Helium, metastables

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)...

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)... 

The principle of an experimental setup used for PglES with helium metastables is shown in Fig. 11. The metastables are produced in a hot cathode discharge, running at approximately 10-2 torr and 0.1 A in a differentially pumped chamber. The quenching lamp allows removal of... 

Figure 11. Schematic drawing of experimental setup used for Penning ionization electron spectroscopy using helium metastables.

The helium metastables differ only in the spin state. Thus pronounced differences in the functions V+(R) and T(R) for Penning systems with He(23S) and He(2 S) should be attributable to an influence of the spin. A quantitative discussion of this influence lies in the regime of the electronic structure and has not been given as yet. Hence we are limited in this section to stressing some of the observed similarities and differences between the helium metastables regarding Pgl and to try to give plausible qualitative explanations. 

Values taken from Hotop 48, except for the new value for He(23S)-H.51 Well depths of potentials with helium metastables are derived from Penning electron spectra. 

Fig. 3. The experimental setup. A beam of helium metastable ions crosses the resonant light inside a power enhancement cavity, which drives the 2S-3S transition. Note PM denotes polarisation-maintaining...

Reactions of helium metastables, e.g. He(2 Sq) with an excitation energy of 20.6 eV, will nearly always involve ionization. For example... 

The time-of-flight distributions for the helium metastable level 2 Sg are shown in Fig.7.14. The velocity intervals are determined by the channel width at detector 1, and the data for detector 2 have been averaged in such a way that the data plotted in Fig.7.14 give the number of metastable atoms in the same velocity intervals. Unfortunately, helium also has another long-lived raetastable level, the 2 S, and it is necessary to find some way of distinguishing between this state and the 2 Sg level. This is the purpose of the lamp shown in Fig.7.12. Metastable atoms in the 2 Sg level can be excited by the absorption of radiation at 20 581 % to the 2 level from which there is a fast... 

Afterglow experiments in helium-neon mixtures. When the afterglow of a helium-neon gas mixture is studied, it is found that the helium metastable population decays very much more rapidly than in pure helium at the same pressure. An examination of the helium and neon term diagram shown in Fig.11.3 quickly reveals that the He(2 S ) metastable level lies very close to the 2s levels of neon. The destruction of the helium metastables is therefore accomplished mainly by excitation transfer reactions of the type... 

The density of excited neon atoms in one of the 2s levels, is determined by the competition between excitation transfer from the helium metastables and radiative decay to the lower 2p levels ... 

Decay rate of spontaneous emission from levels of the 3d 4s configuration of 2n measured in a He-Zn afterglow as a function of zinc vapour density. The decay of the helium metastable... 

This transition is, at present, the shortest wavelength C.W. laser transition available and is widely used in the fields of photochemistry and photofabrication. Unfortunately, lasers pumped by Penning reactions are limited in output power by the saturation of the helium metastable density at high currents, exactly as in the case of the helium-neon system discussed,in section 11.4.2(d). In an attempt to overcome this limitation, considerable effort has been made to develop metal vapour laser systems excited by thermal-energy charge transfer. 

Thermal-energy charge transfer reactions. When the current in a helium discharge tube is increased from 100 mA to 10 A the density of helium ions, He, increases proportionally in contrast to the helium metastable density which... 

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