Showing papers on "Time of flight published in 1998"

Performance characteristics of a chemical imaging time-of-flight mass spectrometer.

Abstract: A chemical imaging time-of-flight secondary ion mass spectrometer is described. It consists of a liquid metal ion gun, medium energy resolution reflectron mass analyzer, liquid nitrogen cooled sample stage, preparation chamber and dual stage entry port. Unique features include compatibility with laser postionization experiments, large field of view, cryogenic sample handling capability and high incident ion beam current. Instrument performance is illustrated by the characterization of scanning electron microscopy grids, silver and functionalized polystyrene beads and the postionization of an organic overlayer on a gold substrate.

High-resolution electron time-of-flight apparatus for the soft x-ray region

Abstract: A gas-phase time-of-flight (TOF) apparatus, capable of supporting as many as six electron-TOF analyzers viewing the same interaction region, has been developed to measure energy- and angle-resolved electrons with kinetic energies up to 5 keV. Each analyzer includes a newly designed lens system that can retard electrons to about 2% of their initial kinetic energy without significant loss of transmission; the analyzers can thus achieve a resolving power (E/ΔE) greater than 104 over a wide kinetic-energy range. Such high resolving power is comparable to the photon energy resolution of state-of-the-art synchrotron–radiation beamlines in the soft x-ray range, opening the TOF technique to numerous high-resolution applications. In addition, the angular placement of the analyzers, by design, permits detailed studies of nondipolar angular distribution effects in gas-phase photoemission.

High-Resolution Time-of-Flight Mass Spectra of Alkanethiolate-Coated Gold Nanocrystals

Abstract: Alkanethiolate-coated gold nanocrystals have been laser desorbed from a solid film, ionized, and analyzed by time-of-flight mass spectrometry. A previously unresolved distribution of peaks is observed and the number of gold and sulfur atoms associated with each feature can be assigned. Alkyl chains are not evident in these high-resolution spectra. Experiments using spatially and temporally separated light pulses demonstrate that more of the desorbed nanocrystals are neutral than are ionized. Pure gold cluster ions of various sizes can be generated from these neutral molecules in the two-laser experiment.

The Berlin time-of-flight ERDA setupe

Abstract: The new mass and energy dispersive Elastic Recoil Detection Analysis (ERDA) spectrometer located at a high-energy target position of the heavy-ion-beam laboratory (ISL) of the Hahn-Meitner-Institut Berlin is presented. Many different projectile ions from hydrogen to xenon with variable energies up to several MeV/u are available. The recoil identification is done by means of a time-of-flight (TOF) energy setup with a relatively large solid angle of 1.57 msr. Due to the long flight path of 123 cm and a time resolution of about 180 ps, a good mass and depth resolution can be achieved.

Multi-pass reflectron time-of-flight mass spectrometer

Abstract: A novel design for a time-of-flight mass spectrometer capable of tandem mass spectrometry measurements with high resolution and high sensitivity using two variable reflectrons in a co-linear geometry. Variably switched reflectrons are oriented coaxially on opposing ends of the ion flight region allowing multiple passes of the ions along the flight region permitting high resolution, tandem mass spectrometry experiments to be performed. An electrostatic particle guide is incorporated to ensure high ion transmission efficiency in a multi-pass system. In addition to permitting the high transmission efficiency of ions, the EPG can be used in a bipolar pulsed mode to isolate ions of interest for structural study.

Femtosecond laser ablation of gallium arsenide investigated with time-of-flight mass spectroscopy

Abstract: We have investigated femtosecond laser-induced ablation of gallium arsenide using time-of-flight mass spectroscopy. At the ablation threshold, we estimated surface temperatures on the order of 3500 K. We observed a clear thresholdlike effect in the number of detected particles and with increasing fluence free flight desorption transforms into a collisional expansion process. Above the ablation threshold, the behavior of gallium particles can be quantitatively described through Knudsen-layer theory.

Time-of-flight photoelectron emission microscopy TOF-PEEM: first results

Abstract: The time structure of the synchrotron radiation at BESSY (Berlin) is used to operate a photoemission electron microscope in a time-of-flight (TOF) mode. The electrons which are emitted from the sample surface with different energies are dispersed in a drift tube subsequent to the imaging optics. The screen of the microscope was replaced by a fast scintillator (tau = 1.4 ns) and the light is detected by an ultra fast gated intensified CCD camera (800 ps gate 1 MHz repetition rate). The resolving power in the energy domain is demonstrated and possible implications on the spatial resolution (chromatic correction) are discussed. Additionally, an improved contrast at very low emission energies is shown. The capability of the setup as an efficient microspectroscopic tool is illustrated. (C) 1998 Elsevier Science B.V. All rights reserved.

