Metastable 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. 

Figure 13 Plots of (

Figure 16 Relation of de-excitation cross sections (ctm) for metastable helium atoms with the Hotop-Niehaus formula for (a) He(2 S) and (b) He(2 S). (From Ref. 154.)...

Recent measurements utilizing the crossed-beam technique have been performed as follows.37 A metastable helium beam is formed by electron-impact excitation of a thermal helium beam effusing from a multichannel array. The optical quenching method12 described earlier is applied to obtain results for He(2 5 ) and He(23S) separately. The target gas beam is... 

Gas-discharge lamps are used to optically pump the metastable helium atoms into a higher excited electronic state, which has a dipole-allowed transition to the ground state. Only He (2 S) can be pumped selectively, thereby producing pure He(23S) beams. For the heavier rare gases, both metastable states are equally pumped by gas-discharge lamps. The use of cutoff filters to selectively pump one state is not adequate because of the temperature dependence of the filter transmission and the low / numbers of the pumping transition. Metastable neon can be selectively pumped by a continuous wave (cw) dye laser,60 whereas Ar, Kr, and Xe have so far only been selectively pumped by pulsed dye lasers.61... 

Figure 7. Time-of-flight distributions of metastable helium beams. Singlet distributions are generally narrower than triplet ones. The 540-meV distribution was obtained with plasma-jet source (see Section III.A.6).

We begin by specializing the discussion of Section II.C to the He -He case. If a metastable helium atom collides with a ground-state helium atom, the interaction to two identical atoms in different electronic states results. This simple fact has a strong influence on the scattering patterns. 

Castello, G., Munari, S. Gas chromatographic analysis of gaseous atmospheric pollutants with a metastable helium detector. Chim. Ind. (Milan) 51, 469 (1969). —... 

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. 

Finally, we wish to mention that, at Harvard University, another laser spectroscopy experiment was carried out in a discharge cell, and not in a clean environment as the metastable helium beam of the other experiments. The uncertainty of this unpublished measurement is 8 kHz [11], probably due to systematic effects from collisions in the helium cell [13]. 

We detected the saturated fluorescence emitted by a beam of 23S metastable atoms as they cross at right angle the slave laser light. A 1015 atoms/s.sterad flux of metastable helium atoms was produced by electronic collisions in a DC discharge of a helium atomic beam, similar to that described in [15]. To improve the precision of the linecenter determination, we increased the signal-to-noise ratio S/N by means of standard frequency modulation the third harmonic demodulated lineshape is shown in Fig. 4. The function expected for a Lorentzian spectrum was fit and linecenters were calculated with an uncertainty ranging between 10 kHz and 20 kHz, that is consistent with the observed S/N, mainly limited by the stability of the reference frequency and of the metastable helium beam. The reproducibility was two or three times worse than the uncertainty,... 

Another ionization process that is the main process when helium is used as the gas is proton transfer. This type of ionization occurs when metastable helium atoms react with water in the atmosphere to produce ionized water clusters that can protonate the sample molecule, leading to the protonated molecule. 

The electric discharge produces both electrons and photons. The electrons can be accelerated to produce metastable helium atoms, which in turn can ionize the components in the column eluent. However, the photons generated in the discharge have, themselves, sufficient energy to ionize many eluent components and so ions will probably be produced by both mechanisms. It is possible that other ionization processes may also be involved, but the two mentioned are... 

The detector has two sections the upper section consisting of a tube 1.6 mm ID. (where the discharge takes place) and the lower section, 3 mm I.D. (where reaction with metastable helium atoms and photons... 

The photons and metastable helium atoms enter the reaction zone where they meet the eluent from the capillary column. The solute molecules are ionized and the electrons produced are collected at the lower electrode and measured by an appropriate high impedance amplifier. The distance between the collecting electrodes is about 1.5 mm. The helium must be 99.9995 pure. The base current ranges from... 

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). 

This cascade effect, unless controlled, results in an exponential increase in ion current. It is clear that the helium must be extremely pure or the production of metastable helium atoms would be quenched by traces of any other permanent gases that may be present. 

The Characterization of Liquid and Solid Surfaces with Metastable Helium Atoms,... 

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