Volume 72, Issue 12, December 2001
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
72(2001); http://dx.doi.org/10.1063/1.1416107View Description Hide Description
The use of synchrotron radiation (SR) as a far- and mid-infrared source in the measurement of photoacoustic Fourier transforminfrared spectra of solids is demonstrated for the first time in this work. Initial experiments were performed at beamline U10A at the National Synchrotron Light Source, Brookhaven National Laboratory. For synchrotron photoacoustic spectroscopy to be feasible, it must yield results superior to those obtained with a conventional thermal (Globar®) source; accordingly, SR and Globar® photoacoustic spectra recorded under similar conditions were compared in detail. The intensities of SR far-infrared photoacoustic spectra were found to be consistently greater than the corresponding Globar®spectra. At shorter wavelengths, SR always underfills the effective aperture (or, alternately, sample size); SR is a superior source in a spectral region that is a function of this aperture. The high wave number limit of this region exhibits a power-law dependence on aperture size. This investigation also showed that the entire mid-infrared photoacoustic spectrum is more intense using SR and apertures smaller than approximately 0.5 mm.
72(2001); http://dx.doi.org/10.1063/1.1409565View Description Hide Description
We describe two very long path absorption cells for sensitive laser absorption spectroscopy which are based on a design first described by Chernin and Barskaya [Appl. Opt. 30, 51 (1991)]. The cells distribute a nearly arbitrary number of spots across both dimensions of the field mirror in an efficient, compact array: confining up to 500 traversals in a 26 cm diam cooled cylindrical vacuum housing. The first cell, constructed at Denison University, is one of several multipass cells in the Laser Spectroscopy Laboratory used in the visible and near infrared (400–1100 nm). Optical paths of up to 1 km have been attained in this 3 m long cell at pressures from near vacuum to several atmospheres. The cell operates at temperatures down to 125 K. Conventional Pyrex optics are used. The second cell is part of a long path laboratory laser absorptionspectrometer used in the thermal infrared: 350–2000 cm−1 at Goddard Space Flight Center. Path lengths up to 476 m have been achieved at frequencies less than 500 cm−1. Two unique features of the Goddard cell are the piezoelectric micrometers, which simplify considerably the controls for the optical alignment, and the diamond-turned spherical mirrors, which significantly reduce the cost and the complexity of cryogenic operation. In addition, we discuss criteria for determining the optimal pathlength of a given multipass cell for the measurement of weak spectral lines.
72(2001); http://dx.doi.org/10.1063/1.1408934View Description Hide Description
An intense ultraviolet and visible radiationsource for various applications has been conceptualized, designed, and developed using a mode microwave cavity filled with plasma immersed in a magnetic field. The radiation intensity emitted from the source, as a function of the magnetic field intensity, approaches maximum at the electron cyclotron resonance. Light intensity increases with pressure inside the lamp and the microwave power.
- PARTICLE SOURCES, OPTICS and ACCELERATION, DETECTORS
72(2001); http://dx.doi.org/10.1063/1.1416112View Description Hide Description
We describe a high speed imagingsystem that is used to track tracer particles in highly turbulent flows. The system uses silicon strip detectors designed for high energy physicsexperiments and is capable of reading two detectors at a frame rate of 70 kHz. Each detector contains 512 strips and measures a one-dimensional projection of the light striking it. The position measurements from this system have a dynamic range of 6400:1. Extensions to higher frame rates and more detectors are possible. We describe the detectors, readout system, supporting systems, and give an evaluation of the measurement accuracy.
72(2001); http://dx.doi.org/10.1063/1.1416109View Description Hide Description
Bradbury–Nielson gates for the modulation of beams of charged particles, particularly ion beams in mass spectrometry, have been produced with an adjustable wire spacing down to 0.075 mm. The gates are robust, they can be fabricated in less than 3 h, and the method of production is reproducible. In time-of-flightmass spectrometers, fine wire spacing leads to improvements in mass resolution and modulation rates. Gates that were produced using this new method have been installed in a Hadamard transform time-of-flightmass spectrometer in order to demonstrate their utility.
