Volume 71, Issue 1, January 2000
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
71(2000); http://dx.doi.org/10.1063/1.1150150View Description Hide Description
We achieved stable single-frequency oscillation in an external-cavity tapered diode laser. The external cavity was constructed from a diffraction grating and an output mirror, which were placed, respectively, in the narrow-area side and the broad-area side of the tapered-stripe chip, with collimatorlenses on both sides. The best position for the output mirror was a focal point of the output beam along the axis parallel to the diode junction plane, while the beam along the axis perpendicular to the junction plane was collimated by the collimator lens. Continuous frequency tuning of 900 MHz and a linewidth below 1 MHz were obtained. A bistability was observed in the output power around the threshold current after the output mirror was installed.
71(2000); http://dx.doi.org/10.1063/1.1150148View Description Hide Description
In the course of 1998, the Spectromicroscopy beamline at ELETTRA completed commissioning and succeeded in performing its first test experiments. The beamline is designed to perform photoemissionexperiments with high spatial resolution, which is obtained by focusing the radiation in a small spot on the sample by means of a multilayer-coated Schwarzschild Objective. Three objectives are currently available; these operate at photon energies of 74, 95, and 110 eV. A review is presented of the performances achieved together with an outlook on the future upgrades of the microscope. The smallest achievable spot size is currently 0.5 μm. At present, the limit to the spatial resolution is due to aberrations caused by figure errors of the objective. Typical counting rates in photoemissionspectra, for example, on the peak, are of the order of counts per second with an energy resolution of the order of 100–200 meV. Among the first experiments in which p- and n-type GaAs layers of 0.25 μm thickness were imaged.
71(2000); http://dx.doi.org/10.1063/1.1150151View Description Hide Description
We performed feasibility tests of photoelectron emission spectromicroscopy of wet samples in the water window (285–532 eV) soft x-rayspectral region. Water was successfully confined in an ultrahigh vacuum compatible compartment with x-ray transparent sides. This water cell was placed in the MEPHISTO spectromicroscope in a transmission geometry, and complete x-ray absorption spectra of the water window region were acquired. We also show micrographs of test samples, mounted outside of the compartment, and imaged through the water. This technique can be used to study liquid chemistry and, at least to the micron level, the microstructure of wet samples. Possibilities include cells in water or buffer, proteins in solution, oils of tribological interest, liquid crystals, and other samples not presently accessible to the powerful x-ray photoelectron emission spectromicroscopy technique.
71(2000); http://dx.doi.org/10.1063/1.1150152View Description Hide Description
A capillary discharge has been developed to produce pulses of intense soft x-ray radiation of tens of nanoseconds duration. The soft x-rayphotons were emitted in a plasma column resulting from polyethylene in capillary wall ablation. The spectrum was dominated by the C IV, C V, and C VI emission lines in the soft x-rayspectral range. The experimental value of the electrical circuit inductance has been measured and compared to the calculated one. The electron temperature was estimated to be higher than 50 eV from the plasmaresistivitymeasurement. The time dependence of the electron density outside the capillary channel has been determined using spectroscopicmeasurement in the visible range. The time dependence of the electron temperature has been determined from the intensity ratio of C V and C VI emission lines, using a collisional radiative equilibrium code.
71(2000); http://dx.doi.org/10.1063/1.1150153View Description Hide Description
Development of a compact beam intensity monitor using gas amplified sample current measurement is described. The monitor can be a powerful tool for x-ray spectroscopy and microscopy when the beam is defined by a small pinhole or slits and when the workspace around the sample is limited. The thickness of the monitor is as small as approximately 3 mm, and the dimension is 10 mm square. The photon flux is monitored by measuring x-ray excited current from an Al foil under atmospheric conditions. Emitted electrons from the Al foil can ionize surrounding air molecules, and the gas amplified current can be measured with the use of a biased grid that prevents created ion pairs from recombination.
71(2000); http://dx.doi.org/10.1063/1.1150154View Description Hide Description
We present and demonstrate a method for balancing the two paths of a two-photon interferometer of the Hong-Ou-Mandel (HOM) variety. This method removes the difficulties of balancing the paths in this type of interferometer where a type-I crystal is used for the generation of the entangled photon pairs. Using this method we show that we are consistently able to balance the ∼2000 mm path lengths in our HOM interferometer on average to within ∼60 μm.
