Dhruv Radhakrishnan brings us up to speed on the way scientists “capture” the motion of electrons and how this technology could improve how we fuel our world.
Everyone wants a better camera. One that never blurs. We can watch a hummingbird beating its wings in crystal-clear slo-mo at 120 fps on an iPhone (please sponsor me Apple), but can we catch the most minute quantum-scale electron motion with that same clarity?
Most students have heard of the uncertainty principle. The whole idea that it's impossible to be able to tell exactly where an electron is in space. But what if I were to tell you that we now have a camera that we can use to ‘see’ electrons move with remarkable accuracy? Let's begin by discussing how a regular camera works.
How cameras work
The principle behind everyday cameras is that images are formed by light reflected off an object being focused by a lens. These rays are focused onto an image sensor composed of a lattice of light-sensitive pixels. The light intensities at each pixel are then converted into digital signals to produce pictures.
However, if the lattice is exposed to light for too long, all pixels will register a high brightness, reducing the contrast and clarity within the picture. This is why the camera needs a device to regulate the exposure time – this is the shutter, which works by opening and closing to allow light in for a certain period. For a bright scene, a small exposure time is needed, as the required intensity contrast will be reached quickly, and vice versa for a dark scene. But, if the scene changes during this exposure time, the image becomes blurred – which becomes an issue when investigating processes on the subatomic scale.
Why an electron camera has to work differently, and what it actually measures
At the super-small level, if we want to determine the motions of particles like electrons, we cannot simply take photos, as the events happen at attosecond scale, meaning that it's impossible to open and close a shutter in time for the image not to be blurred. For context, an attosecond is 10-18 (0.000000000000000001) seconds. There are more attoseconds in one second than seconds that have passed since the start of the universe 13.6 billion years ago.
So, a different angle of attack on the problem must be taken.
The process
An intense laser pulse is directed at an inert gas, which in turn makes the gas emit pulses of higher-order harmonics (higher frequency pulses) of the initial pulse. These emitted pulses are very short: they have a lifetime of about 100 attoseconds. A pulse will hit a sample (typically a gas), and then ionise it, releasing an electron, which initiates the measurement process. Now to take the photograph. After a determined delay, an infrared laser pulse is released, creating an IR field. An electron interacts with the field, and its momentum will change depending on the delay of the IR pulse. An electron's momentum can be detected, and from this, the instant of ionisation can be deduced, allowing us to track the motion of the electron. By this method, we essentially have a camera which records the instants of ionisation of electrons.
Why it's useful
“After all this faff and millions of pounds of investment, what is the point of having an electron camera?”, I hear you say. Well, I'm glad you asked, as there's plenty of reasons why this is, in fact, useful.
Atomic modelling & quantum tunnelling
When we observe electrons after being ionised, we can piece together details like how long it takes electrons to absorb the energy or how the specific orbital an electron is in can change its ionisation time. By collecting these data for different atoms and complexes, a more complete picture of the atomic model can be built, furthering our understanding of small-scale physics.
Let's say you go outside and kick a football against a wall. You'd expect the ball to just bounce back at you right? I wouldn't know because I've never done it before. But, imagine if the ball just appeared on the other side of the wall, yet there was no visible hole you could see. Your jaw would drop, you'd think there was some black magic going on right in front of you. To a quantum physicist though, it would just be a macro-scale quantum tunnelling event occurring. Quantum tunnelling is when particles behave like waves, allowing them to “tunnel” through firm boundaries without having “enough energy”. Previously, tunnelling was shrouded in mystery, with many physicists believing it to be instant, with no clue as to how it occurs. Now, with our attosecond cameras, we can track the exact process, essentially catching the electrons “in the act”, demystifying these curious events.
More practical applications: chemical reactions and solar energy
Electrons are the driving force behind chemical reactions. In molecules, because of the overlap of electron orbitals, charge is often not centralised on specific atoms – rather, it is spread out as a distribution across all atoms. So, by using our attosecond electron camera, the charge distributions within a molecule can be studied, allowing us to work out which chemical reactions will happen in given conditions. From here, reaction pathways can be tinkered with to boost yields and cut down on waste in a plethora of fields, whether it be pharmaceuticals, plastics or petrochemistry.
These cameras even have applications in everyday objects like solar panels. Solar panels work through the photoelectric effect (when light hits them, they release electrons, causing a current), which happens at the attosecond scale (it was previously thought to be instantaneous). Now, on observing interactions on this scale, new materials and structures can be experimented on to maximise the efficiency and speed of the electron flow, which will lead to more potent and profitable energy production.
Attosecond physics has the potential (and is currently fulfilling it) to be a very successful new foray into undiscovered territory. As well as its more obvious purpose in attacking fundamental questions like quantum electron motion and better modelling the nature of the atom, further research will also lead to other, more tangible benefits, like improving efficiency on industrial scales such as energy production and the vast chemical sector. It would certainly be wise to watch this space.