by Philip Ball
Scientists have created ultra-light and ultra-heavy forms of the element hydrogen, and have investigated their chemical properties.
Donald Fleming, a chemist at the University of British Columbia in Vancouver, Canada, and his colleagues generated two artificial analogues of hydrogen: one with a mass a little over one-tenth that of ordinary hydrogen, and one four times heavier than hydrogen. These pseudo-hydrogens both contain short-lived subatomic particles called muons -- super-heavy versions of the electron.
The researchers tested the behaviour of these new atoms in a chemical reaction called a hydrogen exchange, in which a lone hydrogen atom plucks another from a two-atom hydrogen molecule -- just about the simplest chemical reaction conceivable. In a paper in Science, they report that both the weedy and the bloated hydrogen atoms behave just as quantum theory predicts they should -- which is itself surprising.
The experiment is a "tour de force", says Paul Percival, a muonium chemist at Simon Fraser University in Burnaby, Canada.
"I would never attempt such a difficult task myself," he says, "and when I first saw the proposal I was very doubtful that anything of value could be gained from the Herculean effort. Don Fleming proved me wrong. I doubt if anyone else could have achieved these results."
A normal hydrogen atom contains a single negatively charged electron orbiting a nucleus made of a single positively charged proton. About 0.015% of natural hydrogen consists of the heavy isotope deuterium, in which the nucleus contains a proton and an electrically neutral neutron, and which has a mass twice that of normal hydrogen. And there is a third isotope with a proton and two neutrons: tritium, three times as massive as hydrogen, which is produced in trace quantities by cosmic rays interacting with the atmosphere, but is too dangerously radioactive for use in such experiments.
The chemical behaviour of atoms depends on the number of electrons they have rather than their masses, so the three hydrogen isotopes are chemically almost identical. But the greater mass of the heavy isotopes means that they vibrate at different frequencies, and quantum theory suggests that this will produce a small difference in the rates of their chemical reactions.
To rigorously test that theory, isotopes of hydrogen are needed with greater differences between their masses. Fleming and his colleagues created some, using muons produced by collisions in the TRIUMF particle accelerator in Vancouver.
Muons have many properties similar to electrons, but are more massive. "A muon is an overgrown electron -- an electron on steroids -- with a mass about 200 times that of an electron," says Richard Zare, a physical chemist at Stanford University in California. "But unlike the free electron, the free muon falls apart, with a mean lifetime of about 2.2 microseconds." This meant that the researchers had to work fast to study their pseudo-hydrogen.
To make the ultra-light isotope, they swapped the proton with a positively charged muon, which has just 11% of the mass of a proton. And to make ultra-heavy hydrogen, they replaced one of the electrons in a helium atom with a negative muon.
Helium has two electrons, two protons and two neutrons. But because it is more massive than an electron, the negative muon orbits the nucleus much more closely, masking the positive charge of one of the protons. In effect, the atom becomes a hydrogen-like composite: a 'nucleus' made of the existing two-proton, two-neutron nucleus and the muon, orbited by the remaining electron. It has a mass of a little over four times that of hydrogen.
Fleming and colleagues found that the reaction rates for hydrogen exchange involving these analogues that were calculated from quantum theory were close to those measured experimentally. "This gives confidence in similar theoretical methods applied to more complex systems," says Fleming.
The close match between experiment and theory wasn't necessarily to be expected, because quantum calculations use a simplification called the Born-Oppenheimer approximation, which assumes that the electrons adapt their trajectories instantly to any movement of the nuclei. This is generally true for electrons, which are nearly 2,000 times lighter than protons. But it wasn't obvious that it would hold for muons, which have a tenth of the proton's mass.
"It surprises me at first blush that the theoretical treatments hold up so well," says Zare. "The Born-Oppenheimer approximation is based on the small ratio of the mass of the electron to the mass of the nucleus. Yet suddenly the mass of the electron is increased two-hundred-fold and all seems to be well."
Because the muon has such a short lifetime, extending such studies to more chemically complex systems is very challenging. But Fleming and his colleagues propose now to look at the 'hydrogen' exchange reaction between the super-heavy 'hydrogen' and methane (CH4).