According to a study published in Science, EPFL scientists have discovered how quantum interference and symmetry influence molecule behavior in collisions with gold surfaces, providing new insights into molecular interactions. The discoveries may have significant consequences for chemistry and materials science.
The vacuum chamber during a scattering experiment. The detector (grey) and the Au(111) gold surface (yellow). The lines indicate the path of the scattering molecular beam. Image Credit: C. Reilly (EPFL)
When molecules collide with surfaces, a complicated exchange of energy occurs between the molecule and the atoms that form the surface. But, beneath this dizzying intricacy, quantum mechanics rule the day.
Quantum interference is particularly important. It occurs when a molecule's possible paths overlap, resulting in certain patterns of interaction: some pathways magnify each other, while others cancel out completely. This “dance of waves” influences how molecules exchange energy and momentum with surfaces, and hence how efficiently they react.
However, until now, seeing quantum interference in surface collisions with heavier molecules such as methane (CH4) was almost difficult due to the vast number of pathways available for the system to travel en route to the many collision outcomes. Many scientists have even asked if all quantum effects will always “wash out” for these processes, allowing the simpler principles of classical physics, which apply to every day, “macroscopic” objects, to adequately describe them.
To address the issue of witnessing quantum interference in methane surface collisions, researchers in Rainer Beck's department at EPFL, along with colleagues in Germany and the United States, created an approach that cuts through the complexity. They adjusted methane molecules into certain quantum states, dispersed them off a gold (Au) surface, and then measured their states after the contact.
The findings revealed distinct patterns of quantum interference, challenging previous ideas about molecular behavior and opening up new avenues for studying these interactions.
Gold Rush
The researchers did not employ just any chunk of gold to act as a scattering surface; instead, they used a gold sample that was meticulously produced to be completely crystalline and then cut along a particular direction to disclose a surface known as “Au (111)”, which is atomically smooth and chemically inert. They also kept the surface under ultra-high vacuum during the studies to avoid contamination from gas particles found in typical ambient settings.
The exceptional flatness and cleanliness of the Au(111) surface ensured that the observed scattering behavior stemmed from fundamental quantum wave properties rather than surface defects or contaminants. This allowed the team to focus exclusively on interference effects.
Laser Focus
The researchers next utilized a laser-based technique to accurately manipulate the quantum states of methane molecules before they collided with the gold surface, as well as measure the quantum states the molecules occupied following the collision. Methane molecules exist in a variety of energy states; therefore, their internal vibrations and rotations vary.
To ensure that all of the molecules started in the same well-defined quantum state, the researchers fired a pump laser at a beam of methane molecules, causing them to enter a well-defined quantum state.
After that, scientists directed the methane molecule beam towards a spotless Au(111) surface, where it collided and dispersed. Following the impact, the group used a tagging laser set to particular energy levels to strike the dispersed molecules. The researchers employed a highly sensitive detector called a bolometer to track subtle temperature changes in the scattering molecules. These changes occur when a molecule absorbs the laser’s energy while in a matching quantum state.
Quantum Interference Revealed
By using this technique, the researchers were able to determine which quantum states the methane molecules were in following their collision with the gold surface. They discovered that symmetry determined which transitions were permitted and which were prohibited when comparing their findings to quantum theory.
Simply said, symmetry is the ability of something to remain unchanged when rotated, flipped, or mirrored. Every molecule’s state in the quantum universe has a unique symmetry, and changes between states must adhere to stringent symmetry regulations.
The distinct paths followed between two states of a methane molecule would cancel each other out if the two states had incompatible symmetry. Similar to attempting to pass through a doorway leading to a brick wall, the transition in this instance just did not occur.
However, when the states possessed compatible symmetry, the paths amplified one another, and the transitions were robust and clearly visible—similar to doors aligning across rooms, allowing for smooth travel. This proved that quantum interference is more than just an abstract concept; it actually regulates molecular behavior at surfaces.
The Double-Slit Connection
In their study, the scientists draw an interesting analogy to the well-known double-slit experiment, in which particles such as electrons form interference patterns when passing through two slits, behaving like waves, exactly as methane molecules did here.
The study identifies a novel type of quantum interference in molecular scattering. Unlike the more familiar “diffractive” interference, which alters scattering angles (as in the double-slit experiment), this interference altered the rotational and vibrational states of the methane molecules, suppressing some transitions while increasing others.
The study demonstrates one of the most apparent examples of quantum wave effects in molecule-surface interactions, paving the path for advances in surface chemistry, greener energy catalysts, and efficient industrial processes 100 years after quantum mechanics was first introduced. They also offer a novel framework for investigating molecular interactions in both the fundamental and applied sciences.
Quantum interference in molecule-surface collisions
Video Credit: EPFL
Journal Reference:
Reilly, C. S. et. al. (2025) Quantum interference observed in state-resolved molecule-surface scattering. Science. doi.org/10.1126/science.adu1023