One way to obliterate an atom is to shoot it with the planet's most powerful X-ray gun. Linda Young tried that experiment in October 2009, when she was testing the newly opened X-ray free-electron laser at the SLAC National Accelerator Laboratory in Menlo Park, California. A single pulse from the US$420-million machine packs the same energy as all the solar radiation hitting Earth at that moment, but focused down to one square centimeter. “It will destroy anything you put in front of it,” says Young.
When the laser pulse slammed into the neon atoms in that experiment, it made them explode, stripping away each atom's 10 electrons within 100 femtoseconds (1 femtosecond is 10−15 seconds). But it was the manner of this destruction that most interested Young, who heads the X-ray science division at Argonne National Laboratory in Illinois. The X-rays first removed the atom's inner electrons, leaving the outer ones in place. For a brief moment, the neon atoms in the path of the laser became hollow.
That exotic form of neon is one of a number of strange species created by physicists intent on contorting atoms. Some teams have inflated atoms to the size of dust particles. Several research collaborations are creating anti-atoms out of antimatter. And others have loaded atomic nuclei with protons and neutrons in the quest to forge new superheavy elements. Some of the experiments aim to investigate atomic structure; others use atoms as the first steps in modeling more complicated systems. They are all descendants of the revolution in atomic theory catalyzed by Danish physicist Niels Bohr 100 years ago. But Bohr would have had difficulty imagining how far scientists could go in poking and prodding atoms into such extreme forms.
The atom that Bohr proposed in July 1913 looked like a miniature Solar System, with electrons arranged in concentric orbits around a positively charged nucleus. In Bohr's model, electrons were point-like particles that were quantized, meaning that they could jump from one orbit to another but could not exist in between. The advent of quantum mechanics in the 1920s retained the concept of orbits but re-imagined electrons as spreading everywhere around the nucleus. The location of each electron can be described only in probabilities, in the form of a mathematical wavefunction.
Electrons furthest from the nucleus can be kicked free with the least amount of added energy, so are usually the first to be stripped away. Yet X-rays, which pack a concentrated punch, can remove more tightly bound electrons from inner orbits. A medical X-ray takes out just one of those inner electrons before another from an outer shell drops down to fill the space. But the SLAC X-ray laser is in a class by itself. The beam is so intense and focused that every 100-femtosecond pulse sends 100,000 X-ray photons flying past each square ångström of space (1 ångström is 10−10 meters). That allowed Young to blast away all the inner electrons of the neon atoms in her 2009 experiment. When electrons from the outer shells dropped into the abandoned inner shells, the beam soon kicked those out as well.
“If you tune your X-rays properly, you can pick which shell you want to empty out first,” says Young. “Being able to control the inner-shell dynamics is very cool.” The current record for this kind of atom-hollowing was reported last November by a group at the Center for Free-electron Laser Science in Hamburg, Germany, which used the SLAC laser to strip away, from the inside out, the 36 inner electrons of a 54-electron-strong xenon atom.
Young hopes that research on hollow atoms will prove helpful when the laser is ready for one of its intended uses — creating images of biological molecules such as DNA and proteins by scattering X-rays off their atoms. Those pictures come at a price: the beam quickly destroys the molecules it is imaging. Knowing how hollow atoms form during this process may help researchers to interpret how the scattering pattern changes as a molecule explodes, Young says.
Two decades ago, several research groups made hollow atoms using a different process: first stripping almost all of the electrons from atoms, then depositing the resulting highly charged, slow-moving ions onto a surface. When the ions were a few tens of ångströms away from the surface, they attracted electrons from it, creating momentarily hollow atoms with electrons in outer but not inner shells. Those outer electrons then fell inwards, and the hollow atoms expelled a burst of energetic electrons and photons. “A hollow atom is nothing but a fireball of an enormous amount of energy,” says Joachim Burgdörfer, a physicist at the Vienna University of Technology, who worked on developing the theory of the process.
Several research groups pursued hollow atoms in the late 1980s and 1990s, with some scientists exploring how the burst of photons from their formation might clean surfaces by removing the topmost layers without doing deeper damage. Although that procedure has been patented, it has not captured the attention of industry, says Fritz Aumayr, a physicist at the Vienna University of Technology. The closest it has come to an application so far was in 2008, when researchers invoked the process to explain how heavy ions spewed from the Sun can damage the surfaces of planets such as Mercury. The ions become hollow atoms as they drop onto the planet, and release bursts of energy as they land.
This year, Aumayr published a paper showing that the energy expelled from ions dropping onto carbon membranes can create nanoscale pores whose size is controlled by the strength of the ion's charge (that is, how many electrons it was missing). That might be a useful route for making nanosieves for filtering small molecules, he says, or for creating nanopores to pass DNA through for sequencing.