Helium atom ionization potential

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. 

The principal mechanism for analyte response is ionization due to collision with metastable helium atoms. Hetastable helium atoms are generated by multiple collisions with beta electrons from the radioisotopic source. Since the ionization potential of helium (19.8 ev) is higher than that of all other species except neon, then all species entering the ionization chamber will be ionized. 

The helium atom has the highest ionization potential of any atom. It has a nuclear charge of + 2, and the electrons reside in the lowest energy level close to the nucleus. 

An a particle can abstract two electrons from some other atom or molecule (and given the extremely high ionization potential of helium, the highest of any atom, it would be difficult to prevent it) to become a helium atom. Helium also is a constituent in stars as a result of the fusion reaction... 

Figure 7.10 The principle of field ionization. Left the potential for a helium atom near a metal without field, and (right) in the presence of an electric field of strength F (V/cm). Field ionization by electron tunneling becomes possible when the He Is level (ionization potential /) is above the Fermi level of the metal. Tunneling increases when the He atom is closer to the surface. This, however, requires high local fields, which are present at the edges of crystal facets or at adsorbed atoms.

As described in Section 5.6.2, argon/helium atoms are excited to a metastable state by beta radiation from a radioactive source. The species formed is then capable of ionizing all compounds with a lower ionization potential. The products formed are then subject to an electric field (500-1100 V) and the change in current measured. 

PENNING EFFECT. An increase in the effective ionization rate of a gas due to the presence of a small number of foreign metastable atoms. For instance, a neon atom has a metastable level at 16.6 volts and if there are a few neon atoms in a gas of argon which has an ionization potential of 15.7 volts, a collision between the neon metastable atom with an argon atom may lead to ionization of the argon. Thus, the energy which is stored in the metastable atom can be used to increase the ionization rate. Other gases where this effect is used are helium, with a metastable level at 19.8 volts, and mercury, with an ionization level at 10.4 volts. 

The helium ionization detector (HID) is a sensitive universal detector. In the detector, Ti3H2 or Sc3H3 is used as an ionization source of helium. Helium is ionized to the metastable state and possesses an ionization potential of 19.8 eV. As metastable helium has a higher ionization potential than most species except for neon, it will be able to transfer its excitation energy to all other atoms. As other species enter the ionization field the metastable helium will transfer its excitation energy to other species of lower ionization potential, and an increase in ionization will be measured over the standing current. 

Lindgren I, Salomonson S, Moller F (2005) Construction of accurate Kohn-Sham potentials for the lowest states of the helium atom Accurate test of the ionization-potential theorem, Int J Quant Chem, 102 1010-1017... 

A measure of the voltage necessary to remove one (or more) electrons from an atom is called its ionization potential. Thus, the ionization potentials of H, He, and Li have been found to be 13 volts, 24 volts, and 5 volts, respectively. The inner two electrons of lithium are held very tightly (71 volts and 122 volts). It should also be noted that there is no tendency for an extra electron to move into the vicinity of a helium atom since it would have to occupy a relatively unstable state, much further from the nucleus than the other two. 

Experimentally, one finds that it takes only 452 kJ to break apart a mole of hydrogen molecules. The reason the potential energy was not lowered by the full amount is that the presence of two electrons in the same orbital gives rise to a repulsion that acts against the stabilization. This is exactly the same effect we saw in comparing the ionization energies of the hydrogen and helium atoms. 

Since about 10 years ago (thermal) helium beams have been used for the diagnostics of fusion boundary plasmas as they can penetrate relatively far because of the high ionization potential of the atoms (nearly 25eV) [61,62]. From the line ratios of the triplet and singlet lines one can derive local electron temperatures and densities (Fig. 6.18) provided the population rates and their equipartition times are known and allow the application of a steady state model [63], The corresponding rates have been improved during the last few years, and although it is now a well-established technique, there are still open questions and scope for future developments. 

On examining the data closely we find a systematic deviation of the calculated from the observed values. This deviation becomes most marked in the elements of low atomic number. It lies in the same direction as the well-known difference between the observed and calculated ionizing potentials of helium. 

Therefore, intensity of cathode emission increases with greater cathode current and with higher atomic weights of the filler gas species. Helium, with its low atomic weight and relatively high ionization potential, generally is least suitable (Tl). 

In testing the capability of the mass spectrometer to detect metastable components, we excited helium in a discharge and looked for metastable He(2 S) atoms. The ionization curve in Figure 13 shows the presence of metastable He atoms. A rough value of the ionization potential obtained from these data was 5 e.v., which correlates with the spectroscopically calculated ionization potential of He(23S) atoms of 4.77 e.v. In order to observe these atoms it is necessary to maintain the discharge close to the sampling orifice, indicating very rapid destruction of the metastables by wall collision. 

The metastable atoms that must be produced in the argon and helium detectors need not necessarily be generated from electrons induced by radioactive decay. Electrons can be generated by electric discharge or photometrically, which can then be accelerated in an inert gas atmosphere under an appropriate electrical potential to produce metastable atoms. This procedure is the basis of a highly sensitive helium detector that is depicted on the left-hand side of Fig. 1. The detector does not depend solely on metastable helium atoms for ionization and, for this reason, is called the helium discharge ionization detector (HDID). 

The helium atom (Z=2), with two electrons, has just one monoionization energy, hence the two electrons have the same energy. The lithium atom (Z = 3) and the beryllium atom (Z=4) both have two ionization potentials, but B (Z= 5), C (Z=6),..., Ne (Z= 10) have three. For Na (Z= 11) there are four. We are referring to the process... 

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