In the landscape of modern science, the ability to take a picture of an individual atom feels like a solved problem. One of the foundational techniques that made this possible is Field-Ion Microscopy (FIM), a method for both imaging a surface atom-by-atom and plucking those atoms off.
Given our current capabilities, it's easy to assume that the underlying physics of this process is completely understood. But a deep dive into a nearly 50-year-old PhD dissertation from the University of Cambridge reveals a different story. The 1975 thesis by Allen Robert Waugh, titled Field-Ion Energy Analysis and Field-Desorption Microscopy, shows that the seemingly simple act of removing a single atom from a surface is a process brimming with counter-intuitive quantum effects. It reveals that the leading theories of the time were not just slightly off, but wrong by factors of hundreds of millions—a sign that the quantum rulebook was completely different.
This foundational work is a fascinating reminder that even the most fundamental processes are not always what they seem. Here are five of the most impactful quantum surprises that this 1975 dissertation helped bring to light.
One of the most basic tasks in FIM is "field evaporation"—using a powerful electric field to gently lift a single atom off a metal tip. To do this predictably, you need a solid theory that tells you how the rate of atom removal changes when you tweak the strength of the electric field. In 1975, the simplest and most common model was the "Image-Force Theory." But as Waugh's dissertation details, this theory wasn't just a little bit wrong; it was spectacularly wrong.
The theory predicted that the rate of evaporation was incredibly sensitive to the electric field. The mathematics suggested that a tiny 1% increase in the field's strength would cause the evaporation rate to skyrocket. The scale of this predicted change was staggering, as highlighted in the thesis:
"This is in fact much more rapid than the experimentally observed values, which lie nearer 101.5 for a 1% change in field."
To put that in perspective, the theory predicted that a mere 1% increase in the field would make atoms evaporate ten billion times faster. But in reality, experiments showed the rate increased by only about 30 times (101.5). The theory wasn't just inaccurate; it overestimated the effect by a factor of more than 300 million. It also made another false prediction: that elements should only evaporate as one type of charged ion. In reality, the thesis notes, "it is commonly found that elements evaporate as a mixture of charge species." This colossal failure of the leading model was a powerful lesson in how our classical intuition can be wildly misleading when applied to the quantum, atomic scale.
To see the atoms on the metal tip in the first place, scientists use a faint background of an "imaging gas," typically an inert gas like helium. For a long time, researchers assumed this gas was just a passive observer, playing no role in the drama of field evaporation. It was thought to have "little, if any, effect on the evaporation behaviour."
Waugh's dissertation helped confirm a radical new understanding: these gases are anything but inert. The work showed that "image gases are adsorbed on the specimen surface for considerable lengths of time," sticking to the surface even under the intense electric fields. Instead of just bouncing off, the helium atoms were active participants.
This led to the counter-intuitive discovery that metal atoms can evaporate as temporary, composite particles bonded to the inert gas atoms. These complexes, such as "helides" (a helium-metal ion combination), were being detected by the instruments. This had a direct practical impact: the presence of adsorbed helium "will reduce the evaporation field," making it easier to pluck atoms from the surface. This finding underscored a fundamental principle of quantum measurement: you cannot observe a system without fundamentally changing it, even with something as supposedly non-reactive as helium.
Classically, an atom is like a marble stuck in a divot on a wooden plank. To get it out, you have to shake the plank hard enough (add thermal energy) for the marble to hop over the edge. For an atom to escape a surface, it needs to be given enough thermal energy to vibrate violently enough to "hop over" an energy barrier that holds it in place.
However, the thesis emphasises a far stranger mechanism that dominates at the low temperatures used in FIM: quantum tunneling. Because the potential barrier holding the atom to the surface is incredibly narrow (around 1 Ångström, the width of an atom), the atom doesn't need to go over the barrier at all. In the quantum world, if the wall of that divot is thin enough, the marble doesn't need to hop over it—it can simply vanish from the divot and reappear on the other side. Quantum mechanics allows for a "finite probability that an ion may tunnel through the barrier, in spite of the short wavelength of such a heavy particle."
At the cryogenic temperatures of these experiments, this quantum shortcut isn't just a curious edge case; it becomes a dominant way for atoms to escape. This was a critical insight that existing theories had failed to properly incorporate. As the dissertation states, the evidence was clear:
"...it still seems likely that tunnelling should play an important role in field evaporation at the low temperatures used in field-ion microscopy, and any future theory of field evaporation must take tunnelling into account."
This finding fundamentally changed the picture of how atoms break their bonds with a surface, replacing the classical "hop" with a quantum "disappearance" through an impassable wall.
When you strip atoms off a perfectly ordered crystal surface, you might expect the image of those flying atoms to simply reflect the neat, grid-like structure of the crystal they left behind. But the experiments described in the dissertation, using a technique called "field-desorption microscopy," revealed something far more complex and beautiful.
Instead of a simple map of the atomic lattice, the stream of evaporating atoms painted a "marked and unexpected structure" on the detector screen, a canvas where the atoms were painting the invisible forces governing their escape. The patterns observed for tungsten, for instance, were intricate and mysterious, showing a hidden set of rules governing the atoms' flight paths. The key features included:
This meant that the trajectories of the atoms as they flew from the surface were not simple, straight lines radiating outwards. The discovery of these complex patterns, or "aiming errors," showed that the journey of an evaporating ion was far more intricate than anyone had imagined. The beautiful, ghostly images were not just pictures of the surface, but visual records of the complex physics of the evaporation process itself.
According to the quantum theories of the day, there was an invisible wall near the metal surface called the "critical distance." Inside this boundary, an atom simply could not be ionised by the electric field—it was a "forbidden region."
The experimental work detailed in the thesis, however, found ions that seemed to originate from inside this very zone. In experiments using mixtures of gases (like helium and hydrogen), ions were detected that had energies so high they must have been created closer to the surface than theory allowed. This was a major puzzle.
The proposed explanation was a complex, multi-step process. First, a hydrogen atom is ioniseed far from the surface, which is allowed. The electron freed from that hydrogen atom then travels back towards the metal tip and strikes a helium atom adsorbed on the surface, knocking it off in a process called "electron-impact desorption." This finding was initially controversial and was dismissed by some as a measurement artifact or "'ghost' spectral lines." However, the careful work in Waugh's dissertation helped confirm that the effect was "genuine." It was a perfect example of scientific discovery, where a result that seems to break the established rules forces a deeper understanding of a more complex, multi-particle reality.
Exploring a nearly 50-year-old dissertation is a powerful reminder that the processes we think we understand are often built on a foundation of quantum strangeness and complexity. It shows how our classical intuitions consistently fail us at the atomic scale, a world where our best theories can be wrong by hundreds of millions, supposedly inert gases become active participants, and particles can tunnel through walls that should be impenetrable.
The work of a PhD student in 1975, patiently tracking down unexpected signals and puzzling over strange images, helped lay the groundwork for our modern ability to manipulate the atomic world. It makes you wonder, what seemingly "solved" problems in science today are just waiting for a closer look to reveal their own hidden, unexpected complexities?