EPFL scientists explore molecular collisions with gold

When molecules collide with surfaces, they exchange energy with the surface atoms. This complex process is influenced by quantum interference, where different pathways overlap, creating patterns where some paths enhance each other while others cancel out. This affects how molecules exchange energy and react with surfaces.

Observing quantum interference in collisions with heavier molecules like methane (CH4) was challenging due to the many possible pathways. Scientists wondered if quantum effects would disappear, making classical physics enough to describe these processes.

EPFL researchers, with colleagues in Germany and the United States, developed a method to simplify this process. They adjusted methane molecules to specific quantum states, collided them with a gold (Au) surface, and measured their states after the collision.

Their results showed clear patterns of quantum interference, challenging assumptions about molecular behavior and offering new ways to study these interactions.

The team used a gold sample that was carefully grown to perfectly crystalline. They then cut it along a special direction to reveal a surface named “Au(111)”, which is atomically smooth and chemically inert.

The researchers kept the surface under an ultra-high vacuum to ensure accurate results and prevent contamination. The flat and clean Au(111) surface ensured that the observed behavior was due to quantum wave aspects, not surface irregularities or impurities. This allowed the team to focus purely on interference effects.

The team used a laser-based technique to control methane molecules’ quantum states before colliding with the gold surface. Methane molecules naturally exist in different energy states, with varying internal vibrations and rotations. To ensure all the molecules started in the same quantum state, the researchers fired a pump laser at a beam of methane molecules, exciting them into a well-defined state. They then measured the quantum states of the molecules after the collision.

They then aimed the methane molecules at a clean Au(111) surface, where they collided and scattered. After the collision, the team used a tagging laser tuned to specific energy levels. If a molecule was in a matching quantum state, it absorbed the laser’s energy, causing a tiny temperature change that the researchers measured with a bolometer.

Using this method, the scientists identified the quantum states of methane molecules after the collision. They found that symmetry determined which transitions were allowed and which were forbidden.

Simply put, symmetry describes how something stays the same when flipped, rotated, or reflected. In the quantum world, each molecular state has a specific symmetry, and transitions between states must follow strict symmetry rules.

If two states of a methane molecule had incompatible symmetry, the pathways between them canceled each other out, preventing the transition—like trying to walk through a doorway that leads to a brick wall. But when the states had compatible symmetry, the pathways amplified each other, resulting in strong and visible transitions—like doors aligning for smooth movement. This confirmed that quantum interference actively controls molecular behavior at surfaces.

The authors compared their findings to the famous double-slit experiment, where particles like electrons create interference patterns when passed through two slits, behaving like waves. Similarly, methane molecules displayed interference in this study.

The study reveals a new form of quantum interference in molecule scattering. Unlike the well-known “diffractive” interference affecting scattering angles (as in the double-slit experiment), this interference affected methane molecules’ rotational and vibrational states, suppressing some transitions while enhancing others.

This research, marking 100 years since the advent of quantum mechanics, presents one of the clearest examples of quantum wave effects in molecule-surface interactions. It paves the way for advancements in surface chemistry, cleaner energy catalysts, and efficient industrial processes, offering a new framework for exploring molecular interactions in fundamental and applied sciences.

Journal Reference:

1. Christopher Reilly, Daniel Alierbach et al. Quantum interference observed in state-resolved molecule-surface scattering. Science. DOI: 10.1126/science.adu1023

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