Spectrometer aperture

Fig. 3.10 Variation of the spectrometer aperture as a function of the source motion for Mossbauer spectrometers operated in constant acceleration mode with triangular velocity profile, and the resulting nonlinear baseline distortion of the unfolded raw spectra. For simplicity a point-source is adopted, in contrast to most real cases (Rib mm active spot for Co in Rh)...

Figure A3.5.L The fast ion beam photofragment spectrometer at SRI International. L labels electrostatic lenses, D labels deflectors and A labels apertures.

Figure Bl.23.5. Schematic illustration of tlie TOE-SARS spectrometer system. A = ion gun, B = Wien filter, C = Einzel lens, D = pulsing plates, E = pulsing aperture, E = deflector plates, G = sample, PI = electron multiplier detector with energy prefilter grid and I = electrostatic deflector.

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

All experiments were carried out in a VG ESCALAB instrument described previously [23,24]. Volatile products were detected with UTI 100 C mass spectrometer. The ionizer of the mass spectrometer was enclosed in a quartz envelope with a 5 mm aperture for samphng gases from the vacuum. 

The Ti02 (001) surface was cleaned and reduced by cycles of ion bombardment as previously described [3]. The distribution of titanium oxidation states was determined from cxirve fitting the Ti(2p3/2) envelope in x-ray photoelectron spectra [3]. After surface preparation, reaction experiments were conducted in either the TPD or steady state mode. TPD experiments have been described [1]. XPS spectra were also obtained following a saturation exposure of the sample using the same procedure as that for the TPD experiments. After pump down, the crystal was placed under the Mg X-ray source and the Ti(2p), 0(ls), and C(ls) regions were scanned. For steady-state experiments a dosing needle was aligned perpendicular to the axis of the mass spectrometer. It was used to direct a steady beam of methylacetylene (Linde, 95%) at the crystal surface when the sample was placed at the aperture of the mass spectrometer. Steady state reaction experiments were... 

An FIM may be modified so that the imaged atom chosen for analysis can be positioned over a small aperture in the phosphor-coated screen. If the electric field is raised to a sufficiently high value, material may be removed from the surface by field evaporation. The specimen is subjected to a high-voltage pulse, which causes a number of atoms on the specimen surface to field evaporate as positive ions. Only the atom that was imaged over the aperture (or probe hole ) passes into a time-of-flight mass spectrometer, all the other atoms being blocked off by the screen. The applied... 

The mass-to-charge ratio, m/n, of those ions that pass through the probe aperture and are analysed in the mass spectrometer is calculated from the equivalence between the potential energy of the atom on the specimen surface at voltage V0, and the kinetic energy that the atom acquires during acceleration to the grounded... 

Schematic representation of the experimental setup is shown in Fig 1.1. The electrochemical system is coupled on-line to a Quadrupole Mass Spectrometer (Balzers QMS 311 or QMG 112). Volatile substances diffusing through the PTFE membrane enter into a first chamber where a pressure between 10 1 and 10 2 mbar is maintained by means of a turbomolecular pump. In this chamber most of the gases entering in the MS (mainly solvent molecules) are eliminated, a minor part enters in a second chamber where the analyzer is placed. A second turbo molecular pump evacuates this chamber promptly and the pressure can be controlled by changing the aperture between both chambers. Depending on the type of detector used (see below) pressures in the range 10 4-10 5 mbar, (for Faraday Collector, FC), or 10 7-10 9 mbar (for Secondary Electrton Multiplier, SEM) may be established.

Volatile or volatilizable compounds may be introduced into the spectrometer via a pinhole aperture or molecular leak which allows a steady stream of sample molecules into the ionization area. Non-volatile or thermally labile samples are introduced directly by means of an electrically heated probe inserted through a vacuum lock. Numerous methods of sample ionization are available of which the most important are electron impact (El), chemical ionization (CY), field ionization (FI), field desorption (FD), fast atom bombardment (FAB), and radio-frequency spark discharge. 

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