Chemistry requires effort. Whether it’s by raising the temperature, increasing the chances that compatible atoms will collide in a heated smash-up, or increasing the pressure and pushing them together, building molecules usually requires some energy cost.
Quantum theory provides a solution if you are patient. And a team of researchers from the University of Innsbruck in Austria has finally seen quantum tunneling in action in a world-first experiment that measures the merging of deuterium ions with hydrogen molecules.
Tunneling is a feature of the quantum universe that makes it appear that particles can pass through obstacles that are normally too difficult to overcome.
In chemistry, this barrier is the energy required for atoms to bond with each other, or with existing molecules.
Nevertheless, the theory states that in extremely rare cases it is possible for atoms in close proximity to “tunnel” through this energy barrier and connect without any effort.
“Quantum mechanics allows particles to break through the energetic barrier due to their quantum mechanical wave properties, and a reaction occurs,” says first author Robert Wild, an experimental physicist from the University of Innsbruck.
Quantum waves are the ghosts that drive the behavior of objects like electrons, photons and even whole groups of atoms, obscuring their existence before any observation, so that they don’t sit in a precise place but occupy a continuum of possible positions.
This blurring is negligible for larger objects such as molecules, cats and galaxies. But when we zoom in on individual subatomic particles, the range of possibilities expands, forcing the site states of different quantum waves to overlap.
When that happens, particles have a small chance of appearing where they have no business, tunneling into regions that would otherwise require a lot of force to enter.
One of these areas for an electron can be within the bonding zone of a chemical reaction, welding together neighboring atoms and molecules without the boom-crash crushing of heat or pressure.
Understanding the role quantum tunneling plays in the construction and rearrangement of molecules can have important consequences in the calculations of energy release in nuclear reactions, such as those involving hydrogen in stars and fusion reactors here on Earth.
While we have modeled this phenomenon for examples involving reactions between a negatively charged form of deuterium – an isotope of hydrogen containing a neutron – and dihydrogen or H2proving the numbers experimentally requires a challenging level of precision.
To achieve this, Wild and his colleagues cooled negative deuterium ions to a temperature that brought them close to standstill before introducing a gas made of hydrogen molecules.
Without heat, the deuterium ion was far less likely to have the energy required to force hydrogen molecules into a rearrangement of atoms. Yet it also forced the particles to sit still close together, giving them more time to bond through tunneling.
“In our experiment, we allow possible reactions in the trap for about 15 minutes and then determine the amount of hydrogen ions that are formed. From their number, we can deduce how often a reaction has occurred,” explains Wild.
That number is just over 5 x 10– 20 reactions per second occurring in every cubic centimeter, or about one tunneling event for about every hundred billion collisions. So not much. Although the experiment backs up previous modeling, it confirms a benchmark that can be used in predictions elsewhere.
Given that tunneling plays a fairly important role in a variety of nuclear and chemical reactions, many of which are also likely to occur in the cold depths of space, getting a precise grasp of the factors at play gives us a firmer footing to base our predictions on.
This research was published in Nature.