Real-time Observation of Molecule Relaxation Using Ultrafast X-ray Spectroscopy

Designing the next generation of efficient energy conversion devices to power our electronics and heat our homes requires a detailed understanding of how molecules move and vibrate as they undergo light-induced chemical reactions.

Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have now visualized distortions of chemical bonds in a methane molecule after it absorbs light, loses an electron, and then relaxes. Their study provides insight into how molecules interact with light, which could ultimately be useful for developing new ways to control chemical reactions.

Examining how molecules respond to light over extremely fast timescales allows researchers to track how electrons move during a chemical reaction. said Enrico Ridente, a physicist at Berkeley Lab and lead author on the paper Sciences Work report sheet. This means examining how excess energy is redistributed in a molecule excited by light, as electrons and nuclei move as the molecule relaxes to a state of equilibrium.

Examining these subtle motions means making observations of processes that occur on time scales faster than a millionth of a billionth of a second. For decades, researchers have relied on the theory to describe how the extra energy affects the symmetry of a molecule’s bonds that are excited by light — but they don’t break. This theory predicts how the bond lengths and angles between individual atoms should change as the electrons are positioned, and what intermediate structures they should adopt.

Now, using ultrafast X-ray spectroscopy facilities in Berkeley Lab’s Department of Chemical Sciences, Ridente and his colleagues have observed how the structure of ionized methane molecules evolves over time.

“Methane ions are an ideal system for addressing this question because they don’t disintegrate when excited by light,” Radinet said.

First using a laser to strip an electron from a neutral methane molecule, then taking ultrafast X-ray spectroscopy snapshots of the remaining ion, the researchers collected a time series of spectral signals. The signals revealed how an initially symmetric shape becomes distorted over a tenth of a femtosecond (a femtosecond is one-quarter of a millionth of a second) — observational evidence of a long-studied effect called Jahn-Teller distortion.

Longer time observations showed that for another 58 femtoseconds, the deformed shape vibrates coherently in a scissor-like motion while redistributing its energy through other vibrations through geometric changes of structure.

said Stephen Lyon, a chemist at Berkeley Lab and senior author on Sciences paper.

The researchers used the Cori and Perlmutter systems at the National Energy Research Scientific Computing Center (NERSC), a user facility of the Department of Energy’s Office of Science at Berkeley Laboratory, to perform calculations that confirmed their measurements of the molecule’s motions.

“We can now explain how a molecule deforms after losing an electron and how the energies of the electrons respond to these changes,” said Diptarka Hit, a graduate student at Berkeley Lab and lead theoretical author of the study.

The study demonstrated the feasibility of an X-ray approach to studying ultrafast molecular dynamics. Methane is a fundamental yet simple molecule in which one of the most basic types of deformation occurs as expected, but with dynamics richer and more complex than previously understood.

“This research opens the door to studying more complex systems and other types of abnormalities,” says Ridente. Such insights into the dynamics of electrons and nuclei can lead to innovations in new energy conversion devices and photocatalytic applications.