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Imaging The Quantum Motion of Electrons Using Light

Physicists say they’ve worked out how to video the quantum motion of an electron as it moves around a hydrogen atom, a step that has important implications for our understanding of biomolecules

  • July 25, 2012

For decades, physicists have studied the way an electron ought to bind to a proton, the simplest atomic system. The fascinating patterns of hydrogen orbitals that form at different energy levels are static objects, calculated by detailed computer modelling. They are snapshots of hydrogen atoms frozen in time.

But the most advanced computer models can also calculate what hydrogen atoms look like as they switch from one state to another, how the orbitals change shape, how they combine and superpose. The results are videos of hydrogen orbitals in motion–quantum movement.

But all that is just theory. Nobody knows what hydrogen atoms look like in practice because it’s impossible to photograph electrons with light, let alone make movies of them in action. Right? 

Not quite. In recent years, physicists have learnt how to generate pulses of light that are small enough and short enough to tease apart the structure of a hydrogen atom. These pulses consist of x-rays in packets just a few wavelengths long. 

In the next few years, this technique ought to be capable of making movies with a resolution of about an angstrom and a frame rate of an image per femtosecond.  That’s more than good enough to show the movement of hydrogen orbitals.

So what will these moves look like? Physicists have standard techniques for calculating the way x-rays scatter off atoms. The idea here is that they prepare an ensemble of hydrogen atoms in a specific state, or combination of states, using a conventional laser pulse. 

A short time later, they then zap the atoms with packets of x-rays and measure how they are scattered. This gives a snapshot of the hydrogen atom at that instant.

To build up a movie, they take another image but this time leave a little extra time between the preparation pulse and the imaging pulse. And so on. This produces a movie of the quantum motion of electrons in orbit around a proton.

The problem, of course, is that a packet of x-rays inevitably changes the electron orbitals, distorting the shape of the hydrogen atom as it is being imaged. It’s this distortion that makes quantum imaging so troublesome.

In fact, it’s so complex that physicists have simply ignored it; or at least persuaded themselves that it is negligible. The only calculations they’ve ever done to model the quantum motion of electrons assume that the x-rays do not change the behaviour of electrons in any way. 

Today that changes thanks to the work of Gopal Dixit at the Center for Free-Electron Laser Science at DESY in Hamburg and a couple of pals. 

These guys have worked out how x-rays ought to influence the shape of a hydrogen atom and calculated what a video of the resulting quantum motion of electrons would look like.

The figure above shows the results as a series of frames. The middle row shows the way electron orbitals change when in a superposition of 3d and 4f orbitals. The bottom row shows the prediction according to the existing approach–and rather uninteresting it is too.

The top row, on the other hand, shows what the images would look like assuming that x-rays distort the orbitals. They clearly show the kind of asymmetry that x-rays imaging would introduce, something that the existing approach simply does not allow for.

That’s important because these kinds of videos ought to be possible to make in the coming months and years. Knowing how to interpret them will be crucial. 

And hydrogen atoms will be just the start. It won’t be long before we have videos of the quantum motion of electrons in more complex molecules, perhaps even in biomolecules themselves. When that happens, we’ll be watching the quantum motion of life itself.

Ref: arxiv.org/abs/1207.4565: Imaging Electronic Quantum Motion With Light

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