Helium ionisation potential

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

The goal of this work is a calculation of the ionisation potentiad of the helium atom which is as precise as possible. To this end a method for the solution of the Schrddinger equation is used, which resembles the method of Ritz for the solution of the variational problem. The calculations are extended through eleventh order, and the ground term obtained in this way differs from the one obtained by experiment by only 1.5 prom. By contrast, the ionisation potential differs by 4.8 prom, since the subtraction of the known energy of the helium ion leads to an increase in the relative error. 

Recently Edlen in Upsala has measured the I.P. s (ionisation potentials) of Li and Be" " " very accurately with the help of a new vacuum spectrograph. I have therefore extended my previous calculations on helium to these two cases. The most rational way to do this appeared to be the treatment of a complete eigenvalue problem, where is treated as the perturbation function and is the perturbation parameter. Making the ansatz... 

For helium the calculations have been made more precise by three orders, to exclude every possibility that the previously determined value will be changed sig-mficantly. We obtsdn a value for the ionisation potential which exceeds the experimental by 0.003 V, while the one foimd previously was too small by about 0.010 V. The difference is certainly within the experimental error margins, which have unfortunately not been given. For Li" " and Be" " " the theoretical values fall within the very narrow experimental error margins. 

In field ionisation microscopy (FIM), helium at low pressure is introduced into the above system and the polarity of the applied potential difference is reversed. Helium atoms in the vicinity of the now positively charged metal tip are stripped of an electron and the resulting helium ions are accelerated radially to the negatively charged fluorescent screen. 

It is useful to test approximations for the total ionisation cross section of helium, since it is a common target for the scattering and ionisation reactions treated in chapters 8, 10 and 11. Fig. 10.15 compares the data reported as the experimental average by de Heer and Jansen (1977) with the distorted-wave Born approximation and the coupled-channels-optical calculation using the equivalent-local polarisation potential. Cross sections... 

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