Observation of the Molecular Response to Light upon Photoexcitation

Haiwang Yong, Nikola Zotev, Jennifer M. Ruddock, Brian Stankus, Mats Simmermacher, Andrés Moreno Carrascosa, Wenpeng Du, et al., “Observation of the Molecular Response to Light upon Photoexcitation” Nature Communications, vol. 11, iss. 1 (December 2020).

	author = {Haiwang Yong, Nikola Zotev, Jennifer M. Ruddock, Brian Stankus, Mats Simmermacher,
Andrés Moreno Carrascosa, Wenpeng Du, et al.},
	title = {Observation of the Molecular Response to Light upon Photoexcitation},
	journal = {Nature Communications},
	volume = {11},
	number = {1},
	abstract = {When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation.},
	year = {2020},
	type= {Article},

The first step in many light-driven chemical reactions is a shift in the arrangement of a molecule’s electrons as they absorb the light’s energy. This subtle rearrangement paves the way for everything that follows and determines how the reaction proceeds. In a recent experiment, this first step was observed directly for the first time, showing how the molecule’s electron cloud balloons out before any of the atomic nuclei in the molecule respond. A team consisting of researchers from Brown University, the University of Edinburgh and the Department of Energy’s SLAC National Accelerator Laboratory reported their findings in Nature Communications.

“In past molecular movies, we have been able to see how atomic nuclei move during a chemical reaction,” said Peter Weber, a chemistry professor at Brown and senior author of the report. “But the chemical bonding itself, which is a result of the redistribution of electrons, was invisible. Now the door is open to watching the chemical bonds change during reactions.”

This was the latest in a series of molecular movies starring 1,3-cyclohexadiene, or CHD, a ring-shaped molecule derived from pine oil. In a low-pressure gas its molecules float freely and are easy to study, and it serves as an important model for more complex biological reactions like the one that produces vitamin D when sunlight hits your skin.

Four years ago, researchers from Brown, SLAC and Edinburgh used LCLS to make a molecular movie of the CHD ring flying apart, – the first-ever molecular movie recorded using X-rays. This achievement was listed as one of the 75 most important scientific breakthroughs to emerge from a DOE national laboratory, alongside discoveries such as the decoding of DNA and the detection of neutrinos.

But none of those previous experiments were able to observe the initial electron-shuffling step, because there was no way to tease it apart from the much larger movements of the molecule’s atomic nuclei. The experiment used non-resonant X-ray scattering to measure the changes in the distribution of electrons as the molecule absorbed light at 200 nm. The goal for future experiments is to simultaneously track the electronic structure and molecular geometry during a chemical reaction, using ultrafast X-ray scattering.