Helium light emission

Excitation in collisions of ground state helium atoms with He, Ne, H2 and N2 at llOOeV beam energy has been studied.37 Part of the radiation emitted along the beam path was focused onto the entrance slit of a spectrograph equipped with an image intensifier tube. The spectral resolution was about 20 A. No light emission from He was found in He + He nor in He + Ne collisions. Only excitation of the heavier rare gas atom has been... 

In 1989, Hewlett-Packard introduced a modernized and totally automated atomic emission detector (AED) connected with a GC equipped with a capillary column. At present, two names are used for the device GC-AED and (less frequently) GC-AES. The device utilizes a microwave-induced helium plasma for decomposition and excitation of analyzed compounds, and a photodiode array (PDA) for light emission measurement. The GC-AED may be used alone or in conjunction with GC-MS, and the situation allows for more effective use of both devices together than separately. 

X-ray emission spectrography developed in a way that made it convenient to segregate as light elements—elements ordinarily beyond reach of the method—those of atomic number lower than titanium. Hydrogen and helium are, of course, excluded because they emit no x-ray lines (1.19). In the early days of x-ray emission spectrography, its relative uselessness for the determination of elements between helium and titanium counted heavily against it in comparison with emission spectrography in the visible or ultraviolet. 

Helium, the second most abundant element in the universe after hydrogen, is rare on Earth because its atoms are so light that a large proportion of them reach high speeds and escape from the atmosphere. However, it is found as a component of natural gases trapped under rock formations (notably in Texas), where it has collected as a result of the emission of a particles by radioactive elements. An a particle is a helium nucleus (4He2+), and an atom of the element forms when the particle picks up two electrons from its surroundings. 

The emission from iodine vapor excited by monochromatic light may easily be photographed since it consists of a series of doublets. With addition of even a low pressure (e.g. 1 torr of helium) of foreign gas, the iodine molecules become distributed among so many rotational levels that the emission becomes very difficult to resolve even though a phototube may indicate virtually no decrease in total intensity. 

Figure 4.43 Energy- and angle-resolved triple-differential cross section for direct double photoionization in helium at 99 eV photon energy. The diagram shows the polar plot of relative intensity values for one electron (ea) kept at a fixed position while the angle of the coincident electron (eb) is varied. The data refer to electron emission in a plane perpendicular to the photon beam direction for partially linearly polarized light (Stokes parameter = 0.554) and for equal energy sharing of the excess energy, i.e., a = b = 10 eV. Experimental data are given by points with error bars, theoretical data by the solid curve.

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