SPACE is cold, really cold. That should mean very little happens, and yet there are stars, planets and people. Now an ultracold experiment could help explain how chemical reactions get started in the freezing near-vacuum of interstellar space.
It seems that, far from staying aloof, some atoms actually find it easier to get cosy with each other at temperatures close to absolute zero, which is -273.15 °C, or 0 kelvin. The reason could be to do with that well-known mischief-maker, quantum physics, which causes particles to behave like waves and to exist in myriad states at once.
“Some atoms actually find it easier to get cosy with each other at temperatures close to absolute zero”
Advertisement
Textbooks show atomic nuclei surrounded by electrons in different energy levels, or shells. Atoms form bonds by sharing or exchanging electrons, usually so that pairs of electrons fill the outermost shell. So closed-shell atoms, whose outer shells are full, are expected to be fairly complacent and resistant to change. In contrast, so-called radicals have a single unpaired electron and are anxious to give it away or steal another electron to keep it company.
However, this simple picture had never been tested at a fraction of a degree above absolute zero. Now Wade Rellergert at the University of California, Los Angeles, and his colleagues have found that calcium, which has a pair of electrons in its outer shell, is more reactive at low temperatures than the radical rubidium. “The reaction rate can go flying up when things are very cold,” says team member Scott Sullivan, also at UCLA.
The experiment is part of an emerging field in which physicists try to cool particles until they hold the smallest amount of energy allowed by quantum mechanics. Such supercooled particles might be useful for a host of applications (see “Ultracold and ultra-useful”).
The team chose calcium because they expected it would not interact much with the real target of their study, ytterbium ions, a soft, silvery metallic element with one electron in their outermost shell. At room temperature, the two elements are expected to either leave each other alone, or the ytterbium ion steals an electron from the calcium atom (see diagram). They hoped the ytterbium ions would simply bump into the surrounding calcium atoms, which had been cooled to 0.004 K, losing energy and cooling down.
When they put the two together in a vacuum chamber, “we saw the ytterbium start to disappear”, says Sullivan. That suggested the calcium was pairing up with the ytterbium atoms, the team reports in a .
“We went with this thing that shouldn’t react, and in fact it reacted a whole hell of a lot,” says team member of UCLA. “It turned out that this closed-shell atom reacted 10,000 times faster than the open-shell counterpart,” he says, referring to a using ytterbium ions and rubidium, a radical metal with one free outer electron.
To figure out why the calcium was so reactive, Hudson turned to theoretical physicists and Alexander Petrov at Temple University in Philadelphia, Pennsylvania. The duo specialise in predicting how atoms will interact by directly solving the Schrödinger equation, which describes how a quantum system changes with time.
The calculations revealed a quantum-physics explanation for why calcium might be extra-reactive at cold temperatures. Atoms move more quickly at higher temperatures. At room temperature, calcium atoms are moving around so fast that an electron can only jump onto a ytterbium ion very occasionally when they pass close by.
But at ultracold temperatures, the particles are more sluggish, allowing the electron to, in effect, have one foot on its own calcium atom and another on the ytterbium ion. As quantum objects, the particles behave like waves, and they remain close to each other long enough for their wavelengths to overlap. This is essentially equivalent to being a single molecule in an excited, or less stable, energy state. If the quasi-joined particles then emit a photon, they can relax into a lower energy state, becoming a bona fide molecule in a process known as radiative association. “These radiative processes are much more likely at low temperature,” Hudson says. “It’s because of them that we see this enormous reaction rate.”
“At very low temperatures, atoms are more sluggish, remaining close to each other for longer”
That’s exciting to astrochemists like Jean Turner, also of UCLA. “Radiative association is a key starting point of interstellar chemistry,” she says.
As well as having an average temperature just 3 degrees above absolute zero, space is also nearly empty – a cubic centimetre might hold a total of a million particles, which would be considered a good vacuum on Earth. Under those extreme conditions, atoms meet slowly and infrequently.
Yet space contains countless complex molecules, from simple sugars to linked rings of carbon atoms known as polycyclic aromatic hydrocarbons. “We see the chemistry out there, so we know it has to happen,” Turner says. But how would it have got started at all?
Finding that closed-shell atoms can react more quickly than expected might give a clue. Calcium and ytterbium are rare in interstellar space, but similar processes might govern carbon – the backbone of life as we know it – which has two pairs of electrons in its outer shell.
Physicist Christoph Zipkes of the University of Cambridge says it is too soon to make such inferences. But astrochemist Marc Morris of UCLA says the result is a key step forward. “The interstellar medium is chock-full of interesting organic molecules,” Morris says. “We don’t know very well how they form in any detail. If their formation can be assisted by charge exchange reactions, that would be extremely interesting.”
Ultracold and ultra-useful
The practice of supercooling atoms has ushered in atomic clocks and even a new form of matter in which many atoms behave as one.
Molecules can rotate and jitter in more directions than atoms, making them harder to cool, but the extra work could pay off. “There are lots of things you can do with molecules that you can’t do with atoms,” says of Northwestern University in Evanston, Illinois.
Molecules could help reveal if the fundamental physical constants are actually changing. For example, the mass ratio between electrons and protons, called mu, has been accused of varying, a possibility that would confound physicists. Molecular vibrations and rotations depend on this ratio in a straightforward way, making them the ideal lab for the tests, says Odom.
Molecular ions with just the right properties and number of quantum states could be used to run calculations in super-efficient quantum computers, which take advantage of the fact that such “qubits” exist in multiple states at once. Physicists want to find molecules with “enough states to encode all the information you would like, but not so many as to make it difficult to control the molecule”, says Andrew Grier at the Kastler Brossel Laboratory in Paris, France.
One day, chilling molecules might even allow their reactions to be controlled to the point where they could form compounds not found in nature. “Perhaps these techniques could be used to make new drugs or materials,” says Eric Hudson of the University of California, Los Angeles.
