Volume 70, Issue 9, September 1999
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
70(1999); http://dx.doi.org/10.1063/1.1149952View Description Hide Description
The design, construction, and operation of an optical build-up cavity (BUC) designed primarily for use in molecular beamlaser spectroscopy where signal is proportional to laser power, is presented. The design is such that the cavity has no internal adjustment controls, needing only alignment with respect to the laser and molecular beams. When used with a single-mode titanium:sapphire laser, a circulating cw power of 900 W was achieved. The BUC increased the signal to noise of the experiment by a factor of 390 relative to a single crossing of laser and molecular beam without degrading resolution. Performance of the BUC is limited by heating of its mirrors by the high power densities incident upon them (∼3 MW cm−2).
70(1999); http://dx.doi.org/10.1063/1.1149953View Description Hide Description
A low-cost detection system for laser-induced plasma spectroscopy (LIPS) is described. The system comprises a Rowland spectrometer with photomultiplier detection and a miniaturized multiple gated integrator system. The sensitivity and the spectral band pass of the system were characterized and compared to a conventional Czerny–Turner system with an intensified diode array detector for chromium as representative analyte. Both, the sensitivity and spectral resolution were found to be sufficient for the intended simultaneous multielement LIPS analysis of simple and constant matrices.
70(1999); http://dx.doi.org/10.1063/1.1149954View Description Hide Description
We have studied a new and reliable cooling method for diamond crystals in third generation light sources. In order to reduce the strain from brazing between the diamondmonochromator and its Cu block cooling device, another diamond platelet was introduced as an interface. Using a finite element method program based on numerical thermal analysis, the mounting strain was estimated at 0.6 arcsec. Then data was confirmed through preliminary x-ray characterization using laboratory x-ray sources. The strain in the area exposed to a 1 mm×4 mm synchrotron radiation beam (absorbed power 100 W) was estimated by simulation at an acceptable 1.0 arcsec. Applying the new cooling method, the diamond crystal can tolerate synchrotron radiation power up to 100 W.
A study on background subtraction in Auger and photoelectron time coincidence spectroscopy using third generation synchrotron radiation source70(1999); http://dx.doi.org/10.1063/1.1149955View Description Hide Description
Auger and photoelectron time coincidence spectroscopymeasurements on Si(100) have been carried out employing third generation synchrotron radiationsource. This work has been focused on the particular treatment of the collected data required for this kind of experiment. The time structure characteristics of the European Synchrotron Radiation Facility light source have been taken into account in time coincidence data analysis, and compared with other sources. This study has shown the particular influence of the source in the distribution of accidental coincident events, revealing the need to introduce a suitable method for background subtraction in coincidence spectra.
70(1999); http://dx.doi.org/10.1063/1.1149956View Description Hide Description
A linear and a circular zone plate with a blazed zone profile (ZPBP) have been fabricated and characterized using synchrotron x rays. The ZPBPs have significantly improved performances in terms of focusing efficiency and the background near the focus compared to those of a zone plate with a square profile, of which the transmission function can be characterized by a binary square wave. In many respects and practical cases, an x-ray ZPBP may be used in a way analogous to an optical lens in the visible light region. In this article, the experimental characterization of the ZPBPs is presented and some special applications are discussed.
70(1999); http://dx.doi.org/10.1063/1.1149947View Description Hide Description
We describe a fiber-optic interferometer that employs wavelength changes to achieve maximum sensitivity. Wavelength changes are induced by adjusting the operating temperature of the laser, eliminating the need for an actuator to vary the spacing between the sensing fiber and the object to be monitored. The instrument and techniques described are suitable for cryogenic, high vacuum applications such as magnetic resonance force microscopy, where space is limited and micromanipulation can be challenging. The noise floor of is adequate for monitoring subangstrom displacement of force microscope cantilevers.
70(1999); http://dx.doi.org/10.1063/1.1149957View Description Hide Description
We have measured the intensity profile of x rays focused by compound refractive lenses (CRLs) fabricated using acrylic (Lucite) and polyethylene plastics. A linear array of closely spaced holes acts as multiple cylindrical lenses. The important parameters for this type of focusing are the focal length and absorption, and, for wavelengths shorter than 3 Å, low atomic number plastics are suitable. We have experimentally demonstrated that we can achieve one-dimensional focusing for photon energies between 9 and 19.5 keV with focal lengths between 20 and 100 cm. For example, using 12 keV x rays we have achieved focal full width at half maximum linewidths down to 21 μm at a distance of only 20 cm from the CRL. The x-ray source was a synchrotron emitter whose source size in the vertical dimension was 445 μm.
