Volume 70, Issue 8, August 1999
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
Locking the cavity of a pulsed periodically poled lithium niobate optical parametric oscillator to the wavelength of a continuous-wave injection seeder by an “intensity-dip” method70(1999); http://dx.doi.org/10.1063/1.1149895View Description Hide Description
Injection seeding by a single-mode continuous-wave (cw) laser provides a convenient way to achieve narrowband tunable operation of a laser with a broad spectral gain profile, or of an optical parametric oscillator(OPO). Continuous single-mode tunability of the laser or OPO output usually requires the length of the optical cavity to be controlled as the injection-seeding wavelength is scanned. We report a novel variant on established methods of locking the optical cavity length to the seed wavelength. Our approach takes advantage of the resonance properties of an optical cavity. When the cavity is in resonance with the cw seed radiation, the total intensity of that radiation reflected off the cavity displays a pronounced dip; this intensity dip can be used as a locking signal to reset the cavity length piezoelectrically during each interval between the pump pulses that excite the laser or OPO. Our active cavity-locking scheme is realized in the case of a ring-cavity OPO, incorporating periodically poled lithium niobate (PPLN), pumped at 1.064 μm by a single-mode pulsed Nd:yttrium–aluminum–garnet laser and injection-seeded at its signal wavelength by a 1.55 μm single-mode tunable diode laser. The coherent infrared output of this injection-seeded PPLN OPO is shown to be continuously tunable, with an optical bandwidth of ∼130 MHz (0.0045 cm−1) and excellent spatial beam quality.
Ultralow-angle dynamic light scattering with a charge coupled device camera based multispeckle, multitau correlator70(1999); http://dx.doi.org/10.1063/1.1149894View Description Hide Description
We use a charge coupled device(CCD)camera and a multi-tau software correlator to measure dynamic light scattering (DLS) at many angles simultaneously, from 0.07° to 5.1°. Real-time autocorrelation functions are calculated by averaging both over time and over CCD pixels, each corresponding to a different coherence area. In order to cover the wide spectrum of decay times associated with the large range of accessible angles, we adopt the multitau scheme, where the correlator channel spacing is quasilogarithmic rather than linear. A detailed analysis is presented of the effects of dark noise, stray light, and finite pixel area, and methods to correct the data for these effects are developed, making a CCDcamera a viable alternative for a DLS detector. We test the apparatus on a dilute suspension of colloidal particles. Very good agreement is found between the particle radius derived from the CCD data, and that obtained with a conventional DLS setup.
70(1999); http://dx.doi.org/10.1063/1.1149896View Description Hide Description
A laser-induced Rayleigh light-scattering (RLS) system was assembled and used to noninvasively measure the transient molecular number density in an ideal gas. This information was used to find the transient gas temperature when operating at known pressure. The laser was a 4 W argon ion operating on all lines at a total power of about 2.5 W. The theoretically predicted photon arrival rate at the photomultiplier tubedetector was calculated and compared well with the observed photon rates. These rates were high enough that sampling could be averaged over a 1 s period with theoretical uncertainty due to electronic shot noise below 0.1%, and below 2% for a 0.001 s averaging time. A propagated error analysis showed uncertainty in the transient temperature due to all sources was 2–4 K. The RLS system was used to record transient air temperature at several locations above a flat plate during heating from room temperature to 475 K. Results showed buoyancy-induced fluctuations of about 3 Hz, and instabilities in temperature in addition to the overall temperature rise due to plate heating. Excellent transient temperature records were obtained, substantiating the predicted 2–4 K uncertainty.
70(1999); http://dx.doi.org/10.1063/1.1149897View Description Hide Description
We have developed a design method to produce laterally graded multilayerx-ray mirrors based on sputter deposition techniques. The optimization of all relevant parameters yields an ab initio estimation of any layer gradient suitable to achieve precise x-ray focusing devices. The performance and the accuracy of this method are demonstrated. A graded multilayer was deposited on a flat substrate that was bent to a parabola. The obtained nonlinear lateral gradient differed from the theoretical calculations by less than 1%. Focusing experiments performed at an x-ray energy of 8 keV on the ESRF optics beamline revealed an excellent performance with a focal spot size of about 7 μm.