Ion time-of-flight study of laser ablation of silver in low pressure gases

Abstract: The dynamics of ions from a laser-ablated silver target in low pressure background atmospheres have been investigated in a simple geometry using an electrical probe. A simple scattering picture for the first transmitted peak of the observed plume splitting has been used to calculate cross sections of the ablated silver ions in oxygen (σ{O2}=4.8×10−16 cm2) and in argon (σ{Ar}=6.7×10−16 cm2). The dynamics of the blast wave is well described by blast wave theory.

A high-resolution, large acceptance scintillation time-of-flight spectrometer

Abstract: A high-resolution detector array has been developed for the measurement of momenta of charged and uncharged particles via the time-of-flight method. This spectrometer represents one detector arm of a system used to study the photoabsorption mechanisms of (γ, NN) and (γ, πN) reactions. It is comprised of 112 plastic scintillators (NE110) of dimensions 5 × 20 × 300 cm 3 with a photomultiplier attached to each end. A mean time resolution of ≲400 ps (FWHM) and a position resolution of ≲6 cm have been achieved. The modularity and flexibility of the array encouraged its use in other electron-and photo-induced reaction measurements.

The new time-of-flight ERDA setup at the HMI-Berlin

Abstract: The new time-of-flight ERDA (TOF ERDA) spectrometer of the Hahn-Meitner-Institut Berlin is presented. It is located at a high-energy target position of the ion-beam laboratory (ISL). The great variety of ions from helium to xenon with variable energies up to several MeV/amu allows the determination of the distribution of all elements in the samples up to a depth of some micrometer. The measurement of the hydrogen concentration is possible with high efficiency. With the relatively large solid angle of 1.57 msr fast measurements with low ion beam currents are possible. The long flight path of 123 cm and a time resolution of about 180 ps enable a good mass and depth resolution.

Response function of NE213 scintillator for 0.5-6 MeV neutrons measured by an improved pulse shape discrimination

Abstract: Using the pulse shape discrimination method combined with the time of flight technique, we have obtained the response function of a 2″ diameter × 2″ thick NE213 scintillator by measuring directly the proton recoil energy spectrum of 0.5–6 MeV prompt neutrons from a 252 Cf source. Three parameters, time of flight (TOF), pulse shape discrimination (PSD) and recoil energy (RE), were recorded in an event-by-event mode with a TOF gate. We attempted to improve the determination of maximum proton recoil energies equal to incident neutron energies by employing two analysis methods. First, we attempted to separate better neutrons from coexisting gamma rays in the PSD spectrum by projecting neutron channels of the PSD spectrum onto both TOF and RE spectra in a cubic matrix constructed by the three-parameter data. The resulting two-dimensional matrix composed of TOF and RE channels was free from gamma rays and corresponding Compton-recoiled electrons, and then projected with neutron energy bins of 0.05 MeV wide determined by TOF. Finally, to determine the maximum proton recoil energies from each RE spectrum with a realistic function, accounting for the nonlinear response of the NE213 scintillator due to the quenching effect, we performed a least-squares fit to the RE spectrum using the four-parameter function. The response function obtained in the present work agrees well with previous experimental results obtained by Gul et al. (Nucl. Instr. and Meth. A 278 (1989) 470) and a Monte Carlo study by Cecil et al. (Nucl. Instr. and Meth. 161 (1979) 439).

Some applications of high-resolution RBS and ERD using a magnetic spectrometer

Abstract: We have developed a high-resolution Rutherford backscattering spectroscopy (RBS) system with a 90°C sector magnetic spectrometer ( δE E ∼ 0.1% , including the energy spread of the incident beam). Energy spectra of sub-MeV He1 ions backscattered from single crystal surfaces are measured with the high-resolution RBS system. The RBS spectra observed at grazing exit angles (several degrees) consist of several well-defined peaks which correspond to the ions scattered from successive atomic layers indicating the achievement of monolayer resolution. Examples of the application of this novel technique are presented. The spectrometer is also used for elastic recoil detection (ERD) measurements. By installing an electrostatic deflector between the magnet and the ion detector, recoiled ions are distinguished from the scattered ions. This method has a considerable advantage over the conventional ERD concerning the depth resolution and also has an advantage over the recently developed high-resolution ERD, which combines an electrostatic or magnetic spectrometer with a time of flight (TOF) technique, regarding the measurement time. A depth resolution of about 0.8 nm is achieved in the measurement of the hydrogen distribution in silicon crystals.