Spectral response measurements of an x-ray sensor camera by a fluorescence wavelength dispersive spectrometer70(1999); http://dx.doi.org/10.1063/1.1149958View Description Hide Description
Measurements of the spectral efficiency of an x-raycamera in the 5–25 keV range are presented. The camera consists of a slow scan transfer device (a charge coupled device) optically coupled to a scintillator screen A straightforward method, using laboratory x-ray sources, allows one to carry out measurements in a larger energy range, typically from a few keV to tens of keV by means of a fluorescence wavelength dispersive spectrometer. The spectral responsivity of the x-ray imaging detector, in units of analog-to-digital converter in keV, is discussed. In particular, the assumption of a linear conversion from incident x-ray energy to visible photon energy is confirmed by the experiment.
New high temperature furnace for structure refinement by powder diffraction in controlled atmospheres using synchrotron radiation70(1999); http://dx.doi.org/10.1063/1.1149948View Description Hide Description
A low thermal gradient furnace design is described which utilizes Debye–Scherrer geometry for performing high temperaturex-raypowderdiffraction with synchrotron radiation at medium and high energies (35–100 keV). The furnace has a maximum operating temperature of 1800 K with a variety of atmospheres including oxidizing, inert, and reducing. The capability for sample rotation, to ensure powder averaging, has been built into the design without compromising thermal stability or atmosphere control. The ability to perform high-resolution Rietveld refinement on data obtained at high temperatures has been demonstrated, and data collected on standard powder is presented. Time-resolved data on the orthorhombic to rhombohedral solid state phase transformation of is demonstrated using image plates. Rietveld refinable spectra, collected in as little as 8 s, opens the possibility of performing time-resolved structural refinements of phase transformations.
70(1999); http://dx.doi.org/10.1063/1.1149959View Description Hide Description
We demonstrate the detection of a molecular beam by means of a micromechanical momentum transfer detector operated in vibrational resonance. With a sensitive surface area of the small paddle allows us to detect a beam with He atoms hitting the surface per second. The detector response time equals the damping time of the paddle oscillation of about 1 s. The detector is sensitive enough to measure intensities in molecular beamscattering experiments. The novel detection scheme has the potential to allow the development of a position sensitive molecular beamdetector.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
70(1999); http://dx.doi.org/10.1063/1.1149960View Description Hide Description
The performance of a quadrupole mass filter (QMF) generally degrades when using electrodes of circular cross section in place of mathematical ideal hyperbolic electrodes. The circular cross section of electrodes produces nonlinear resonances resulting in distortion and peak splitting in mass spectra. In addition, resonances reduce the actual working cross section, resulting in limited ion yield. In this article we study nonlinear resonances and intensities of resonance lines passing through the tip of the stability diagram of the QMF. We have found that balancing of multipole terms, rather than eliminating individual multipole terms, improves the sensitivity of the QMF considerably. The theory for assessing intensities of nonlinear resonances is presented in detail along with rescaling laws to adjust current QMF parameter settings. A general formula is presented from which the location and intensity of nonlinear can be derived, which then may be used for the design of special purpose QMFs.
70(1999); http://dx.doi.org/10.1063/1.1149961View Description Hide Description
The low energy electron reflectivity from a magnetic surface depends on the relative orientation between the electron spin and the sample magnetization. This effect has been exploited for realizing a new electron spin polarizationdetector. As a scatteringsurface we have used a well ordered surface, which is stable against surface contamination and gives rise to very large spin dependent effects. We describe in detail the preparation in vacuum of such a target surface and show that it can be transferred in a separate system, without performance losses. The analyzing power S of the detector is given by the relative variation of its response when the target magnetization is reversed. We find extremely large S values for electrons reflected at 3–6 eV kinetic energy, with maximum above 45%, i.e., roughly 3 times better than other polarimeters presently used.
70(1999); http://dx.doi.org/10.1063/1.1149962View Description Hide Description
The superconducting electron cyclotron resonanceion source SERSE of INFN-Laboratori Nazionali del Sud has been recently upgraded with an 18 GHz generator which takes the place of the 14.5 GHz generator, used up to now. In order to further extend the validation of high B mode to higher frequency, some comparative tests have also been carried out, aimed at understanding the role of the magnetic field and frequency on the ion yield at higher levels than were ever done before. The results at the frequencies of 14.5 and 18 GHz are compared and the trend already observed elsewhere is here confirmed. Preliminary observations of the “two frequency heating” have contributed to increase further the currents of the highest charge states.
- NUCLEAR PHYSICS, FUSION and PLASMAS
70(1999); http://dx.doi.org/10.1063/1.1149963View Description Hide Description
We describe a simple laboratory device for establishing a vacuum arc plasma discharge that can serve to simulate a unipolar arc. The technique makes use of a triggered vacuum arc plasma gun to generate a plasma plume that in turn causes breakdown of a secondary discharge. The device is in fact a secondary vacuum arc discharge that is triggered by a primary vacuum arc discharge, with some of the features of the secondary plasma discharge having similarities with a unipolar arc configuration. Here we describe the experimental setup and outline how the device can be used for some areas of unipolar arc materials research.