72(2001); http://dx.doi.org/10.1063/1.1412859View Description Hide Description
The use of resistive charge division based solid-state detectors for position sensing of electrons has been studied. The operational conditions having importance for device performance are considered for one- and two-dimensional detectors manufactured by SiTek®. The parameter of main interest has been position resolution. Good linearity with a spatial resolution of 0.2 mm is obtained under optimum conditions. The advantages and limitations of the present approach and alternative techniques that could be considered for individual electron detection are discussed. A possible application of the detector is foreseen in its use for lattice location studies of impurity atoms by emission channeling.
- NUCLEAR PHYSICS, FUSION and PLASMAS
72(2001); http://dx.doi.org/10.1063/1.1412856View Description Hide Description
A method has been devised to extract the two-dimensional ionization profiles from spatially resolved ion density measurements made in a capacitively coupled argon rf plasma. This technique is valid if the production process is ionization and the loss process is ambipolar diffusion. As this procedure calculates the total production (ionization) term, processes such as two-step ionization and photoionization are included. The main benefit of this new technique is that it extracts ionization rates that pertain to the total ionization in the discharge. It follows that this technique could be used to determine when other ionization processes (e.g., photo- and two-step ionization) are important. On testing with experimental data, the inversion process was observed to reproduce the original source term, thereby verifying that the iteration process converges to a real solution. The calculated ionization rate using the integral method is in good agreement with the ionization profiles obtained from the density measurements. Hence, there is now a method of determining the ionization rate that is independent of electron energy probability function measurement and that includes all ionization processes, not just ground state ionization by electron–neutral collisions.
Absolute calibration of space- and time-resolving flat-field vacuum ultraviolet spectrograph under both P and S polarized light conditions for plasma diagnostics72(2001); http://dx.doi.org/10.1063/1.1416105View Description Hide Description
Measurement of spectra in the wavelength range from vacuum ultraviolet (VUV) to soft x ray is an important means to diagnose impurity behavior in magnetically confined plasmas used in fusion plasmas. Recently, a space- and time-resolving flat-field grazing-incidence VUVspectrograph was constructed for the simultaneous observation of spatial, temporal, and spectral distributions of plasma radiation in the wavelength range of 150–1050 Å. Absolute calibration experiments were performed at beamline 11C and beamline 12A in the Photon Factory at the High Energy Acceleration Research Organization. The absolute sensitivity of the VUVspectrograph was measured under both P and S polarized light conditions.
Nonlinear optical tagging diagnostic for the measurement of Fokker–Planck diffusion and electric fields72(2001); http://dx.doi.org/10.1063/1.1419221View Description Hide Description
Strong optical pumping of ions can result after a few microseconds of resonance excitation in a laser beam. However, both Fokker–Planck diffusion and acceleration due to macroscopic electric fields can remove an ion from resonance by changing the ion velocity on a similar timescale. Therefore, the time dependence of laser induced fluorescence can be influenced by particle acceleration and velocity–space diffusion. This effect which has already been used to measured Fokker–Planck diffusion, is extended to include the influence of an electric field and used to measure the electric field associated with an electrostatic shock in a multipolar gas discharge.
72(2001); http://dx.doi.org/10.1063/1.1419228View Description Hide Description
The characterization of a helicon plasmasource has been done in argon plasmas using a rf compensated Langmuir probe. It is now well known that helicon discharges exhibit three modes of operation: a capacitive mode (known as the E mode), which is a low-density mode (around an inductive mode (known as the H mode) with higher densities and a helicon-wave sustained mode (known as the W mode) with very high densities By increasing the injected power, the transitions between these modes can be observed and correspond to sudden increases of the plasma density and of the glow brightness. In this article the performances of the helicon source in terms of density and plasma potential are presented and discussed. Transitions between the different coupling modes are observed and explained thanks to two phenomenological models and a good agreement is found between the experimental values and the model results. It has been shown that the main parameter, which determines the coupling mode, is the plasma density, which depends on the rf power and the gas pressure. So the transition between E and H modes is observed when the density reaches a critical value: for this value the skin depth δ is equal to the radius of the Pyrex tube. A similar result has been shown for the transition: a minimum density, the value of which depends on the magnetic field amplitude, is necessary to observe the first longitudinal helicon-wave mode. This value is also directly linked with the geometry of the reactor and of the antenna.
