Helium, field ionization

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.

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. 

The energy relations that must be obeyed to make field ionization possible are indicated schematically in Fig. 52a. In free space, the potential well of an atom placed in a uniform field is distorted symmetrically. At high fields (with helium, F > 4.5 volts/A) the barrier behind which the electrons are trapped (shaded in the illustration) is sufficiently thinned and electrons can tunnel through. The rate of tunneling has been evaluated explicitly for hydrogen atoms (70, 71) and hydrogen molecule ions, H (72) for the former, the rate constant for field ionization can be written as... 

J.S. Parker, G.S.J. Armstrong, M. Boca, K.T. Taylor, From the UV to the static-field limit Rates and scaling laws of intense-field ionization of helium, J. Phys. B 42 (2009) 134011. 

Riley DJ, Mann M, MacLaren DA, Dastoor PC, Allison W, Teo KBK, Amaratunga GAJ, MUne W (2003) Helium detection via field ionization from carbon nanotubes. Nano Lett 3 1455-1458... 

McClintock, P. V. E. and Read-Forrest, H., Angular variation of current from field emission and field ionization sources in liquid helium. Cryogenics, 13, 363,1973. 

Fig. 10. Photoeleotron spectrum of oxygen using the helium resonance line (21-21 e.v.) obtained with a magnetic electron energy analyser (May and Turner, unpublished work). Ionization energy increasing from left to right. The spectrum reveals four levels of ionization and the vibrational structure associated with each state of the ion can be clearly distinguished. This spectrum may be compared with that obtained using an electrostatic retarding field analyser (Al-Joboury et al., 1965).

The most frequently studied samples with FIM are refractory metal tips, such as W, Mo, Pt, Ir, etc. The field evaporation threshold for refractory metals is appreciably higher than the field to ionize helium atoms, which is 4.5 V/A. Field evaporation is also used for forming and cleaning the FIM sample, which is the tip end, to make it a sharp end and to remove adsorbed exotic atoms. A typical FIM image is shown in Fig. 1.34. 

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