Beam detectors electron bombardment

Lee Y T, McDonald J D, LeBreton P R and Herschbach D R 1969 Molecular beam reactive scattering apparatus with electron bombardment detector Rev. Sol. Instrum 40 1402-8... 

Fig. 1.—Schematic view of the photofragment spectrometer. In a high-vacuum chamber a molecular beam is perpendicularly crossed with pulses of polarized light from a laser. Photofragments recoiling upward from the intersection region are monitored by an electron bombardment quadrupole mass spectrometer detector as a function of fragment mass, recoil time (after the laser pulse) over a known flight path, and angle 0 between the electric vector of the laser light and the detection direction.

Except for the alkali beams, where surface ionization can be used, the weak beam is difficult to detect. Cohen et al.51 succeeded in detecting Ba and A1 beams with a specially designed electron bombardment detector. In spite of these problems sputtering is for many substances, like light metal atoms and any other material of high melting point, the only way to produce a fast neutral beam. 

The velocity distribution has been measured in two ways Parks and Wexler59 have used a time-of-flight method combined with a specially designed electron bombardment detector, while Haberland et al.60 made a retarding field analysis of the ionized fragments produced by electron bombardment of the primary beam. For Kr and Xe the velocity resolution has been found to be 7 % f.w.h.m. or better over the whole velocity range from 0-2 to 5 eV for Kr and up to 10 eV for Xe. 

Because of the very high beam intensity, typically 1017 to 1018 atoms/ sterad sec, there is no difficulty in detecting seeded beams, and simple electron bombardment detectors may be used. 

Lamb and Retherford [81] proposed to make a beam of atomic hydrogen, and to excite it by electron bombardment. Most of the atoms in the 22S level, but not those in the 22P, levels, would live. long enough to reach a detector. A radiofrequency field of the correct frequency in the path of the atoms would induce transitions from 22 to one of the 22P levels. Decay of the latter would result in a reduction in the number of atoms detected, which would indicate radiofrequency resonance. We shall give here a brief account of the work and quote the results. Details of the experiments are given in a series of six papers [82], [83], [80], [84], [131], [30] and a review [79]. 

In 1967, Yuan T. Lee (1936- ) joined Herschhach as a researcher after completing his Ph.D. at Berkeley. Lee designed and built an apparatus with supersonic beam nozzles and an electron bombardment ionizer that functioned as a universal detector. Supersonic beams propel species in the same direction at nearly the same speed and allow very few collisions. For this reason, chlorine atoms, much more reactive than potassium atoms, could be employed in the new and even more sensitive apparatus ... 

In a later, more sophisticated generation of beam devices, the detector can be rotated with respect to the reactant ion beam direction. An apparatus used by Champion et is shown in Fig. 3. Primary ions were formed by electron bombardment, extracted, accelerated to about 200 eV by a series of five cylindrical lens elements, and mass-analyzed by a 60°, 13.3-cm-radius mass spectrometer. The ions were then retarded to the desired collision energy by another five-lens system, formed into a ribbonlike beam by a pair... 

Lee YT, McDonald JD, LeBreton PR, Herschbach DR. 1969. Molecular beam reactive scattering apparatus with electron bombardment detector . Rev. Sci. Instrum. 40(11) 1402-1408. 

The detector, which usually can be rotated to various scattering angles, must be able to differentiate among the species present. Surface-ionization detectors with efficiencies of approximately 10% are employed for reactions of metal atoms yielding metal halide products. Laser-induced fluorescence has proved to be an excellent detector for group Ilb halides and a few other systems. In general, products can be detected by electron-bombardment ionization with subsequent analysis by a mass spectrometer. Unfortunately, these mass spectrometer or universal detectors have a low detection efficiency ( 0.1%,) thus, beam machines with such detectors require several differentially pumped chambers to reduce the background pressure (of the mass of interest) in the detector to torr. Modulation tech-... 

For this purpose, the authors used a special vacuum cell with a controlled focused electron beam incident on a zinc oxide film target. In these experiments, the role of the film was twofold. It served as an adsorbent and as a high-sensitivity detector of hydrogen atoms (10 at/cm ). Hydrogein atoms were produced due to surface dissociation of adsorbed molecular hydrogen. This process was induced by heating or bombardment of the adsorbed layer by an electron beam. 

Excitation of sample by bombardment with electrons, radioactive particles or white X-rays. Dispersive crystal analysers dispersing radiation at angles dependent upon energy (wavelength), detection of radiation with gas ionization or scintillation counters. Non-dispersive semiconductor detectors used in conjunction with multichannel pulse height analysers. Electron beam excitation together with scanning electron microscopes. 

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