Atomic Helium Beams

Since about 10 years ago (thermal) helium beams have been used for the diagnostics of fusion boundary plasmas as they can penetrate relatively far because of the high ionization potential of the atoms (nearly 25eV) [61,62]. From the line ratios of the triplet and singlet lines one can derive local electron temperatures and densities (Fig. 6.18) provided the population rates and their equipartition times are known and allow the application of a steady state model [63], The corresponding rates have been improved during the last few years, and although it is now a well-established technique, there are still open questions and scope for future developments. 

The influence of high-n shells, electron loss processes and level mixing should be further investigated. Also, the line emission from the n = 5 (4,3) levels should additionally be measured and compared with the model. In ADAS there should be an update of the He adf04 data set with respect to ionization, excitation, charge exchange, and n = 5 contributions. As the 

Species ionizat. rates excitat. rates coll.rad. models remarks 

D + + + ionizations/photon for Hq, Hp, H7 for edge plasmas available more data needed for CEX (Ne,Si,B.) 

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Fig. 4.12. Atomic-beam diffraction. A nearly monochromatic beam of helium, generated by a nozzle, falls on the solid surface with an angle of incidence. The diffracted beam is collected at an outgoing angle. The angular distribution of the diffracted helium beam contains the information about the topography and structure of the...

Another important property of the specularly scattered fraction of atoms is their great sensitivity to surface disorder. On scattering from a well ordered surface, nearly 15% of the scattered helium atoms appear in the specular helium beam. This fraction decreases to 1 to 5% when the surface is disordered. Thus measurements of the fraction of specularly scattered helium can provide information on the degree of atomic disorder in the solid surface. 

Scattering studies with metastable atoms are in many cases easier (and less expensive) than experiments with ground-state atoms, The discussion that follows is mainly concerned with helium, as most of the information is available for this atom. Figure 2 shows a skeletal setup of the experiment. A helium beam from a supersonic nozzle source is excited by electron impact to its two metastable states. The singlet state can be quenched by the 2g radiation from a helium-gas discharge lamp ... 

Figure 3. Newton diagrams for the two sets of measurements shown in Figs. 4 and 5. On left, He beam source is cooled to liquid-nitrogen temperature and ground-state helium beam is at room temperature, whereas beam temperatures are interchanged on right. This gives same center-of-mass kinetic energy but different laboratory energies for scattered atoms.

We detected the saturated fluorescence emitted by a beam of 23S metastable atoms as they cross at right angle the slave laser light. A 1015 atoms/s.sterad flux of metastable helium atoms was produced by electronic collisions in a DC discharge of a helium atomic beam, similar to that described in [15]. To improve the precision of the linecenter determination, we increased the signal-to-noise ratio S/N by means of standard frequency modulation the third harmonic demodulated lineshape is shown in Fig. 4. The function expected for a Lorentzian spectrum was fit and linecenters were calculated with an uncertainty ranging between 10 kHz and 20 kHz, that is consistent with the observed S/N, mainly limited by the stability of the reference frequency and of the metastable helium beam. The reproducibility was two or three times worse than the uncertainty,... 

This study used helium scattering to locate energy levels of bound states of He/LiF((X)l). If the angle and kinetic energy of the helium beam are varied, dips in the surface reflectivity will occur whenever the atom s kinetic energy perpendicular to the surface equals one of the discrete bound-particle energies of the surface-potential well. In the case in question, four bound levels were observed this information could compare to several different theoretical models. 

An interesting application of the theory was made by Devonshire (73) in an attempt to explain the anomalous diffraction of helium beams at crystal surfaces observed by Frisch and Stem (74). It was found that at suitable angles of incidence impinging helium atoms need not be reflected from the surface, but could move along it in a mobile state for some distance before being emitted. It could also be shown that... 

A gun is used to direct a beam of fast-moving atoms or ions onto the liquid target (matrix). Figure 4.1 shows details of the operation of an atom gun. An inert gas is normally used for bombardment because it does not produce unwanted secondary species in the primary beam and avoids contaminating the gun and mass spectrometer. Helium, argon, and xenon have been used commonly, but the higher mass atoms are preferred for maximum yield of secondary ions. 

Elemental chemical analysis provides information regarding the formulation and coloring oxides of glazes and glasses. Energy-dispersive x-ray fluorescence spectrometry is very convenient. However, using this technique the analysis for elements of low atomic numbers is quite difficult, even when vacuum or helium paths are used. The electron-beam microprobe has proven to be an extremely useful tool for this purpose (106). Emission spectroscopy and activation analysis have also been appHed successfully in these studies (101). 

Gas lasers are not unlike fluorescent light bulbs and neon signs. Gas is confined to a hollow tube, and electricity passing through it excites the atoms. The most common gas lasers use carbon dioxide, argon, and helium-neon. Gas lasers are relatively inexpensive and can produce very high-powered beams. 

The experiment conducted by Rutherford and his co-workers involved bombarding gold foil with alpha particles, which are doubly charged helium atoms. The apparatus used in their experiment is shown in Figure 14-9. The alpha particles are produced by the radioactive decay of radium, and a narrow beam of these particles emerges from a deep hole in a block of lead. The beam of particles is directed at a thin metal foil, approximately 10,000 atoms thick. The alpha particles are delected by the light they produce when they collide with scintilltaion screens, which are zinc sulfide-covered plates much like the front of the picture tube in a television set. The screen... 

Since the demonstration by Schumacher et al ) of the use of alkali metal vapor inclusion into a supersonic beam to produce clusters, there have been a number of attempts to generalize the approach. It has recently been recognized that instead of high temperature ovens, with their concommitant set of complex experimental problems, an intense pulsed laser beam focused on a target could be effectively used to produce metal atoms in the throat of a supersonic expansion valve. ) If these atoms are injected into a high pressure inert gas, such as helium, nucleation to produce clusters occurs. This development has as its most important result that clusters of virtually any material now can be produced and studied with relative ease. 

The earlier classical publications in this field belong to Oliphant [120]. In his survey he used a beam of rapid metastable atoms of helium obtained by neutralizing ions on the walls of platinum capillary. Oliphant was the first to observe emission of electrons from a surface of magnesium and molybdenum under the action of metastable atoms, and also rebounding of metastable atoms from a molybdenum surface. 

The presence of adsorbed layers also affects the other parameters of the interaction between metastable atoms and a metal surface. Titley et al. [136] have shown that the presence of an adsorbed layer of oxygen on a W( 110) surface increases the reflection coefficient of helium metastable atoms. The reflection is of irregular nature and grows higher when the incidence angle of the initial beam increases. A series of publications [132, 136, 137] indicate that the presence of adsorbed layers causes an increase in the quantum yield of electron emission from a metal under the action of rare gas metastable atoms. 

Resonance ionization spectroscopy is a photophysical process in which one electron can be removed from each of the atoms of a selected type. Since the saturated RIS process can be carried out with a pulsed laser beam, the method has both time and space resolution along with excellent (spectroscopic) selectivity. In a recent article [2] we showed, for example, that all of the elements except helium, neon, argon, and fluorine can be detected with the RIS technique. However, with commercial lasers, improved in the last year, argon and fluorine can be added to the RIS periodic table (see figure 2). 

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