Using Muons as a Disguise, Chemists Fool Helium Into Thinking It's Hydrogen

The quantum trick helps illustrate how atomic mass can affect chemical reactions

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Chemists have messed with the constituent parts of a helium atom and fooled it into behaving like it was hydrogen. This form of alchemy allows a physical test of how atomic mass affects chemical reaction rates.

The trickery involves a particle accelerator, a heavy subatomic particle and some knowledge of quantum mechanics.

Donald Fleming of the University of British Columbia in Vancouver and his colleagues took muons produced at Canada's TRIUMF particle accelerator, and smashed them into a cloud of helium, molecular hydrogen (two hydrogen atoms) and ammonia. Positive muons resulted in muonium, a light version of hydrogen. Negative muons resulted in a heavy version of hydrogen, which has the nucleus of helium but behaves chemically like hydrogen. The latter effect has to do with fooling the electrons.

In the heavy version, the helium atoms captured the muons, which are much heavier than electrons. The negatively charged particles orbited very close to the helium atoms' nuclei, where they effectively canceled out one of the protons. The other electron is thereby tricked into seeing a nucleus with just one positively charged particle, just like a hydrogen atom. But the nucleus is really 4.1 times heavier than normal. The light and heavy versions of hydrogen differ in size by a factor of 36, according to Fleming's study, published Friday in the journal Science.

This is useful for studying the quantum effects at work during chemical reactions, according to the study. A single hydrogen atom can form molecular hydrogen by stripping away one of the atoms from a pre-existing H2 hydrogen molecule. This is accomplished at high energies, or through quantum tunneling, whereby an atomic particle doesn't have to overcome the kinetic energy barrier that keeps it in its place.

Different atomic energies result in different chemical reaction rates, and Fleming et. al wanted to test this with the light muonium and heavy hydrogen. As they expected, the chemical reaction with the disguised helium was the slowest, as New Scientist explains. The normal hydrogen's reaction was faster, followed by the light hydrogen. The rates perfectly matched predictions from quantum mechanical calculations led by Fleming's teammate Donald Truhlar of the University of Minnesota in Minneapolis.

The proof-of-theory confirms scientists' ability to predict the energies involved in chemical reactions, and could be useful in studying quantum effects.