- MICROSCOPY and IMAGING
72(2001); http://dx.doi.org/10.1063/1.1416104View Description Hide Description
A new analog frequency modulation (FM) detector (demodulator) for dynamic force microscopy (DFM) is presented. The detector is designed for DFM by utilizing the FM detection method where the resonance frequency shift of the force sensor is kept constant to regulate the distance between a tip and a sample surface. The FM detector employs a phase-locked loop (PLL) circuit using a voltage-controlled crystal oscillator (VCXO) so that the thermal drift of the output signal is negligibly reduced. The PLL is used together with a frequency conversion (heterodyne) circuit allowing the FM detector to be used for a wide variety of force sensors with the resonance frequency ranging from 10 kHz to 10 MHz. The minimum detectable frequency shift was as small as 0.1 Hz at the detection bandwidth of 1 kHz. The detector can track a resonance frequency shift as large as 1 kHz. We also present some experimental results including the observations of the Si(111)-7×7 reconstructed surface and fullerene molecules deposited on the surface by DFM using this FM detector.
72(2001); http://dx.doi.org/10.1063/1.1416120View Description Hide Description
A scanning tunneling microscope(STM) comprised of two mechanically and electrically independent probes is described. This dual-probe STM is capable of atomic resolution imaging with either tip. The two probes have five combined degrees of freedom, which allow them to be positioned together at the same surface location without alterations in tip shape. The positioning capabilities of the microscope are demonstrated by obtaining images with each tip of a unique location on a graphite surface. Stable tip–tip imaging is demonstrated by directly recording the tunneling current between probe tips and the sample.
72(2001); http://dx.doi.org/10.1063/1.1416108View Description Hide Description
A procedure for operating an Eigler-style, low temperature scanning tunneling microscope(STM) at variable temperatures has been developed. A critical exchange gas pressure regime was found to allow for controlled variation of the STM temperature while it is encapsulated in a liquid helium Dewar. The sensitivity of various parameters to the ability to generate stable variable temperatures above 4 K is discussed.
72(2001); http://dx.doi.org/10.1063/1.1416113View Description Hide Description
The design and performance of an ultrahigh vacuum compatible cryogenic dual-tip scanning tunneling microscope is described. The microscope is attached at the bottom of a low-loss liquid helium Dewar and can be operated down to 4.2 K. The coarse positioning system consists of five linear steppers driven by piezo-tubes. The displacement of each stepper can be monitored by its own embedded capacitive position sensor with a submicron resolution, thus allowing accurate control of the tip navigation process. An alignment procedure, using a specimen made of three mutually nonparallel planes, is introduced to bring the two tips into overlapped scan ranges without the help of an additional guiding device such as an electron microscope. The overall system exhibits good mechanical rigidity and atomic resolution has been achieved with either tip. This instrument is well suited for investigating low temperature quantum properties of atomically clean nanostructures in a three-terminal configuration.
- CONDENSED MATTER; MATERIALS
72(2001); http://dx.doi.org/10.1063/1.1419219View Description Hide Description
We present the design and performance tests of a miniaturized pulsed low-energy electron gun. Electrons photoemitted from a gold cathode are accelerated over a distance of 75 μm and then collimated by a microchannel plate. According to calculations, this novel concept will allow the time spread of the electron pulses to be kept below 5 ps for kinetic energies as low as 100 eV. The achievement of a minimum angular beam divergence (≈1°) along with an energy resolution of 1.1 eV has to be paid for by low signal intensities. We demonstrate the performance of the gun and the high electron-beam coherence by presenting low-energy-electron diffraction images taken from a submonolayer of lead adsorbed on the germanium (111) surface. We anticipate that this electron gun will open up new possibilities for following structural changes on solid surfaces in real time.
72(2001); http://dx.doi.org/10.1063/1.1416106View Description Hide Description
Experiments conducted in a modified Langmuir trough are described. Measurements of surface tension were obtained using a Wilhelmy plate, while infrared (IR) images of the water surface adjacent to the Wilhelmy plate were simultaneously recorded. A contaminating surfactant film was allowed to form on the surface while the experiments were being conducted. The data reveal a substantial change in the IR imagery due to the film in all cases. The difference between the appearance of the clean and surfactant-covered regions of the surface is clear and distinct. For some of these experiments the change in the IR imagery is accompanied by no observable change in surface tension. This, along with several other aspects of this work, suggest that IR imagery may be a superior tool, compared to traditional surface tensionmeasurements, for ascertaining the cleanliness of a liquid substrate.
