Atoms in the Spotlight

We can now settle that your high school chemistry teacher did not lie to you when talking about atomic structure and (perhaps if you were lucky enough to get that far) molecular orbitals. Recently, scientists have been able to observe the actual electron orbitals of a hydrogen atom  and see the rearrangement of atoms in chemical reaction.

Being able to observe the different orbitals represents a new sort of “resolution” record in physics. Atomic force microscopy (AFM) can show you individual atoms and even the atomic bonds between atoms, but that is probably hitting the absolute limit of what we can do with AFM techniques. Why? Because AFM is basically like poking something with an atom and you can’t resolve features that are much smaller than whatever you’re poking with. And since the electron cloud basically makes up the whole force that prevents atoms from overlapping, AFM could never look at the orbitals in a detailed way.

So how did this impressively multinational team (seriously, there are universities from five different countries) do it? Think of an atomic scale projector, or maybe like an old cathode ray TV.  In the original paper, it’s called a “photoionization microscope”. Ionization is giving an electron so much energy it leaves the atom it is attached to, photoionization is doing this process with light. The figure below shows a schematic of the experiment.

The experimental set-up of Stodolna, et al. Source: Physical Review Letters

The experimental set-up of Stodolna, et al, to view hydrogen orbitals. Source: Physical Review Letters

The H-S on the left is a beam of hydrogen sulfide molecules, which they split to get a source of hydrogen atoms. The plate labeled (a) is an aperture to narrow the beam of hydrogen. That trippy wave at the inset between (b) and (c) is a set of two lasers, one that excites the electrons into certain orbitals the team wanted to image and another one that ionizes the atoms. (b) and (c) are a set of electrodes that generate an electric field to direct the ejected electrons to the detector at (d). (e) is a special electronic components that acts as an ion lens to focus the beam. In this case, it is being used to broaden the path electrons can take so it is easier to see the different orbitals. And they definitely see them.

The orbitals of hydrogen. The number of nodes (regions of virtually no signal) corresponds to the first number in parentheses, the quantum number.

The orbitals of hydrogen. The number of nodes (regions of virtually no signal) corresponds to the first number in parentheses, the quantum number.

Now you might be wondering why this matters, since the idea of electron orbitals and wave functions has been established for decades. The end of this article points out there are still some weird quantum effects physicists would like to understand better. Directly observing of of the wave function could be a new tool to investigate some processes, like the bizarre Ahronov-Bohm effect that can allow for interesting manipulation of light which could be used in new communications technologies. And this could also help in molecular nanotechnology by allowing researchers to understand the nature of the bonds in their devices.

Speaking of bonds, another team has seen them break and reform using AFM and another technique called scanning tunneling microscopy(STM).  While AFM is basically touching things with atoms to see them, STM is like electrocuting things. Okay, that’s a slight exaggeration. It actually measures the current as electrons quantum mechanically tunnel across the gap between the STM’s tip and the sample surface. The microscope uses a feedback loop to keep the current constant and moves the tip up or down as needed (the tunneling current is exponentially dependent on distance, so this is actually a very sensitive technique). One might think that STM measuring electrons would make it great at studying chemical bonds, but this is complicated by how many different energy states the electrons can be in, so it ends up being a measure of both molecular structure and electronic properties. So AFM is needed to get a higher resolution image to supplement the STM (I’m still a bit unsure about what the STM is needed for, actually). But they’re also pushing AFM to very tight limits, and end up using it as an nano tuning fork basically, as interactions between the probe and sample would change how it vibrates. And the images are gorgeous. And highly reminiscent of intro chemistry. You can even see the carbon atoms in the middle of the chains!

Imaging hydrocarbon products

Top row: STM images. Middle: AFM images. Bottom: Ball and stick model of molecules.

This also gets us more than amazing images. As the research team points out, chemistry studies atomic and molecular reactions, but traditional chemistry techniques focus on bulk averages of billions of billions of molecules (if you’re working on any scale approaching grams). But a single chemical reaction may actually have multiple paths it could take with the atoms and molecules in different intermediate states. Techniques that can observe these different states directly will help chemists research reactions in more detail and also enable ways to improve chemical processes that are economically important (not all reaction pathways are equally efficient). For instance, this study saw products that you wouldn’t expect to see in bulk quantities.

There is one major limit to this technique. They needed to do it on a flat surface, and it also helped that the molecules they were looking at were flat. It’s predicted that new tips and combining STM and AFM techniques into one instrument could help in studying molecules with more complicated chemistries and geometries. The surface problem is harder to overcome; you can’t do this experiment on say something like hydrogen and oxygen combining in air to make water. But a lot of interesting stuff happens on surfaces (surface chemistry is an entire research area). And a lot of things that affect people’s lives depends on surface chemistry. Many of the major industrial catalysts are metals that speed up reactions when the reactants stick to their surfaces, like the catalytic converter in your car. And a lot of nanotechnology is also surface technology, In fact, that inspired the research here. The Berkeley team was making nanomaterials from graphene and did this to study the structure.

Links to original papers: Hydrogen Atoms under Magnification: Direct Observation of the Nodal Structure of Stark States. A.S Stodolna, et al.

Direct Imaging of Covalent Bond Structure in Single-Molecule Chemical Reactions. D.G. de Oteyza, et al.

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