Post by kieransan on Jun 5, 2013 6:57:56 GMT
AN ATOM'S electrons are an ever-shifting quantum melee, but it turns out you can still take their photograph as if they were standing still. A quantum-style microscope has imaged the hydrogen atom's wave function, the equation that determines its electrons' positions – and in turn the atom's properties.
The electrons that dance around an atomic nucleus help determine how the atom bonds with others, but they are notoriously difficult to pin down. Thanks to quantum theory, which says that tiny particles are in multiple places simultaneously, you can never say where a given electron actually is. The best you can do is say how likely it is to be in a given spot.
Not all positions are equally available: electrons can only reside at certain distances from the nucleus, with these distances related to how much energy the electron holds. In principle, the wave function, denoted by Greek letter psi, can be used to reveal these energy levels for any given atom or molecule, although in practice this has only been done for the very simplest – the hydrogen atom and molecule (made of two hydrogen atoms bonded together).
But how on earth do you make an image of such an object? Measuring the position of a single electron "collapses" the wave function, forcing it to pick a particular position, but that alone is not representative of its normal, quantum presence in the atom. "Wave functions are difficult to measure. They're exquisite quantum objects that change their appearance upon observation," says Aneta Stodolna of the FOM Institute AMOLF in Amsterdam, the Netherlands.
Her team decided to make a picture using a technique dreamed up 30 years ago that can be thought of as a quantum microscope. Rather than taking an image of a single atom, they sampled a bunch of atoms. This removes the quantum nature of each individual atom's electron, forcing it to choose a particular location from those it is allowed to reside in. Do it with enough atoms and the number choosing each spot will reflect the quantum probabilities laid out by the wave function.
Stodolna's team made a beam of atomic hydrogen and zapped it with two separate lasers that excited the atoms' electrons by precise amounts. An applied electric field then pushed the excited electrons away from their respective nuclei, towards a detector about half a metre away.
The electrons emitted waves that produced an interference pattern on the detector (see "An atom undressed"). Crucially, the pattern was a projection of the spacings of the energy levels in the hydrogen atom, as laid out in the wave function, with bright rings where electrons were present and dark lanes where they were not (Physical Review Letters, doi.org/mmz). "You can think about our experiment as a tool that allows you to look inside the atom and see what's going on," Stodolna says.
The result is reminiscent of an intriguing controversy: whether the wave function in the atom is a real physical waveMovie Camera or a very useful piece of mathematics. "When you see pictures like that you think, how can anyone not think psi is real?" says Terry Rudolph of Imperial College London, who is currently working to prove that it is. But he adds that further tests are needed to answer the question once and for all.
"This addresses a fundamental system in quantum physics," says Christopher Smeenk of the University of Ottawa in Canada. The technique may be used to study larger atoms, he says. "Because of the simplicity of the hydrogen atom, it acts as a benchmark for multi-electron atoms or molecules."