TAKE nature鈥檚 building blocks back to the drawing board: the atom is being rebuilt. In laboratories in the US and the Netherlands, physicists are ripping away the electrons that clad atomic nuclei, and rearranging them into a series of hollow shells. Already, these hollow atoms are bringing glimpses of the quantum world at work. Eventually, they could make their mark in technology too, opening the way to a new generation of information-packed CDs and the first practical X-ray lasers.
Atoms can be thought of as miniature solar systems, with a nucleus at the centre and electrons orbiting at certain specific distances from it. Electrons, being negatively charged, usually orbit as close as possible to the positively charged nucleus. This means that it takes energy 鈥� from light, for instance 鈥� to pull them away from the nucleus and force them into outer orbits. When that happens, the light鈥檚 energy is effectively stored in the atom as potential energy, and it can be released again if the electron drops back to its original orbit. In fact, quantum theory says that electrons don鈥檛 quite have orbits as neat as those of planets around the Sun 鈥� the true picture is fuzzier, and only the probability of finding an electron can be specified. But the principle remains the same.
A hollow atom is simple enough to envisage: electrons that belong in the innermost orbits have been moved out to more distant ones. But making hollow atoms is trickier, because you can鈥檛 just scoop out the innermost electrons and dump them where you want them to go. The problem is that when an atom absorbs a burst of energy, it is the outermost electrons that are the first to go. The inner ones remain firmly in place. This is because the closer an electron is to the nucleus, the more strongly bound it is and the harder it is to remove.
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First strip your atom
So physicists have adopted a different approach 鈥� starting with a bare nucleus. They make this with a machine known as the electron beam ion trap, or EBIT, developed in 1987 by Mort Levine and Ross Marrs at Lawrence Livermore National Laboratory in California. The EBIT, small enough to fit on a benchtop, traps atoms inside an intense beam of high-energy electrons which knock the atom鈥檚 electrons out and create highly charged ions. Last year, Marrs and his colleagues reported that they had managed to strip all 92 electrons from an atom of uranium 鈥� no easy task when you consider that the more electrons an atom loses, the more tightly it grips its remaining ones.
It was while studying the interaction of such nuclei with surfaces that hollow atoms were first discovered by Jean-Pierre Briand at the Centre for Nuclear Studies in Grenoble. Bare nuclei have such high positive charges that they mop up negatively charged electrons from wherever they can find them. 鈥淥ne of the fundamental characteristics of a bare nucleus is its thirst for electrons,鈥� says Fred Meyer of Oak Ridge National Laboratory in Tennessee.
A metal surface provides the ideal opportunity for the bare nucleus to quench its thirst, because it contains large numbers of loosely bound electrons. In fact, the highly charged ion captures electrons so efficiently that it acquires its full quota 鈥� and so becomes electrically neutral 鈥� long before it hits the surface. It turns out that the binding energy of electrons in the outer levels of the atom is around the same as for the electrons in the metal surface. 鈥淭he really interesting thing here is that the electron capture takes place most easily into outer shells of the atom,鈥� says Meyer. The result is a neutral atom in which most or all of the electrons are at large distances from the nucleus 鈥� a hollow atom.
Such an arrangement is highly unstable. As soon as a hollow atom has formed, it immediately begins to decay to its normal state as the electrons make their way back to the innermost orbits, losing energy in the process. Rather than jumping down directly, the outer electrons must pass through each successive orbit in turn, like descending the rungs of a ladder, losing their potential energy as they go. But this process takes time, and the atom hits the target surface before it has been completed. This means that the hollow atom is still carrying potential energy when it hits, and most of this energy is transferred into the surface of the target.
This finding has encouraged scientists to explore whether this potential energy can be put to practical use. When Dieter Schneider and his colleagues at Lawrence Livermore bombarded mica surfaces with hollow atoms of xenon and uranium, they found that tiny blisters just a few nanometres in diameter were created on the surface. They say that these blisters could eventually be used for storing digital data. Because they are less than one-hundredth of the diameter of the pits used to represent one bit of information on today鈥檚 CDs, this would bring a tremendous breakthrough in information storage technology, increasing the density at which data could by stored by a factor of over 10 000. The complete works of Shakespeare could then reside on a square less than 0.2 millimetres across.
