Helium metastable atoms

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. 

Allowing for the fact that the secondary electron current is in proportion to the concentration of metastable atoms in gaseous phase [62], one may infer that the Au/ZnO structures are highly sensitive proportional sensors of helium metastable atoms. Similar results were obtained for other rare gases. 

Fig. 5.21. Volt - ampere characteristic of the secondary election collector with the Au/ZnO film being acted upon by helium metastable atoms of different concentration (d) and neon (/) metastable atoms (2) (6) [164]...

Fig. 5.25. Temperature dependence of the starting velocity of change in Au/ZnO film being acted upon by helium metastable atoms after different preliminary treatment [174] (/ - film treated by active hydrogen 2- starting film 3 - film treated by active oxygen)...

Lx>ng radiative lifetimes of metastable states support the high density of these particles in slightly ionized plasma, or in excited gas. Thus, according to Fugal and Pakhomov [18, 19] the density of metastable atoms of helium at pressure of the order of a few Torrs, at temperatures ranging from 4 to 300 K, is about two orders of magnitude above the density of electrons. The density of metastable atoms and molecules in... 

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. 

This relationship of the metastable atom deactivation mechanisms is valid for atomically pure metal surfaces and is proved true in a series of works [60, 127, 128]. Direct demonstrations of resonance ionization of metastable atoms near a metal surface are given by Roussel [129]. The author observed rebound of metastable atoms of helium in the form of ions from a nickel surface in the presence of an adsorbed layer of potassium. In case of large coverages of the target surface with potassium atoms, when the work of yield becomes less than the ionization potential of metastable atoms of helium, the signal produced by rebounded ions disappears, i.e. the process of resonance ionization becomes impossible and the de-excitation of metastable atoms starts to follow the mechanism of Auger deactivation. 

The experiments carried out by Kupriyanov et al. [178] support the validity of equation (5.33). Figure 5.24 shows the temperature dependencies of the starting velocity of changes in the electrical conductivity of Au/ZnO film being acted upon by metastable atoms of helium and xe-... 

PENNING EFFECT. An increase in the effective ionization rate of a gas due to the presence of a small number of foreign metastable atoms. For instance, a neon atom has a metastable level at 16.6 volts and if there are a few neon atoms in a gas of argon which has an ionization potential of 15.7 volts, a collision between the neon metastable atom with an argon atom may lead to ionization of the argon. Thus, the energy which is stored in the metastable atom can be used to increase the ionization rate. Other gases where this effect is used are helium, with a metastable level at 19.8 volts, and mercury, with an ionization level at 10.4 volts. 

Figure 2. Central part of apparatus used for measurement of differential elastic cross sections for scattering of helium metastables from ground-state atoms (reproduced from Brutschy et al.30)... 

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 ... 

Abstract. The antiprotonic helium,pe He2+ (= pHe+), is a peculiar metastable atom, interfacing between matter and antimatter. A series of metastable states axe composed of the He nucleus, one electron in the ground Is configuration and one antiproton orbiting with large quantum numbers (n, l), where n l e 38. They possess... 

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,... 

The metastable atoms that must be produced in the argon and helium detectors need not necessarily be generated from electrons induced by radioactive decay. Electrons can be generated by electric discharge or photometrically, which can then be accelerated in an inert gas atmosphere under an appropriate electrical potential to produce metastable atoms. This procedure is the basis of a highly sensitive helium detector that is depicted on the left-hand side of Fig. 1. The detector does not depend solely on metastable helium atoms for ionization and, for this reason, is called the helium discharge ionization detector (HDID). 

Lamb and Skinner [86] were able to measure the interval 2in He4- by a method which, as in the experiment on hydrogen, depended on the metastability of the 22 level, but in which the method of detection of radiofrequency resonance was different. In the case of hydrogen, the method of detection depended on the possibility of distinguishing metastable atoms from atoms in the ground state through their power of liberating electrons from tungsten. Helium ions can also liberate electrons from metals, but the relative ejection efficiency of metastable and non-metastable ions is not known. Lamb and Skinner... 

If the metastable atoms get close to the surface without R1 taking place, then AD can occur. This is the dominant de-excitation process if R1 is suppressed, which occurs if the level of the excited helium atom lies below the Fermi level (/eff > with /eff being the ionization potential, see Fig. 2.7) or if there is an insufficient overlap with empty states due to an adsorbate layer. In this case the li hole is hlled by an electron from the solid or the adsorbate layer with a simultaneous ejection of the excited 2s atomic electron. 

If the electronic excitation energy of a metastable atom A exceeds the ionization potential of another atom B, their collision can lead to an act of ionization, the so-called Penning ionization. The Penning ionization usually proceeds through the intermediate formation of an unstable excited quasi molecule in the state of auto-ionization cross sections of the process can be very high. Cross sections for the Peniung ionization of N2, CO2, Xe, and Ar by metastable helium atoms He(2 S) with an excitation energy of 19.8 eV reach gas-kinetic... 

A gas (usually helium) flows through a corona discharge where it can be ionized or form excited (metastable) atoms. 

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