actually suppressed during arc operation as compared to room temperature gas at the same pressure. This is attributed to the high degree of ionization in the plasma. The relative strength of the white line to the edge jump does not seem to be a strong function of arc current at the argon K edge. Away from resonance (∼3 times the edge energy) x-rayattenuation was negligible.
A laser photodetachment technique for the measurement of in a high frequency traveling wave discharge72(2001); http://dx.doi.org/10.1063/1.1319861View Description Hide Description
A probe based laser photodetachment system has been developed to measure the density in a high frequency hydrogen discharge sustained by a traveling surface wave. Problems associated with application of this technique in high frequency discharges in general are pointed out. The influence of the time varying voltage across the plasma probe sheath on the probe measurement has been minimized by applying a passive method of compensation. An emissive probe, placed inside the launcher, has been used as a reference electrode in order to minimize the discharge disturbance and to reduce the additional resistance in the measuring system. A filtering technique has been incorporated in the detection system to separate the photodetachment signal from the signal at the discharge stimulating frequency. The big heating of the probe, set at electron saturation condition, has been avoided by using a convenient time sequence of the detection system operation. The photodetachmentdensity measurements are confirmed by applying a high sensitivity second derivative probe technique as a second reference method. Axial variation of density, as measured, correlates with the axial distribution of hydrogen atoms. Experimentally obtained radial density profile confirms the well known fact that the negative ions are confined in the existing radial potential well.
- MICROSCOPY and IMAGING
72(2001); http://dx.doi.org/10.1063/1.1344602View Description Hide Description
Superconductingscanning tunneling microscope tips have been fabricated with a high degree of reproducibility. The fabrication process relies on sequential deposition of superconductingPb and a proximity-coupled Ag capping layer onto a Pt/Ir tip. The tips were characterized by tunneling into both normal-metal and superconducting films. The simplicity of the fabrication process, along with the stability and reproducibility of the tips, clear the way for tunneling studies with a well-characterized, scannable superconductingelectrode.
An apertureless near-field scanning optical microscope and its application to surface-enhanced Raman spectroscopy and multiphoton fluorescence imaging72(2001); http://dx.doi.org/10.1063/1.1347975View Description Hide Description
We describe a home-built apertureless near-field scanning optical microscope and present preliminary results of its operation. Raman scattering from samples of polydiacetylene para-toluene sulphonate, and two-photon-induced fluorescence from crystallites of coumarin I dye are strongly enhanced in the presence of a sharp gold-coated atomic force microscope tip. We verify the dependence of the scattered intensity on the polarization of the incident beam relative to the tip axis. Finally, we show near-field fluorescence images taken in the presence of a strong far-field background whose spatial resolution is limited by the size of the tip.
72(2001); http://dx.doi.org/10.1063/1.1347978View Description Hide Description
This article describes details of the operation and performance of fast spectral correlationinterferometry, which allows for considerably improved operation of the surface forces apparatus. Up to 150 interference fringes can be simultaneously tracked over a wide spectral range, leading to considerably more precise and simultaneous extraction of multiple optical quantities, over a greatly extended distance range. When used for surface-separation measurement, a precision of 25 pm is readily achievable over a distance ranging from 0 to >10 μm. Data acquisition rates are also considerably improved, allowing for dynamic measurements. Automated actuation of optical deflectors introduces a multidimensional scanning capability to the optical probe with a lateral resolution of 1 μm. The entire process is computercontrolled and features unattended batch processing of complex measurements. This publication illustrates experimental setup, methodology, measurements, and detailed error calculation for a selection of practically relevant situations in the extended surface forces apparatus.
72(2001); http://dx.doi.org/10.1063/1.1347376View Description Hide Description
This article presents an add-on secondary electron energy spectrometer for scanning electron microscopes(SEMs). The add-on unit fits on to the specimen stage of a conventional SEM, and the SEM is operated as normal. As an objective lens, the add-on unit improves the SEMsspatial resolution by around a factor of 4 at a primary beam energy of 1 keV. The add-on unit functions as a multichannel open-loop voltage contrast spectrometer by employing a bandpass deflector/filter unit whose pass energy is ramped with time. The unit is designed to deflect the secondary electrons while leaving the primary beam unaffected. Initial experimental results show that significant voltage and material contrast can be obtained. In a data acquisition time of 0.32 s, a minimum detectable voltage difference of around 54 mV was obtained. Regions of copper and brass that were indistinguishable via the normal secondary electron image, were distinguished with a signal to noise ratio of around 12 by using the add-on spectrometer.
- CONDENSED MATTER; MATERIALS
72(2001); http://dx.doi.org/10.1063/1.1347981View Description Hide Description
We describe an apparatus for measuring the molecular density and orientation of confined, ultrathin complex fluids under static and dynamic flow conditions. The device essentially couples the utility of the surface forces apparatus—ability to control surfaceseparation and alignment under applied loads—with in situstructural characterization of the intervening material utilizing neutronreflectivitymeasurements. The apparatus is designed such that single crystal substrates of quartz or sapphire with areas up to tens of square centimeters can be kept parallel at controlled and well-defined separations from millimeters to less than 100 nm. The large substrate surface area enables direct structuralmeasurements of the density profile of “soft” material placed between the aligned substrates. In addition, the cell is also designed to enable steady shear rates from 0.001 to 20 Hz to be applied in order to follow the dynamic structural response of the confined material, especially at the solid-solution interface. Faster shear rates of order can be obtained using oscillatory motion. Current design specifications focus on the use of neutronreflectivity to characterize the structure of end-grafted polymer brush layers, but the device can be employed to probe the structure of any complex fluid of interest and is amenable to other characterization techniques.
High-pressure vessel for elastic and inelastic x-ray diffraction experiments for liquids over a wide temperature range72(2001); http://dx.doi.org/10.1063/1.1338487View Description Hide Description
A new x-ray scattering environment is presented which can be employed at high temperatures up to 2000 K and high pressures up to 150 bar. This device is especially designed for angle-dispersive scatteringexperiments. It consists of a thick closed-end aluminum cylinder equipped with a steel flange. Incident and scattered radiation passes through continuous Be windows which cover an angle range up to 55°. The high potential of this scattering setup is demonstrated by representative example experiments at high temperatures and high pressures, which could be carried out in combination with a single-crystalsapphire cell. Inelastic-, elastic-, and anomalous scatteringexperiments using synchrotron radiation and energy dispersive static measurements employing a conventional tube device are reported.