Methane's collision with gold surfaces reveals how quantum interference and symmetry dictate molecular behavior

The interior of the vacuum chamber during a scattering experiment. The detector is shown in gray (top right) and the Au(111) gold surface is shown in yellow. The lines indicate the path of the scattering molecular beam. Credit: Christopher Reilly (EPFL)

The quantum rules shaping molecular collisions are now coming into focus, offering fresh insights for chemistry and materials science. When molecules collide with surfaces, a complex exchange of energy takes place between the molecule and the atoms composing the surface. But beneath this dizzying complexity, quantum mechanics, which celebrates its 100th anniversary this year, governs the process.

Quantum interference, in particular, plays a key role. It occurs when different pathways that a molecule can take overlap, resulting in specific patterns of interaction: some pathways amplify each other, while others cancel out entirely. This "dance of waves" affects how molecules exchange energy and momentum with surfaces, and ultimately how efficiently they react.

But until now, observing quantum interference in surface collisions with heavier molecules like methane (CH₄) was nearly impossible because of the overwhelming number of pathways available for the system to take en route to the different collision outcomes. Many scientists have even wondered if all quantum effects would always "wash out" for these processes so that the simpler laws of classical physics, which apply to everyday, "macroscopic" objects, might be enough to describe them.

Addressing the challenge of observing quantum interference in surface collisions of methane, researchers in Rainer Beck's group at EPFL, with colleagues in Germany and the United States, have developed a method to cut through the complexity. They tuned methane molecules to specific quantum states, scattered them off a gold (Au) surface, and measured their states after the collision.

The results, published in Science, revealed clear patterns of quantum interference, challenging assumptions about molecular behavior and providing new ways to study these interactions. 

A video summary of the study. Credit: Christopher Reilly (EPFL)

Gold rush

The team didn't use just any chunk of gold to serve as a scattering surface; they used a gold sample carefully grown to be perfectly crystalline and then cut along a special direction to reveal a surface named "Au(111)", which is atomically smooth and chemically inert. They also kept the surface under ultra-high vacuum during experiments to prevent contamination from gas particles present under normal ambient conditions.

The exceptional flatness and cleanliness of the Au(111) surface ensured that the observed scattering behavior arose from fundamental quantum wave aspects rather than random surface irregularities or impurities, allowing the team to focus purely on interference effects.

Laser focus

The researchers then used a laser-based technique to precisely control the quantum states of methane molecules before they collided with the gold surface and measure the quantum states the molecules occupy after the collision.

Almost 100 years after physicists Clint Davisson and Lester Germer at Western Electric first announced their observation of interference effects associated with the wave nature of the electron (1927), a new study shows a novel form of interference in the scattering of methane molecules from a metal surface. Whereas in Davisson and Germer's experiment the effects of interference appear in the form of preferred angles of scattering, the interference effects observed in this study, which relate to the methane molecules' rotational and vibrational motion, appear in the form of preferred absorption of specific wavelengths of light by the scattered molecules. Credit: Christopher Reilly (EPFL)

Methane molecules naturally exist in a mix of different energy states, meaning their internal vibrations and rotations vary. So to make sure all the molecules started in the same well-defined quantum state, the researchers first fired a pump laser at a beam of methane molecules, exciting them into a well-defined quantum state.

They then aimed the beam of methane molecules at a pristine Au(111) surface, where they collided and scattered. After the collision, the team hit the scattered molecules with a tagging laser tuned to specific energy levels. If a molecule was in a matching quantum state, it absorbed the laser's energy, creating a tiny change in temperature of the scattering molecules that the researchers could measure with a highly sensitive detector called a bolometer. 

by Ecole Polytechnique Federale de Lausanne

Source: Methane's collision with gold surfaces reveals how quantum interference and symmetry dictate molecular behavior