Study of the capability of energy dispersive small angle x-ray scattering with synchrotron radiation70(1999); http://dx.doi.org/10.1063/1.1149898View Description Hide Description
Energy dispersive small angle x-ray scattering is a unique method that takes advantage of a continuous spectrum of a synchrotron radiationsource. In this study, the capability of this scattering measurement is explored. Using a detector mask with three acceptance rings of different ring radius and a sample to detector distance of 440 mm with the usable x-ray energy ranging from 4 to 20 keV, we can cover a q range from 0.01 to 0.4 Å−1. However, sample transmission and detector response function limit the application range at low energy. Slit scattering is the dominant source of background. The signal to background ratio for a standard cross-linked polyethylene sample can be more than 100. For a time-resolved experiment, data acquisition rate is limited by the throughput of the detector.
70(1999); http://dx.doi.org/10.1063/1.1149899View Description Hide Description
We report on the design of a diffractometer, which offers improved capabilities for x-rayexperiments in ultrahigh vacuum. Its main features are (i) the possibility to follow the evolution of diffraction spots in situ during adsorption or film deposition; (ii) the measurement of reflections at high exit angles, i.e., large perpendicular momentum transfer. This goal is achieved by placing a movable x-ray detector inside the vacuum chamber. (iii) Other surface analysis equipment, e.g., a low-energy electron diffraction or an electron energy analyzer can be moved in front of the sample and operated simultaneously with x-ray diffraction. (iv) A load lock system—currently in preparation—will allow the quick exchange of samples without breaking system vacuum. In addition, a new design of the chi circle used for sample alignment provides a compact, space-saving design of the diffractometer.
70(1999); http://dx.doi.org/10.1063/1.1149900View Description Hide Description
An experimental setup is described, which is used to multi-ionize neutral clusters and to study their stability and the importance of different decay processes. Clusters are ionized in collisions with slow multiply charged ions (projectile chargez ranging from 1 to 30, kinetic energies ranging from 1 to 20 keV/charge). Both ion and cluster beamlines, as well as the characteristics of the analysis and detection systems, are described. Collisions with highly charged ions such as or turn out to be efficient tools in preparing clusters in high charge states without increasing significantly their internal temperature. Measurements performed in coincidence with the number of electrons stabilized by the projectile ion after the collision allowed us to control the charge and the excitation energy of the ionized system. The efficiency of the method is demonstrated for sodium clusters and molecules.
The generation of a tunable laser emission in the vacuum ultraviolet and its application to supersonic jet/multiphoton ionization mass spectrometry70(1999); http://dx.doi.org/10.1063/1.1149901View Description Hide Description
An optical parametric oscillator and a Ti:sapphire laser are used as a pump source for the generation of high-order vibrational stimulated Raman emission in the vacuum ultraviolet region. This tunable laser is employed as an excitation/ionization source in a supersonic jet/multiphoton ionization/time-of-flight mass spectrometric study of benzene. The merits and potential advantages of this approach are discussed in this study.
70(1999); http://dx.doi.org/10.1063/1.1149944View Description Hide Description
An apparatus has been constructed to study radical–radical reactions in the gas phase. It consists of a tubular quartz reactor in which radicals are produced by flash photolysis using an excimer laser as light source. The composition of the gas mixture is analyzedin situ by photoionizing sampled gases using the vacuum ultraviolet emission of a hollow cathode lamp and subsequent time-of-flight mass spectrometry. A simple arrangement of grids at the entrance to the flight tube is used to interrupt the constant flux of ions by application of a combination of constant and pulsed voltages. Individual mass spectra can be taken at a repetition rate of around 20 kHz following each photolysis event. Signal counts from a specified number of consecutive mass spectra are fed into a 2 GHz multiscaler and accumulated as a sampling-time-indexed series of mass spectra. This allows simultaneous observation of the concentrations of multiple transient or stable species on a millisecond time scale. To achieve a suitable signal-to-noise ratio, signals were typically accumulated over several tens of thousands of laser shots at a pulse rate of 10–15 Hz.
70(1999); http://dx.doi.org/10.1063/1.1149902View Description Hide Description
A new crossed molecular beam apparatus with a high-resolution ion imaging detector is described. Two pulsed supersonic molecular beams are crossed at right angles in a vacuum of Torr. The collision region is irradiated with a tunable laser pulse that ionizes the scattered particles state selectively. The generated ions are accelerated by stacked electrodes in a two-dimensional (2D) space focusing mode that increases the velocity resolution of the apparatus. A cylindrical hexapole deflector is placed in the middle of the time-of-flight mass spectrometer to compensate the center-of-mass velocity of the ions and to direct them to the center of the 2D imaging detector. Real-time image processing of the charge coupled device camera signal eliminates blurring of the image detector. The performance of the apparatus was examined by observing the inelastic scattering of NO+Ar at a collision energy of 66 meV. The observed multiple rainbow peaks clearly demonstrate the high performance of the apparatus.
