Helium ionization yield

Figure 5.14 The helium ionization yield for the ion being left in the He+(2s) and He+(2p) excited states as a function of the time delay between the initial XUV pump pulse and the IR probe pulse. The oscillations are due to breathing between different doubly excited states (resonances).

The temperature dependence of the cross section for ionization and excitation by fast electrons is very weak. Consequently, the main kinetic parameter characteristic of the chemical conversion rate in radiation chemistry is the value G, the number of molecules converted per unit absorbed energy (conventionally accepted as 100 eV). This value is known as the radiation chemical yield. The ionization yield for various groups ranges from 2.39 for helium to 4.46 for butane [224] and only slightly depends on the type of irradiation [387]. 

A tabulation of the ECPSSR cross sections for proton and helium-ion ionization of Kand L levels in atoms can be used for calculations related to PIXE measurements. Some representative X-ray production cross sections, which are the product of the ionization cross sections and the fluorescence yields, are displayed in Figure 1. Although these A shell cross sections have been found to agree with available experimental values within 10%, which is adequate for standardless PKE, the accuracy of the i-shell cross sections is limited mainly by the uncertainties in the various Zrshell fluorescence yields. Knowledge of these yields is necessary to conven X-ray ionization cross sections to production cross sections. Of course, these same uncertainties apply to the EMPA, EDS, and XRF techniques. The Af-shell situation is even more complicated. 

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. 

Electric discharges and ultraviolet light give no organic compounds with a mixture of C02 + H20. Ionizing radition (e.g., 40 MeV helium ions) gives small yields of formic acid and formaldehyde.25... 

The evolution of hydrocarbon products from shale as the pyrolysis temperature increased was monitored with a flame ionization detector and a yield versus temperature profile produced. Initially, pyrolysis was done with the CDS pyroprobe, but major limitations were observed, mainly with ensuring prompt removal of products. Subsequently, an automated concentrator and high-temperature pyrolysis furnace produced by Envirochem, Inc. was employed. This system permitted use of larger (50-100 mg) samples and the helium sweep gas effectively removed pyrolysis products (pyrolysates). However, the pyroprobe system is still used to measure YP as it provides a rapid, consistent determination. 

Field ion microscopy, or FIM, is a classical technique for the study of surface structure that can yield a structural image with a magnification factor on the order of 10. About 10,000 V are applied between the sample, shaped into a pointed tip (positive pole), and a fluorescent screen in a low-pressure atmosphere of helium. The helium atoms that approach the sample tip loose an electron due to the strong eleelrie field and the resulting He ions are accelerated towards the fluorescent sereen where they form an image. The ionization probability depends on the local intensity of the electric field at the sample, and therefore on the atomic structure. The method is limited to refractory metals such as tungsten, tantalum or iridium. 

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