Ionization potential of helium

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. 

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... 

Both detectors use the same chamber design so that fundamental differences are with respect to the gases themselves. That is, the ionization potential of helium is significantly higher than argon and thus has the capability of ionizing some species which argon cannot. In this sense it is more universal. 

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. 

In a cylindrical gas counter with a central wire radius equal to 25 /j,m (0.001 in), outer radius 25 mm ( 1 in), and 1000 V applied between anode and cathode, what is the distance from the center of the counter at which an electron gains enough energy in 1 mm of travel to ionize helium gas (Take 23 eV as the ionization potential of helium.)... 

For the first ionization potential of helium we therefore obtain... 

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 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. 

Fig. 4. Potential energy versus distance from the surface. Data is appropriate for He and tungsten. E, is the ionization potential for helium and ( > is the work function of tungsten. E (e") is the kinetic energy of an emitted secondary electron. The symbol He + nej implies a system composed of an helium ion and n conduction electrons in tungsten. The lower potential curve results from an Auger neutralization process where both electrons were originally at the Fermi level. (The figure is similar to one published in Ref. )...

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. 

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. 

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. 

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 ionization potentials of molecules are several electron volts, even for the outermost valence electrons, and thus it is necessary to work in the UV region of the spectrum. Usually, the excitation source is a discharge through helium, which gives a band at 584 A, corresponding to a photon energy 21.24 eV (UPS). If the electrons lie in the core of the molecule, more energy is... 

Since the introduction of ECD, the Ni 3-ray radioactive electron source has remained unchanged, and considerable efforts have been made to develop nonradioactive alternatives. A new version of such a detector — the pulsed discharge electron capmre detector (PDECD) — employs a pulsed discharge in helium as the primary source of electron generation. A modified version of PDECD which makes use of methane as the dopant gas and of a sapphire and quartz insulation was used for detecting OCPs. The relatively low ionization potential of methane allows reduction... 

If the electronic excitation energy of a metastable atom A exceeds the ionization potential of another atom B, their collision can lead to an act of ionization, the so-called Penning ionization. The Penning ionization usually proceeds through the intermediate formation of an unstable excited quasi molecule in the state of auto-ionization cross sections of the process can be very high. Cross sections for the Peniung ionization of N2, CO2, Xe, and Ar by metastable helium atoms He(2 S) with an excitation energy of 19.8 eV reach gas-kinetic... 

The singlet ground state (para-helium case) requires particular spatial symmetry of the function. Therefore, only even powers of t were considered here. Even very short expansions of this type [Eq. (4)] led to very good results reducing the discrepancy between theory and experiment from 0.12 to 0.01 eV in terms of the ionization potential of the helium atom. 

Associative ionization is not restricted to the formation of dimer ions by reaction (1) in pure gases. On the contrary, associative ionization is now known to be a quite general phenomenon that occurs in a variety of gas mixtures. In 1957, Pahl and Weimer observed that HeNe was formed in the positive column of a discharge in helium-neon mixtures. It was subsequently shown in mass spectrometric studies " that the appearance potential of the HeNe" ion is of the order of 23.0 eV (Table IV). Since this minimum excitation energy lies above the ionization potential of neon and is nearly the same as the appearance potential of it is most probable that the same set of excited states of helium that produce He2 also form HeNe, namely... 

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