Diagram of a large, roughly square molecule.
Enlarge / Phthalocyanine, the big molecule used in these experiments.

Wave-particle duality is a fundamental fact of the Universe. But we don’t see many objects moving around as waves. This is why it hurts when a golf ball hits you on the head: you and the golf ball are both behaving like particles.

In principle, that wave-like nature is there to be observed. Researchers have now demonstrated that with a couple of relatively heavy, complicated molecules.

Wave to me

The wave-particle debate started out in the time of Isaac Newton. Water waves were just beginning to be understood, and a series of experiments revealed that light had more to it than meets the eye. So is light a stream of particles or a wave? The debate raged on until Thomas Young presented the results of his classic double slit experiment in 1803, showing that light is a wave.

That neat ending was thrown into chaos when it was discovered that light could cause a solid to emit electrons from its surface. This was explained by Albert Einstein in 1905 by describing light as a particle. From there, it was a short leap of the imagination to realize that light is both a particle and a wave. Wave-particle duality was born to the anguished screams of physicists everywhere.

If light is both a particle and a wave, why not everything else? Louis De Broglie proposed this idea, and electron wave-particle duality was confirmed in 1927 by George Thomson. Wave-particle duality was here to stay, but observing it in heavy particles is very challenging because the particles’ wavelengths are very short.

Wave to you

In the new work, researchers were aiming to observe diffraction—a property of waves—using heavy molecules. Imagine a screen with a sharp edge. When light shines on the screen, there are three areas behind it: the area where light hits the screen and is blocked (this causes a shadow); the area where light misses the screen and passes by; and a third area created by light that hits the sharp edge. The edge will re-radiate the light in all directions, including into both the shadow and bright areas.

This re-radiated light has taken a different path compared to the light that didn’t encounter the edge. The result is that some areas that look like they should be light are actually dark, while dark areas may appear brighter. The pattern of light and dark is called a diffraction pattern. Diffraction is what allows imaging systems (including our eyes) to work.

For a diffraction pattern to be measured, the waves have to be preserved properly. Think of it like this: imagine that you are sitting at the sharp edge and are able to measure the amplitude of the light wave as it comes in and again as it is re-radiated. Using the laws of wave motion, you can then predict the amplitude later at any other position in space, but this is only true if nothing disturbs the wave. In this case, you will observe a diffraction pattern. However, the more disturbed a wave is, the weaker the diffraction pattern. Weak enough, and it finally vanishes.

Heavy particles, like molecules, consist of a collection of smaller, charged particles, which are easily bashed about. This means that the molecule’s wave nature is randomly changed with every collision, making the amplitude unpredictable and washing out the diffraction pattern.

That makes measuring a diffraction pattern from two molecules with masses that are 331 and 515 times heavier than hydrogen quite impressive.

Molecules gently wave

To obtain a diffraction pattern from molecules, the researchers needed to create a beam in which all of the molecules have similar velocities. To do this, the researchers used a laser to blast molecules off of a glass window. The cloud of molecules will race off into the vacuum with a huge range of velocities. A vertical slit was placed downrange to only allow molecules traveling in the right direction to hit the target. The target is not an edge as described above, it’s the light from a laser beam that creates a standing wave pattern. The high intensity of the peaks in the standing wave pattern deflect the molecules exactly as if they had passed through a gap, creating a diffraction pattern. The molecules then continue downrange.

At this point, the molecules all have different energies (speed), so the diffraction pattern will wash out. But the molecules are also falling due to gravity. A second slit, oriented horizontally, selects molecules that have fallen after traveling a certain distance. This also selects a velocity because fast molecules will overshoot the slit, while slow molecules will fall short.

After the second slit, the molecules run into a screen, where they stick. The molecules that the researchers chose glow when subjected to blue or UV light, so the positions of the molecules on the screen could be observed after letting the experiment run for a while.

The researchers showed that they could observe different diffraction patterns for different molecular velocities and for different angles of incident on the standing wave pattern. The diffraction pattern is rather weak, but that’s to be expected in a first demonstration. However, even these noisy results agreed reasonably well with theory.

What I found most remarkable is that the internal state of the molecule didn’t seem to matter at all. These are molecules that have just been violently kicked off a glass plate. They are spinning wildly and vibrating like a kid on a sugar high. Yet they also calmly glide through the experiment, allowing their wave nature to spread out and interfere with each other. I would not have imagined that this would be the case.

Physical Review Letters, 2020, DOI: 10.1103/PhysRevLett.125.033604 (About DOIs)