- BASIC PHENOMENA
70(1999); http://dx.doi.org/10.1063/1.1149964View Description Hide Description
There is currently much interest in the thermoelectricproperties of materials. A method for measuring the thermoelectric power of small single-crystal or polycrystalline samples is described. For high-pressuremeasurements, the small samples are loaded into a diamond anvil cell and compressed. An infrared laser system is used to induce a temperature gradient in the sample. The thermoelectric power is measured with a pair of small thermocouples contacting the sample. Reported here is the thermoelectric power of Ni and the previously reported intermetallic compound up to 10 GPa. Other standards measured by this method included: Bi, Yb, and
Investigation of a microwave differential cavity resonator device for the measurement of humidity in gases70(1999); http://dx.doi.org/10.1063/1.1149995View Description Hide Description
A resonant cavity based microwave differential device has been developed as a sensor for the measurement of small quantities of water vapor in gases. The presence of a contaminant is assessed by induced variations in the relative permittivity due to a shift in the resonant frequency of the measuring resonator. The measured output signal is related to the difference in the reflection coefficients of the measuring resonator and the reference one. A simple modeling approach of the system shows the proportionality between the difference of the reflection coefficients of the cavities and the variation in the dielectric constant. The evaluation of the minimum detectable change in the permittivity is possible using the Clausius–Mossotti equation for a binary gas mixture at a given concentration and pressure. The detection threshold is then determined by taking into account the signal-to-noise ratio of the differential setup. The estimation of the limit of detection for some practical moisture contaminated gases yields values in the low ppm level at K and atmospheric pressure. Experimental data suggest a detection threshold of 6 ppm for moisture.
- MICROSCOPY and IMAGING
70(1999); http://dx.doi.org/10.1063/1.1149965View Description Hide Description
A time-resolvedoptical imagingsystem using near-infrared light has been developed. The system had three pulsed light sources and total 64 channels of detection, working simultaneously for acquisition of the time-resolved data of the pulsed light transmitted through scattering media like biological tissues. The light sources were provided by high power picosecond pulsed diode lasers, and optical switches directed one of the light sources to the object through an optical fiber. The light signals reemitted from the surface of the object were collected by optical fibers, and transmitted to a time-resolved detecting system. Each of the detecting channels consisted of an optical attenuator, a fast photomultiplier, and a time-correlated single photon counting circuit which contained a miniaturized constant fraction discriminator/time-to-amplitude converter module, and a signal acquisition unit with an A/D converter. The performance and potentiality of the imagingsystem have been examined by the image reconstruction from the measured data using solid phantoms.
High resolution photothermal imaging of high frequency phenomena using a visible charge coupled device camera associated with a multichannel lock-in scheme70(1999); http://dx.doi.org/10.1063/1.1149966View Description Hide Description
We have developed a technique using a photothermal microscope from which we can make a thermal image of an electronic component working at a “high frequency” using a charge coupled device(CCD)camera and a multichannel lock-in scheme. To do this, we have created an electronic “stroboscope”: the frequency F of the thermal signal induced by a high frequency electrical excitation and the frequency of the light that illuminates the device are next to each other; the signal reflected at the surface of the device whose amplitude is proportional to the variation of reflectivity and hence to the variation of temperature and whose frequency is the blinking one f is analyzed by a visible CCDcamera. Amplitude and phase images of the high frequency thermal phenomenon can then be made. Moreover, this technique presents a great advantage: the spatial resolution is better than 1 μm. The amplitude and phase images presented show, with a very good spatial resolution, Joule and Peltier heating of a polycrystalline silicon 2.5 kΩ resistor across which a sinusoidal current is forced.
70(1999); http://dx.doi.org/10.1063/1.1149967View Description Hide Description
The interaction between the tip of a near-field scanning optical microscope (NSOM) and the sample it scans is analyzed and compared to a simple tapping model. The approach curves acquired with the NSOM are in excellent agreement with the model, and additional experiments strongly point against a noncontact interaction (such as shear force). Based on this model we are also able to explain the oscillations pattern of the feedback loop. We conclude that our straight-fiber tip, operating under “shear-force” control, intermittently contacts the surface it is scanning, in a way similar to the tapping mode in atomic force microscope.
70(1999); http://dx.doi.org/10.1063/1.1149968View Description Hide Description
A fast low-cost device to detect the phase shift between the excitation and the response of a cantilever in tapping-mode atomic force microscopy is described. For cantilever signals with a good signal to noise ratio, as is commonly found, the device presented can replace a lock-in amplifier. The setup is based on indirect time measurements realized by a combination of commonly used analog and digital integrated circuits. Phase measurement can already be achieved within one cycle. Signal output rates up to 100 kHz allow the use of the phase shift as an auxiliary imaging channel. Cantilever frequencies may range from 6 to more than 500 kHz. The principle of the setup is illustrated together with technical data. Images of a hydrophobic–hydrophilicstructuredsiliconsurface obtained in air and of purple membrane obtained in fluid are presented.