Visualizing Chemical Bond Distortions in Methane Molecules After Absorbing Light

Efficient energy conversion devices for powering electronic devices and heating homes necessitate a detailed understanding of how molecules move and vibrate when undergoing light-induced chemical reactions. Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have successfully visualized the distortions of chemical bonds in a methane molecule after it absorbs light, loses an electron, and then relaxes. The ability to monitor how a molecule responds to light on extremely fast timescales allows researchers to track how electrons move during chemical reactions. Examining how excess energy is redistributed in a molecule that has been excited by light means observing processes that occur on timescales faster than a millionth of a billionth of a second.

Methods

For decades, researchers have relied on theory to describe how excess energy affects the symmetry of a molecule that’s been excited by light. This theory predicts how the bond lengths and angles between individual atoms should change while electrons shift position and what intermediate structures it should adopt. Using ultrafast X-ray spectroscopy facilities at Berkeley Lab’s Chemical Sciences Division, the researchers observed how the structure of ionized methane molecules evolves over time. Methane ions were an ideal system to investigate the question of how a molecule dissipates energy without breaking apart when excited by light. The researchers used a laser to strip an electron from the neutral methane molecule, then took ultrafast X-ray spectroscopic snapshots of the remaining ion, collecting a time series of spectral signals. The signals revealed how the initially symmetric shape becomes distorted over a ten-femtosecond period.

Results and Implications

Observational evidence of a long-studied effect called Jahn-Teller distortion was discovered in the study. Longer time observations showed that for another 58 femtoseconds, the distorted shape vibrates coherently in a scissoring-like motion while redistributing its energy via other vibrations through the structure’s geometric changes. The researchers used the Cori and Perlmutter systems at the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility at Berkeley Lab. The researchers were able to perform calculations that confirmed their measurements of the molecule’s movements. The study demonstrated the viability of an X-ray approach for studying ultrafast molecular dynamics. Methane is a fundamental yet simple molecule where one of the most basic types of distortions occurs as predicted, but with richer and more complicated dynamics than previously understood. Such insights about the dynamics of electrons and nuclei can lead to innovations in new energy conversion devices and photocatalysis applications.

Researchers at Berkeley Lab have visualized the distortions of chemical bonds in a methane molecule after it absorbs light, loses an electron, and then relaxes. This study provides insights into how molecules react to light, which can ultimately be useful for developing new methods to control chemical reactions. By examining how a molecule responds to light on extremely fast timescales, researchers can track how electrons move during a chemical reaction. Understanding how excess energy affects the symmetry of a molecule that’s been excited by light is critical to developing efficient energy conversion devices for powering electronic devices and heating homes.