High order quarter-wave plates and analysis of the directions of rotation in the circular polarizations71(2000); http://dx.doi.org/10.1063/1.1150155View Description Hide Description
The advent of new research subjects, such as plasmas, optical pumping, and atom cooling, has led to an increased use of quarter-wave plates in order to produce circularly polarized beams with the correct direction of rotation. The recent commercialized quarter-wave plates work in high orders, which leads to a drastic reduction of their pass band. Moreover, the direction of rotation in circular polarization can be affected. The purpose of the present article is to describe new techniques allowing an accurate determination of the optical properties of the used plates.
71(2000); http://dx.doi.org/10.1063/1.1150156View Description Hide Description
We have developed a laser beamanalyzer for quantitative studies in resonance ionization experiments. The laser beam profiles, which can be represented as images or as graphs, were used to compute the overlap of a number of laser beamsinteracting with an atom beam. In our experimental setup, resonance ionization signal was monitored as a function of overlap of laser beams employing the laser beamanalyzer developed. The dependence of resonance ionization signal was explained with the help of an interaction model that includes the effects of geometry of the atom beam.
71(2000); http://dx.doi.org/10.1063/1.1150157View Description Hide Description
In order to circumvent the limitations of a quadrupole mass filter, which so far has been the most common mass analyzer for use with electrospray ionization, a mass spectrometer coupling electrospray with time-of-flight mass analysis has been constructed. The design of this spectrometer, including a simple electrospray ion source, an atmosphere–vacuum interface without a rf-only quadrupole/multipole that discriminates against low mass ions which can be used for internal calibration, orthogonal extraction, and a reflectron, is presented together with its performance characteristics. A resolving power of 7500 was obtained for the +5 ion of bovine insulin and the detection limit for the same molecule was shown to be 1 f mol. Mass accuracy using a conventional two-point calibration is 60 ppm for internal calibration on low mass ions and better than 100 ppm for external calibration on peptide ions. An improved mass accuracy equal to 10 ppm was achieved using internal three-point calibration on low mass ions naturally present in the spectra. The dynamic range of a single sample was found to be at least 400:1.
71(2000); http://dx.doi.org/10.1063/1.1150158View Description Hide Description
A new supersonic molecular beam-surface scattering apparatus permits eigenstate-resolved measurements of gas-surface reactivity. Infrared light from a narrow-bandwidth tunable laser intersects a supersonic molecular beam and prepares an ensemble of molecules in a single rotational and vibrational quantum state. The energized molecules, with their well-defined translational, vibrational, and rotational energies, pass into an ultrahigh vacuum chamber and impinge on a single crystalmetal surface where their reactivity is quantified. The apparatus provides independent control over translational, vibrational, and rotational degrees of freedom and permits highly detailed studies of gas-surface reactivity. In this article we describe the design and characterization of our apparatus and illustrate its use to study the dissociative chemisorption of methane on Ni(100).
71(2000); http://dx.doi.org/10.1063/1.1150159View Description Hide Description
A new dual-target dual-laser vaporization source for the production of binary metallic clusters is presented. Clusters of the type (X=Al, Fe, Co, Ni) were produced. Excellent control over the mixing process was achieved by varying the delay time between the firing of the two lasers and their energy densities. Having identified these critical parameters, their influence over the production process is shown in detail for the system. The production of bimetallic clusters in this source is due to the spatial and temporal overlap of the two laser vaporized materials in the source.
71(2000); http://dx.doi.org/10.1063/1.1150160View Description Hide Description
The design and performance of a mechanical shutter for laser light based on low voltage piezoelectric transducers is described. By focusing the beam through a pair of piezo-mounted slits, a transmission of 80%, a switching time of 10 μs, and an extinction ratio of 300:1 are obtained.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
71(2000); http://dx.doi.org/10.1063/1.1150161View Description Hide Description
A wakefield is generated when electron bunches pass through discontinuities of beam pipes, such as rf cavities. We analyze the relation between the timing jitter of the bunches and the wakefield. From the analysis we derive a mathematical expression through which the jitter as well as the amplitude fluctuation of the bunches can be obtained by measuring the power spectrum of the wake. A measurement of the bunch jitter using this frequency-domain based wakefield analysis is compared with that measured by a time-domain based sampling oscilloscope. The two measurement results correspond well each other.
71(2000); http://dx.doi.org/10.1063/1.1150162View Description Hide Description
A low-pressure and high-density microwaveion source created and sustained by evanescent waves emitted from a holey plate, and using permanent magnet multicusp fields, has been studied. This source is called a holey-plate ion source.Microwave power at 2.45 GHz is supplied from a coaxial cavity and then converted into an evanescent mode through the use of a holey plate. This source can produce low-pressure and high-density plasma using an octopole magnetic field. The ion current densities are in excess of 4.8 mA/cm2 at an extraction voltage of 3 kV, an argon gas pressure of 0.3 Pa in the plasma chamber, and an input microwave power of 80 W. An 18-mm-diam ion beam is extracted and accelerated from the ion source.