Mapping of vibrational wave-packet motion by femtosecond time-resolved kinetic energy time-of-flight mass spectroscopy

Abstract: Kinetic energy time-of-flight (KETOF) mass spectroscopy in combination with femtosecond pump-probe and molecular beam techniques is used to map molecular dynamics along an internuclear coordinate. By recording transient KETOF mass spectra we monitor the vibrational wave-packet motion on a neutral electronic state for all possible internuclear distances simultaneously. The power of this method is demonstrated by the one dimensional wave-packet motion on the double minimum state of the sodium dimer. For comparison with the experiment quantum mechanical simulations were performed.

A high resolving power ion selector for post-source decay measurements in a reflecting time-of-flight mass spectrometer.

Abstract: An electrostatic deflector has been designed and constructed that can be used in a reflecting time-of-flight mass spectrometer for either single-deflector or dual-deflector velocity selection in post-source decay measurements. The deflector consists of an interleaved set of parallel deflection electrodes as in a Loeb/Cravath/Bradbury device, but thin metal ribbon instead of wire is used for the deflection electrodes. The time for reversing the electric field, which depends on various factors such as the electronics for pulsing the voltage and the time constant of a particular electrode geometry, is about 19 ns for the deflectors used in this study. By properly timing the reversal of the electric field, the time-window for ion transmission can be made substantially less than the switching time of each individual deflector. In conjunction with matrix-assisted laser desorption/ionization, the single-deflector's resolving power and transmission are robust with respect to laser fluence, i.e. they remain high even when the fluence is raised well above threshold. By contrast the operational features of the dual-deflector gate offer more versatility in locating and sizing the selection window. Operating the ion selector in a single-deflector mode, we have achieved a resolving power of approximately 710 full width at half maximum (FWHM) for different isotopes of protonated, sodiated, and potassiated substance-P (m/z 1348.6, 1370.6 and 1386.6 respectively; 10.073 keV). Operating it in the dual-deflector mode under two different sets of conditions, we have succeeded in obtaining resolving powers of approximately 1100 (FWHM) for protonated substance-P (m/z 1348.6; 10.8 keV) and approximately 5200 (FWHM) for an isotopomer of PEG 6000 (approximately m/z 6000; 10.04 keV). This accomplishment implies that high-resolution ion selection can be coupled to post-source decay analyses.

Time-of-flight mass spectrometer with constant flight path length

Abstract: The time-of-flight mass spectrometers which must demonstrate a high constancy of the calibrated mass scale even under changeable ambient temperatures and thermal loads due to pumps or electronics Time-of-flight mass spectrometers calculate the masses of ions from the measured time of flight in a long flight tube that is normally manufactured of stainless steel These flight tubes are subject to temperature-related length changes which affect the flight time and therefore the mass determination The thermal expansion of spectrometer parts between ion source and ion detector, thus keeping the flight path for the ions at a constant length Length compensation can be produced by design of the spacing system made of materials of different thermal expansion coefficients, the length changes of which balance out in opposite directions

A fast atomic oxygen beam facility with in situ testing/analysis capabilities

Abstract: A fast atomic oxygen beam facility consisting of a beam source, a mass spectrometer, an Auger electron spectroscope, a scanning tunneling microscope, and a friction tester has been developed to investigate interaction of energetic atomic oxygen with solid surfaces. The fast atomic oxygen beam has been characterized by time of flight distribution, quadrupole mass spectrometry, and quartz crystal microbalance. The time of flight distribution of the beam has shown that the average translational energy of the atomic oxygen beam reaches 4.7 eV and that the full width at half-maximum is 5.5 eV. A flux of the atomic oxygen is calculated from the frequency shift of the quartz crystal microbalance with silver electrodes, and typical flux of the atomic oxygen beam being 4.0×1012 atoms/cm2 s. The flux of atomic oxygen of this source is fairly low, but is corresponding to that in the altitude of 500 km in low Earth orbit. The surface sensitive analysis methods equipped with the facility, such as Auger electron spectr...