70(1999); http://dx.doi.org/10.1063/1.1149903View Description Hide Description
A new optical high voltage sensor using a Pockels microsingle crystal in a longitudinal modulation arrangement is presented. A prototype of the sensor is assembled and shows an advantage of directly measuring voltage levels up to 50 kV without any potential divider in a wide frequency bandwidth of direct current to 116 MHz. In conventional Pockels sensors,acoustic resonance driven by piezoelectric effect introduces oscillatory components to signals obtained from measurements of lightning impulse voltages. In the new sensor, the measured signal is free from the oscillatory components. An accuracy of 1.9% in good agreement with predicted values is obtained from the lightning impulse voltage measurement.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
70(1999); http://dx.doi.org/10.1063/1.1149904View Description Hide Description
This work reports on electron beam characteristics obtained from a Cuphotocathode utilizing an UVexcimer laser of 222 nm for the first time. The values of the current, emittance, and brightness of the electron beamsgenerated are reported. A suitable slit–slit emittance meter was developed to assess the electron beam emittance. This instrument allowed us to determine the electron beam phase-space areas by slicing the beam in small beamlets and measuring the beamlet direction distributions. Small Faraday cups placed after the slits were utilized to this reason. The laser beam was focused in a 4 mm2 spot and the maximum energy laser was established to 0.5 mJ. The highest current was 410 mA and the corresponding normalized emittance value was 10 π mm mrad corresponding to 80% of the total points. From these data the beam quality, expressed as normalized beam brightness, resulted in and the quantum efficiency was By increasing the KrCl laser spot and its energy a very charge electron beam was extracted of 16.4 A.
Spatial distribution of electron cloud footprints from microchannel plates: Measurements and modeling70(1999); http://dx.doi.org/10.1063/1.1149905View Description Hide Description
The measurements of the electron cloud footprints produced by a stack of microchannel plates (MCPs) as a function of gain, MCP-to-readout distance and acceleratingelectric field are presented. To investigate the charge footprint variation, we introduce a ballisticmodel of the charge cloud propagation based on the energy and angular distribution at the MCP output. We also simulate the Coulomb repulsion in the electron cloud, which is likely to cause the experimentally observed increase in the cloud size with increasing MCP gain. Calculation results for both models are compared to the charge footprint sizes measured both in our experiments with high rear-field values (∼200–900 V/mm) and in the experiments of Edgar et al. [Rev. Sci. Instrum. 60, 3673 (1989)] (acceleratingelectric field ∼30–130 V/mm).
70(1999); http://dx.doi.org/10.1063/1.1149906View Description Hide Description
A new fast beam photofragment apparatus for studies of electronic and nuclear dynamics is described. The experimental setup involves a chopped ion beam crossed at 90° by a pulsed laser beam. Photofragments are extracted parallel to the ion beam direction and imaged on a multisphere plate (MSP) detector. The time of flight of a photofragment from the interaction point to the MSP detector is determined by a coincidence technique. The setup allows determination of the photofragment energy and the direction of emission for each event of photofragmentation. In photoelectron spectrometry the energy resolution is tunable down to the meV region. For testing, the apparatus has been applied to photodetachment of at 1064 nm.
70(1999); http://dx.doi.org/10.1063/1.1149907View Description Hide Description
A simple instrument to determine field emission tip work functions and shape functions from simultaneous current–voltage characteristics and field emission energy distributions of field emitter tips and tip arrays is described. This instrument uses a cylindrical energy analyzer with a few correcting elements to simulate a hemispherical analyzer and provides a low cost and more compact alternative to a commercial hemispherical spectrometer. curves and energy distributions may be automatically obtained as a function of time to study field emission tip degradation with usage and/or exposure to gases of interest.
70(1999); http://dx.doi.org/10.1063/1.1149908View Description Hide Description
In this article, by solving the nonlinear envelope equation of the intense beam in the cylindrical coordinate system, we have obtained the varying relations of the beam radius and the beam divergent angle with the paramedics (the magnetic induction strength in solenoid lens, the characteristic quantity of beam intensity, the beam emittance, the initial beam radius, and initial beam divergent angle). By theoretical analysis for the relations, we have obtained the beam peak radius and beam peak position in solenoid lens, the beam waist radius and beam waist position in the case of perfect focusing. In the ideal focusing condition, the radius of intense beam solenoid lens should not be less than the beam peak radius; the length of the solenoid lens must not be less than the beam peak position; and in order to gain optimum focusing, an optimal length should be selected; the magnetic induction strength must not be less than the threshold for getting a focusing beam through solenoid lens, and its optimum value should be selected so as to obtain the minimum value of beam divergent angle at the exit port for getting an perfect focusing output beam. The conclusions in this article have some guiding significance for the design and operation of the intense beam solenoid magnetic lens.
