Ionization Coefficients in Helium at High Pressures

Abstract

IN recent years there has been considerable effort devoted to the study of the ionization processes which give rise to ionization growth in different gases. Of particular interest from the fundamental point of view is the growth of ionization in helium, because in this case quantum mechanical methods may be more easily applied than in the more complex case of diatomic gases. Consequently, direct comparison may be made between experimental data on ionization coefficients, for example, and the quantum mechanical computations. Despite its simple atomic structure, however, the existence of high-energy metastable states of the helium atom, as well as of the helium molecule, results in the possibility of highly complex ionization processes occurring in this gas^1–3. Moreover, experimentally, helium is a difficult gas on which to make observations because it has the highest ionization potential (24.6 eV) of any known gas. Most common impurities have ionization potentials (<16 eV) appreciably lower than the ionization potential of helium, and are thus preferentially ionized by direct electron impact even when present in only small quantities. Furthermore, ionization of the impurity atoms and molecules can be produced by collisions of the second kind with helium atoms in the high-energy metastable states, and by high-energy photons produced in the gas by the transition of excited helium states (which all have energies > 20 eV) to the ground-state. The effects of impurities, even when present in quantities as low as 1 part in 10⁶, become increasingly important as the parameter E/p (E, the electric field, p, the gas pressure) decreases; because, as E/p decreases, the mean electron energy decreases and the ratio of ionized impurity atoms to ionized helium atoms then increases. These considerable experimental difficulties probably explain why there are so far no published data on the measurement of ionization growth in helium at high gas pressures and low values of E/p.