When hydrogen molecules and charged atoms of deuterium collide, they can exchange a proton through a quantum process called tunnelling, but this only happens once in 10 billion collisions.
Quantum tunnelling is an odd effect that allows particles like electrons to travel through barriers in nano-sized devices or protons to move between molecules. The latter is important for chemistry, where a tunnelling proton can change the makeup of molecules even when they don鈥檛 have enough energy for a conventional reaction that would do the same. at the University of Innsbruck in Austria and his colleagues wanted to measure how often quantum tunnelling between a molecule and a charged atom, or ion, actually happens in the lab.
They chose hydrogen molecules and deuterium ions because hydrogen is the simplest element and deuterium is like hydrogen with one extra neutron. They put deuterium ions into a metal cylinder devoid of air and surrounded by 22 rods. Each rod emitted microwave radiation specially tuned to exert electromagnetic forces on deuterium ions, trapping them in a stable and predictable state. The researchers then added hydrogen molecules into the cylinder to give deuterium something to interact with.
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To prevent reactions other than quantum tunnelling from happening, they used liquid helium to cool the whole setup to -258掳C, where deuterium and hydrogen have very little energy. After waiting for 15 minutes, Wester and his colleagues then ejected everything out of the cylinder and measured the masses of the ions and molecules to see whether they were lighter or heavier than before 鈥� indicating they had lost or gained a proton through quantum tunnelling.
Wester says his team has been working on this experiment for over 15 years and this was the first time they not only found evidence of tunnelling but could also use their measurements to calculate at what rate it occurred. They found that only one in 10 billion collisions between a hydrogen molecule and a deuterium ion results in a proton tunnelling from one to the other, and this number was a good match for past theoretical predictions.
How often tunnelling happens can only be predicted and understood from quantum theories of chemistry, so measuring it is an important check of those theories, says at the University of Cologne in Germany. Because it happens so rarely, it has also been exceedingly difficult to study in an experiment, he says.
鈥淭o my knowledge, there are only two other experiments with ions where some sign of tunnelling was observed,鈥� says Wester. His team鈥檚 success came from using more hydrogen and waiting for a longer time than in past experiments. The team is now planning similar experiments with other kinds of hydrogen, some of which may relate to processes that hydrogen participates in inside of interstellar clouds.
Nature