Using the induced axial magnetic field to measure the root mean square beam size and beam density uniformity of an electron beam in an induction linac70(1999); http://dx.doi.org/10.1063/1.1149909View Description Hide Description
An intense electron beam in a solenoidal axial magnetic field rotates azimuthally, and generates a diamagnetic axial field that counteracts the applied field. However, the magnetic flux within the beam pipe is conserved over sufficiently short times, and this induced diamagnetic field must lead to an increase in the axial field outside the beam, which is measurable with a loop or an optical Faraday rotator. If the applied axial magnetic field is uniform across the beam pipe, the measurement directly leads to the rms transverse beam size. However, if the axial magnetic field has sinusoidal components, measurement of the diamagnetic field at two axial positions yields both the beam’s rms transverse size and the beam’s fourth radial moment. Comparison of these moments can give a figure of merit of the beam’s density uniformity.
- NUCLEAR PHYSICS, FUSION and PLASMAS
70(1999); http://dx.doi.org/10.1063/1.1149910View Description Hide Description
A toroidal crystal spectrometer was designed with the purpose of measuring the aluminum K-absorption edge shift, in the wavelength range around 7.9 Å, in strongly compressed matter. The expected shift is about 100 mÅ (∼20 eV). The x rays reflected from the crystal are focused onto a streak camera slit of 16 mm high and 100 μm width, to obtain a time-resolved spectrum. High resolution and dispersion of about 1–16 Å/mm on the detection window are obtained. A crystal with a toroidal surface is used to enhance the focusing power in the spatial dimension as in the spectral one. Numerical simulations are performed by means of one- and two-dimensional codes for the determination of the crystal characteristics like the dimension and the curvature radius with respect to the geometric constraints. Some experimental results are presented concerning the obtained spectra.
70(1999); http://dx.doi.org/10.1063/1.1149911View Description Hide Description
A gold neutral beam probe system which uses a microchannel plate (MCP) as a beam detector is being constructed in order to measure the electrostatic potential at the inner mirror throat (IMT) of the plug/barrier cell in the tandem mirror. The MCP detector was newly designed so as to be able to be used in the IMT strong magnetic field. The characteristics of the MCP gain were measured on a test stand with the strong magnetic field. The characteristic curves of the relative MCP gain were described as functions of the magnetic field strength, the MCP bias voltage, and the angle between the magnetic field and the capillary axis of the MCP. It was found that the newly designed MCP detector was useful for the potential measurement at the IMT region.
Ion beam mass spectrometer for compositional analysis of plasma assisted surface processes in the pressure range of 1–50 mbar70(1999); http://dx.doi.org/10.1063/1.1149912View Description Hide Description
Ion beammass spectrometry (IBMS) has been developed for in situ, real-time monitoring of the ion composition of a plasma in the vicinity of the substrate surface up to a pressure of 50 mbar. Our system was designed for the specific case of diamond deposition in a microwave plasma enhanced chemical vapor deposition chamber, however, it can be adapted to other applications as well, without significant change in the process chamber. Ion beammass spectrometer is a quadrupole mass spectrometer (QMS) based plasma sampling system installed into a separately pumped small analysis chamber. The interface between the two chambers is the top part of the IBMS, specially designed to serve as the sample holder of the reactor chamber (sample heating and biasing is possible). A technique containing wet etching and laser treatment has been developed to form a sufficiently small orifice in various substrates. This orifice samples the particles arriving onto the surface from the plasma but is small enough to maintain the flow effusive into, and an appropriate pressure in the analysis chamber. Directly behind the orifice specially designed ion-optics direct the ions coming from the process chamber into the QMS for mass and energy separation. Since the total flow from the reactor chamber is low, the mean free path in the analysis chamber is sufficiently longer than the distance from the orifice to the QMS, therefore, ions do not collide, thus they do not react after sampling. As a consequence, nonstable species are also detected. The carefully designed ion-optics ensure that a high ion current reaches the QMS. In this article, it is shown that the intensity ratios in the spectra correspond to the concentration ratios of the ions arriving to the substrate surface. The QMS is also equipped with a conventional cross electron beam ion source allowing measurements of neutral species by conventional ionization.