Meanwhile, scientists are obtaining insights into the fundamental physics of atoms by studying how hollow atoms decay. An atom鈥檚 outer electron orbits lie much closer to each other than inner ones, so outer electrons in different orbits often interact with one another, sometimes with strange results. In 1992, Luuk Folkerts and Reinhard Morgenstern of the Nuclear Physics Accelerator Institute in Groningen, the Netherlands, discovered one such effect, known as a three-electron Auger process. It involves two of the outer electrons simultaneously making a downward transition from an outer to an inner orbit. The energy released as a result is acquired by a third electron, and is enough to allow this electron to break free from the atom. By studying this type of collective process in hollow atoms, physicists are able to test some of the predictions of quantum theory concerning the ways in which electrons can 鈥渃ommunicate鈥� with each other.
Last year, another route to producing hollow atoms was found. Charles Rhodes and his research group at the University of Illinois in Chicago created hollow atoms by zapping clusters of around 100 atoms with extremely intense light from the latest ultraviolet lasers. The laser pulses have a power of up to 800 gigawatts 鈥� more than ten times the peak power demand for the whole of Britain 鈥� focused into a beam only around 3 micrometres across. These extreme conditions last for only 300 femtoseconds (0.3 millionmillionths of a second), but they turn out to be just what is needed to create hollow atoms. But what mechanism could be at work?
The answer seems to lie in the way the incident wave of laser radiation interacts with the outer electrons in the cluster. Although the light has almost no effect on the inner electrons, which are tightly bound, the outer electrons are loosely bound and free to oscillate under the force of the waves alternating electric field. At sufficiently high intensity, the laser radiation can accelerate these outer electrons to extremely high energies. Rhodes calculates that the laser pulse can accelerate an outer electron to an energy of 10 kiloelectronvolts 鈥� thousands of times greater than the energy needed to release it from the grasp of the nucleus 鈥� within a distance of only 20 nanometres. The motion of all the outer electrons within the cluster seems to be identical, so the electrons collectively swing from one side of the cluster to the other, with one transit for each half-cycle of the incident wave. When a number of outer electrons act coherently in this way, Rhodes says that their collective energy might focus by some unknown mechanism onto the tightly bound inner electrons, ejecting them and so producing hollow atoms.
The hollow atoms created this way promptly turn back into normal atoms by emitting X-rays. This raises hopes that they could lead to the development of the first practical X-ray laser. A laser works by exciting electrons from a low energy level close to the nucleus to another one farther away, using energy from high-intensity light, for example. When a large number of atoms have been excited in this way, they are then induced to return to their original states and emit light in the process. Because the excited electron in every atom undergoes the same downward transition, the light emitted is of a single wavelength or colour. It is also 鈥渃oherent鈥�, with the crests and troughs of all the waves in step with each other.
Infrared, visible and ultraviolet lasers are now widely available but, so far, the development of X-ray lasers has been fraught with difficulty. X-rays are much more energetic than visible light, so for atoms to emit them their electrons have to be given enough energy to shift them out of their inner, tightly bound energy levels. The key point of Phodes鈥檚 work is that the collective electron motion induced by the intense laser radiation focuses enough energy onto the inner shell electrons within a sufficiently short time to produce X-rays.
The close proximity of a large number of hollow atoms is just what is needed for the production of coherent X-ray radiation.
Like existing lasers, which are used for everything from reading bar codes at super-market checkouts to correcting short-sightedness, X-ray lasers that give out intense bursts of coherent light would almost certainly find a wide range of applications. One exciting possibility is that they could be used to study microscopic biological structures such as proteins and viruses, revealing more detail than ever before. One problem with imaging using conventional, less intense X-ray sources is that the radiation often damages biological material, breaking numerous chemical bonds and changing the structure before exposures are completed. But with the very intense subnanosecond bursts of radiation from X-ray lasers, the exposure could be completed before any changes have had time to take place. By combining X-ray images from several lasers, it may even be possible to produce three-dimensional pictures 鈥� X-ray holograms 鈥� of the structures within organisms.
X-ray lasers could also be a key step in the evolution of CDs. Today鈥檚 CD players use infrared lasers to read data. X-ray lasers, with their much shorter wavelengths, would be able to read the much more densely packed information on CDs of the future 鈥� which may one day be recorded using hollow atoms. Whatever the outcome, these curious, re-engineered atoms are sure to have more surprises in store.