A TOF spectrometer for elastic recoil detection

Abstract: A time-of-flight spectrometer was built for the ERD-TOF (Elastic Recoil Detection by Time Of Flight) and heavy ion RBS (Rutherford Backscattering Spectrometry) experiments with a 17 MV tandem Van de Graaff accelerator The spectrometer consists of two time pick-off detectors and a SSB (Silicon Surface Barrier) detector with variable flight lengths The time detector uses an electrode to accelerate and to focus the electrons from a thin carbon foil to a MCP (Micro Channel Plate) The advantage of this type of time detector is the good efficiency and no obstacles in the beam path at the cost of a small uncertainty in the flight length The efficiency of the spectrometer was measured for the ions lighter than neon in the energy range of a few hundred keV to 7 MeV, and was better than 98% for the particles heavier than B The intrinsic resolution of the time detector is about 220 ps The time resolution of the spectrometer was measured as a function of incident particle type and energy The mass resolution is below 1 amu for the particles of interest

The Electron Drift Technique for Measuring Electric and Magnetic Fields

Abstract: The electron drift technique is based on sensing the drift of a weak beam of test electrons that is caused by electric fields and/or gradients in the magnetic field. These quantities can, by use of different electron energies, in principle be determined separately. Depending on the ratio of drift speed to magnetic field strength, the drift velocity can be determined either from the two emission directions that cause the electrons to gyrate back to detectors placed some distance from the emitting guns, or from measurements of the time of flight of the electrons. As a by-product of the time-of-flight measurements, the magnetic field strength is also determined. The paper describes strengths and weaknesses of the method as well as technical constraints.

Laser ablation time-of-flight mass spectrometric probing of the surface states of SiO2-based porous materials

Abstract: -based materials of different porosity and surface states have been studied by time-of-flight mass spectrometry (TOF-MS) under 308 nm excimer laser ablation. Sequences of SiO2-based negative ion clusters [(SiO2)nX]- are observed and variations of their abundance distributions with sample properties are studied. The unique roles of surface states in cluster formation are discussed.

Quantification of metal contaminants on GaAs with time-of-flight secondary ion mass spectrometry

Abstract: We have shown that small concentrations of metal atoms on UV/ozonized GaAs wafer surfaces can be quantitatively determined by time-of-flight secondary ion mass spectrometry (TOF-SIMS). As standard reference materials, we produced defined submonolayer concentrations of Ca, Mg, Al, Cu, Zn, Si, Ni, Cr, Co, and Fe on the wafer surface by sputter deposition from different single- and multielement targets. The concentrations of the elements with m>27 u involved in these calculations were verified independently by total reflection x-ray fluorescence spectroscopy. For all ten metals, a linear relation exists between the relative metal signal Me+/71Ga+ and the surface concentration of the metal. By this relation we established TOF-SIMS sensitivity factors for these metals on UV/ozonized GaAs. The detection limits for almost all elements are in the order of 109 atoms/cm2.

The time of flight wall for the HADES spectrometer

Abstract: In the framework of the HADES (High Acceptance DiElectron Spectrometer) collaboration we are developing the Time Of Flight (TOF) wall subdetector. In order to efficiently select events with e/sup +/e/sup -/ pairs produced in the nuclear matter, the TOF wall must be able to resolve electrons from pions up to 0.5 GeV/c and from protons up to 2 GeV/c. Because the short flight path of about 2 m requires a 100-150 ps time resolution, we are building a granular system made of scintillator rods readout on both ends by means of photomultipliers, capable of such performance.

Time-of-flight mass spectrometer

Abstract: PROBLEM TO BE SOLVED: To provide a TOFMS(time-of-flight mass spectrometer) having a wide dynamic range through the use of a MCP(microchannel plate) as a detector SOLUTION: In this time-of-flight mass spectrometer, using an ion detector 5 capable of controlling a multiplication factor by an impressed voltage as an ion detection part, the impressed voltage on the ion detector 5 is varied interlockingly with the generation of ion pulses from an ion source, for measuring mass spectra with different multiplication factors After the measurement, gains in the mass spectra are calibrated with the multiplication factor at measurement time, then the mass spectrum measured with a low multiplication factor is adopted in the portion of a high spectrum intensity In addition, at the locations of low spectrum intensity, the mass spectrum measured at a high multiplication factor is adopted Thus, as a result a single continuous mass spectrum is produced