- NUCLEAR PHYSICS, FUSION and PLASMAS
71(2000); http://dx.doi.org/10.1063/1.1150163View Description Hide Description
The present work is concerned with the optimization of the sapphire fast neutron filter thickness used in neutron diffraction instruments. The optimization is based on maximization of the slow neutron transmission, minimization of the fast neutron transmission, and also taking into consideration the neutron background at the vicinity of an instrument. Scattering properties of the sapphire in the fast and slow neutron regions are discussed.
71(2000); http://dx.doi.org/10.1063/1.1150164View Description Hide Description
A new O-mode dual frequency heterodynereflectometer has been installed on Tore Supra [Tore Supra Team, Fusion Technol. 29, 417 (1996)] and it has greatly improved the phase determination due to its high dynamic heterodyne detection and ultrafast sweep capabilities (10 μs) provided by its solid state source HTO. This system operates with O-mode electric fieldpolarization in the range of 26–36 GHz and has been designed for density profile measurements. The reflectometer launches two frequencies separated by 320 MHz simultaneously into the plasma. Heterodyne detection improves the dynamics up to 60 dB, and is associated with a sin/cos detection to allow separate analysis of the amplitude and phase of each reflected signal of both probing waves. Therefore, to calculate the density profile, the group delay can be defined in two different ways: (i) from the derivative of the absolute phase of one of the two probing waves or (ii) by calculating the phase difference between the two probing waves. We explain how ultrafast sweep operations (down to 10 μs) significantly reduce the influence of plasma turbulence on the phase measurements. We also point out the importance of carefully filtering the signal detected in order to keep only information coming from the reflection at the cutoff and get rid of parasitic reflections. It is shown that the phase difference technique does not completely remove fluctuations such as long radial correlation magnetohydrodynamic perturbations. The density profiles determined by the heterodynereflectometer are in good agreement with the measurements from the other diagnostics of Tore Supra.
71(2000); http://dx.doi.org/10.1063/1.1150165View Description Hide Description
An eight-channel soft x-rayspectrometer has been developed, manufactured, and calibrated. Each channel consists of a calibrated bandpass filter,multilayermirror, and vacuum x-ray diode. The accuracy of channel sensitivity calibration is ⩽ 15%. The time resolution of the spectrometer is and energy resolution is
71(2000); http://dx.doi.org/10.1063/1.1150166View Description Hide Description
An imaging spectrometer with a convex rubidiumacid phthalate (RbAP) crystal is designed and examined. Using the ray tracing technique based on the kinematical theory of diffraction, resolution power, dispersion, linearity, spatial resolution and dynamic range of the monochromatic image are discussed. Broadening by a rocking curve is also taken into account. Performance of the spectrometer is successfully examined using the so-called hot spots as the soft x-ray source which are generated in the pinched plasma by the plasma focus facility with an additional gas puff.
71(2000); http://dx.doi.org/10.1063/1.1150167View Description Hide Description
A differential pressure pseudospark device is developed to produce a discharge at a pressure of near the anode. This pressure range is two orders in magnitude lower than conventional pseudospark devices. In this device a pressure gradient is maintained between the cathode and the anode by providing a gas flow through the discharge column. The pressure gradient helps in shifting the Paschen curve more towards the left in comparison to the conventional case. The empirical relationship valid without a gas flow, is not applicable when the pressure difference between the cathode and the anode is over two orders in magnitude. Self-biasing collector technique reveals the presence of energetic electrons (0.4–1.2 keV) present in the plasma downstream of the anode. The nature of this plasma at two distinct pressure ranges of operation of the device shows marked difference in properties. A qualitative discussion is presented that explains the possible discharge mechanism in this device.
- BASIC PHENOMENA
Analysis of dopant concentration effects in praseodymium-based fluorescent fiber optic temperature sensors71(2000); http://dx.doi.org/10.1063/1.1150168View Description Hide Description
An analysis of concentration effects in praseodymium-doped glasses used as the sensor element of several fiber optic temperature sensors has been performed. Results show the dependence of a range of relevant parameters on concentration, and the determination of concentration-independent parameters used in the design of effective probe sensor elements with both doped and co-doped rare earth materials. Low concentrations of commensurate with an acceptable signal-to-noise ratio in the optical system, are preferable for the design of the most effective optical fiberfluorescencesensors based upon this species.