72(2001); http://dx.doi.org/10.1063/1.1419225View Description Hide Description
The control of the thermal gradient in single crystal grown by the laser-heated pedestal growth method is demonstrated through focalization of the laser beam into two different regions of the starting feed stock. To create these conditions, an optical component consisting of two incomplete spherical mirrors with the same focal distances was designed and built in copper and coated with aluminum. The performance of this component was tested in the crystal growth of lithium niobate rods with a radius larger than the critical radius calculated from the theoretical model for single focalization of the laser beam. Thermal gradient and temperature profiles along the crystal axis were measured with single and double focalization of the laser beam during the crystal growth process. The measured data indicate that the main differences in the thermal gradients of both configurations are found in the first two millimeters away from the melt–crystal interface. Single crystal rods of lithium niobate grown along the c axis with radius larger than the critical one are demonstrated with the use of this optical component.
72(2001); http://dx.doi.org/10.1063/1.1419226View Description Hide Description
Containerless solidification of Si K), Zr K), Nb K), and Mo K) has been carried out by the electrostatic levitation (ESL) process in which a bulk sample was electrostatically charged and levitated, then melted by laser irradiation in high vacuum. Multiple cycles of melting–undercooling–recalescence–cooling of Si (23.1 mg), Zr (41.9 mg), Nb (40.4 mg), and Mo (45.7 mg) were reproducibly repeated. Among them, Mo is a refractory material with the highest melting temperature ever containerlessly melted and solidified by electrostatic levitation. The ESL-processed Mo sample showed a thermal dendritic structure throughout the entire surface uniformly without any apparent grain boundaries implying heterogeneous nucleation must have been suppressed due to the containerless condition in the ESL process. The back reflection Laue pattern also revealed a homogeneous quality of the ESL-processed Mo sample.
72(2001); http://dx.doi.org/10.1063/1.1408931View Description Hide Description
A frequency response device has been designed and built based on previous efforts by Y. Yasuda, L. C. V. Rees, F. Meunier, and Ph. Grenier. This design uses the best attributes of the three previous devices and is improved in five specific ways. First, the device makes use of the fast response capacitance manometer. Second, the device uses a servomotor to push and pull a metal bellows pump, which drives the sinusoidal input function, removing the chance of “floating” the bellows. Third, both sinusoidal and step-change volume perturbations can be studied. Fourth, the overall cost of the device was kept below $30 000. Finally, the pressuretransducer is mounted only 8 cm from the sample, instead of ∼30 cm, thus removing any averaging of the pressure signal at higher frequencies. The device is currently capable of measuring frequencies between 0.005 and 5 Hz, a range of three full orders of magnitude. The system can operate at temperatures between room temperature and 473 K and pressures between 0.3 and 300 Torr. Two systems were used to test the device, n-hexane/silicalite and methanol/silicalite. The model developed by Yasuda is used to analyze the frequency response of these systems. This model can describe a system as having more than one diffusivity and accounts for the presence of a surface resistance to diffusion. The results for n-hexane/silicalite and methanol/silicalite are compared to those of van den Begin and Nayak, respectively, and are in good agreement. Also, the diffusivities measured using this device are comparable to those estimated in 1997 by Rees using the frequency response method. In addition to measuring the diffusion rate processes, the kinetic parameters associated with a surface resistance to diffusion were also estimated.
72(2001); http://dx.doi.org/10.1063/1.1419218View Description Hide Description
In order to determine ppm orders of air pollutants, e.g., halide impurities, spectroscopy of the photons emitted from the plasma generated by a focused microwave beam in the air has been applied. The microwave beam was supplied by a pulsed magnetron (microwave frequency: 9.4 GHz; peak intensity: 100 kW; pulse width: 5 μs; pulse repetition frequency: 10 Hz), focused in a vacuum chamber, where pressure and impurities had been controlled to target values, and a plasma was generated using the air there. At the same time, the energy spectrum of the photons from the plasma was measured to determine the concentrations of chlorine and fluorine in the air by a measurement system consisting of a diffraction grating, streak camera, and charge-coupled-device camera. As a result of preliminary experiments, where the concentration of was changed from 0.05 to 2 vol % in decompressed air (pressure: 10 Torr), it was demonstrated that 1700 ppm of chlorine could be determined from the background spectrum caused by the first positive band of nitrogen its second positive band and first negative band The experimental data demonstrated that ppm orders of halide impurities can be determined by improving pulse repetition frequency of the microwave source.