Metastable atoms

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 emission methods of EEP detection are based on the ability of some metastable particles (N2(A ZJ, metastable atoms of rare gases) to knock electrons out of the surface of metals [59]. The present-day technique of measuring small currents allows one to record any small fluxes... 

Interaction between Metastable Atoms of Rare Gases and Surface of Oxide Semiconductors... 

Most of the publications dedicated to the interaction between the RGMAs and a solid surface refer to the rare gas - metal system. The secondary electron emission that occurs in the system allows one to judge of the mechanism that deactivates metastable atoms on a metal surface, as well as to evaluate the concentration of metastable atoms in the gaseous phase. 

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. 

Oliphant and Moon theoretically considered the possibility of electron emission by resonance ionization of metastable atoms near a metal surface. Shekter [122] investigated the Auger-neutralization of ions on a metal surface. Hagstrum [124, 125] carried out an generalized analysis of metastable atoms with a metal surface. 

Fig. 5.18. Energy diagrams of possible mechanisms for deactivation of rare gas metastable atoms on a metal surface [126]...

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. 

Adsorbed layers, thin films of oxides, or other compounds present on the metal surface aggravate the pattern of deactivation of metastable atoms. The adsorption changes the surface energy structure. Besides, dense layers of adsorbate may hamper the approach of metastable atom sufficiently close to the metal to suppress thus the process of resonance ionization. An example can be work [130], in which a transition from a two- to one-electron mechanism during deactivation of He atoms is exemplified by the Co - Pd system (111). The experimental material on the interaction of metastable atoms with an adsorption-coated surface of... 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

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. 

The pattern of interaction between metastable atoms of rare gases and a semiconductor or dielectric surface is not yet clear, the literature data in this field are incomplete and uncoordinated, a fact that is primarily associated with the lack of convenient techniques suitable for studying these systems. 

Because of these factors, the fundamental experimental information about the interaction of metastable atoms with semiconductors and dielectrics is meant for the reflection coefficients that are determined with the aid of beam methods and for the coefficients of heterogeneous deactivation which are evaluated under diffusion conditions. However, the data in this event are fairly scarce and conflicting. The results obtained by the methods of electronic beams do not agree with diffusion experiments. Thus, Allison et al [ 137] report that the coefficients dealing with... 

As evident from the above-given data the two above-mentioned methods of evaluating the coefficients of metastable atom reflection from nonmetallic surfaces yield essentially different results. The information found in the literature is yet insufficient to understand the cause of this discrepancy. 

Doyen [158] was one who theoretically examined the reflection of metastable atoms from a solid surface within the framework of a quantum- mechanical model based on the general properties of the solid body symmetry. From the author s viewpoint the probability of metastable atom reflection should be negligibly small, regardless of the chemical nature of the surface involved. However, presence of defects and inhomogeneities of a surface formed by adsorbed layers should lead to an abrupt increase in the reflection coefficient, so that its value can approach the relevant gaseous phase parameter on a very inhomogeneous surface. 

From the above-made review of literature, one may infer that the interaction of metastable atoms of rare gases with a surface of semiconductors and dielectrics is studied, but little. The study of the mechanism of transferring energy of electron-excited particles to a solid body during the processes under discussion is urgent. The method of sensor detection of rare gas metastable atoms makes it possible to obtain new information about the heterogeneous de-excitation of metastable atoms inasmuch as it combines high sensitivity with the possibility to conduct measurements under different conditions. 

Rare Gas Metastable Atom Sensors Based on the Au/ZnO Structure... 

Panesh et al. [157] were the first to make an attempt to detect rare gas metastable atoms (RGMAs) with the aid of semiconductor sensors. The sensing element (a sensor) was represented by a sintered polycrystalline film of ZnO metastable atoms were obtained in a neon ambient by electron impact. It was shown that electrical conductivity of ZnO film irreversibly increases under the action of RGMAs. However, the signals obtained were too small and that did not allow one to utilize the sensing technique to survey the processes with participation of metastable atoms. 

The most obvious way to raise the sensitivity of sensors to RGMAs is by activating their surface with additives that actively interact with metastable atoms and have some electron coupling with semiconductor. These additives can be microcrystals of metals. As previously shown, the de-excitation of RGMAs on a metallic surface truly proceeds at high efficiency and is accompanied by electron emission. Microcrystals of the metal being applied to a semiconductor surface have some electron coupling with the carrier [159]. These two circumstances allow one to suppose that the activation of metals by microcrystals adds to the sensitivity of semiconductor films to metastable atoms. 

Fig. 5.19. Experimental set-up to study the influence of metastable atoms of rare gases on electrical conductivity of Au/ZnO films [160]...

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. 

Another methodical trait of the Au/ZnO sensor application to detect metastable atoms of rare gases is the limitation of the range of operating temperatures. When heated to above 500 K, these sensors irreversibly loose their sensitivity to RGMAs. The loss of sensitivity is associated with the coalescence of Au microcrystals applied to a ZnO surface. The causes of this will be discussed later. 

---