My graduate research was done in the lab of Professor F. Barry Dunning at Rice University. I worked under the tutelage of the senior student on the experiment, James Lancaster, until 2002. When he graduated, I took over the experiment. My research involved studying the interaction between a singly-charged He+ (helium) ion and a metal surface. In this interaction, the He+ ion is neutralized by a metal electron and becomes a ground-state He0 atom. During the interaction, it is possible for electrons to be ejected from the metal surface. It is these ejected electrons that are collected and studied in our experiment.
In 1954, Homer D. Hagstrum wrote an article in Physical Review entitled "Theory of Auger Ejection of Electrons from Metals by Ions"1. In that article, he proposed four processes, shown above, that can take place when an excited helium atom or ion approaches a metal surface: (1) resonance ionization or (2) Auger (pronounced OH-shay) deexcitation of an excited helium atom, or (3) resonance neutralization or (4) Auger neutralization of a helium ion. Which of these processes occurs depends on whether the helium is an excited atom or an ion when it reaches the surface and on whether the metal surface has a high work-function or a low work-function. A metal's work-function, represented in the figures above with the symbol Φ, is a quantity that expresses how easy or difficult it is to remove an electron from the metal. For metals with a high work-function, it takes a lot of energy, i.e., it is difficult, to remove an electron, while the opposite is the case for metals with a low work-function. In Hagstrum's analysis, Auger deexcitation and resonance neutralization take place at metals with low work-functions while Auger neutralization and resonance ionization take place at metals with high work-functions. Gold, copper and tungsten are examples of metals with high work-functions. Alkali metals such as potassium, sodium and cesium have low work-functions. High work-function metals tend to be more stable and easier to study than low work-function metals, so most of our experiments have been on high work-function metal surfaces. Also, as I mentioned earlier, the particles that are directed at the surface in our experiment are helium ions, not excited helium atoms. Though it is sometimes possible for He+ ions to be neutralized into an excited atomic state before reaching the surface, that does not occur at the high work-function metal surfaces we have studied. Therefore, of the four processes that Hagstrum proposes, the one that applies to our experiments with He+ ions at high work-function metal surfaces is Auger neutralization.
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