Breaking the Born-Oppenheimer approximation – experiments reveal long-theorized quantum phenomena

Ultrafast lasers and X-rays have revealed the coupling between electronic and nuclear dynamics in molecules.

Nearly a century ago, physicists Max Born and J. Robert Oppenheimer developed a hypothesis about the workings of quantum mechanics inside molecules. These molecules are composed of complex systems of nuclei and electrons. The Born-Oppenheimer approximation assumes that the motions of nuclei and electrons within a molecule occur independently and can be treated separately.

This model works the vast majority of the time, but scientists are testing its limits. Recently, a team of scientists demonstrated the collapse of this assumption on very fast time scales, revealing a close relationship between the dynamics of nuclei and electrons. This discovery could impact the design of molecules useful for solar energy conversion, energy production, quantum information science, and more.

The team, which includes scientists from the US Department of Energy’s Argonne National Laboratory, Northwestern University, North Carolina State University, and the University of Washington, recently published their discovery in two related papers in nature And Angewandte Chemie International Edition.

“Our work reveals the interplay between the dynamics of electron spin and the dynamics of nuclei vibrations in molecules on ultrafast time scales,” said Shahnawaz Rafique, a research associate at Harvard University. Northwestern University The first author is Ali nature paper. “These properties cannot be treated independently; they mix together and affect electronic dynamics in complex ways.”

A phenomenon called the vibrational spin effect occurs when changes in the movement of nuclei within a molecule affect the movement of its electrons. When nuclei within a molecule vibrate — either due to their own energy or due to external stimuli, such as light — these vibrations can affect the movement of their electrons, which can in turn change the spin of the molecule, a quantum mechanical property associated with magnetism.

In a process called crossing over between systems, a molecule or molecule is excited corn It changes its electronic state by reversing the direction of the electron’s spin. Cross-system crossing plays an important role in many chemical processes, including those in photovoltaic devices, photocatalysis, and even bioluminescent animals. For this crossing to be possible, it requires certain conditions and energy differences between the electronic states involved.

Since the 1960s, scientists have hypothesized that the spin-vibrational effect could play a role in the crossover between systems, but direct observation of this phenomenon has proven difficult, because it involves measuring changes in electronic, vibrational and spin states at very high levels. Fast timelines.

“We used ultrashort laser pulses—up to seven femtoseconds, or seven millionths of a billionth of a second—to track the motion of nuclei and electrons in real time, showing how the vibrational spin effect can trigger crossovers between systems,” said Lin Chen, a colleague. Arjun Distinguished Professor of Chemistry at Northwestern University and co-author of both studies, “Understanding the interplay between the vibrational spin effect and the intersection between systems could lead to new ways to control and exploit the electronic and spin properties of molecules.”

The team studied four unique molecular systems designed by Felix Castellano, a professor at the University of California North Carolina State University And co-author of both studies. Each system is similar to the other, but they contain controlled and known differences in their structure. This allowed the team to access slightly different crossover effects between the systems and vibrational dynamics to get a fuller picture of the relationship.

“The geometric changes we designed into these systems caused crossing points between interacting electronic excited states to occur at slightly different energies and under different conditions,” Castellano said. “This provides insight into tuning and designing materials to enhance this crossing.”

The effect of vibrational rotation in molecules, caused by vibrational motion, changes the energy landscape within the molecules, increasing the probability and rate of crossover between systems. The team also discovered key intermediate electronic states that were integral to the vibrational spin-impact process.

The results were predicted and reinforced by quantum dynamics calculations performed by Xiaosong Li, a professor of chemistry at the University of California. University of Washington and a laboratory fellow at the Department of Energy’s Pacific Northwest National Laboratory. “These experiments showed very clear and beautiful chemistry in real time that is consistent with our expectations,” said Li, who participated in the study published in the journal. Angewandte Chemie International Edition.

The profound insights revealed by the experiments represent a step forward in designing molecules that can take advantage of this powerful quantum mechanical relationship. This could be particularly useful for solar cells, better electronic displays, and even medical treatments that rely on interactions between light and matter.

References:

“Rotational-vibrational coherence drives singlet-triplet conversion” by Shahnawaz Rafiq, Nicholas P. Weingartz, Sarah Cromer, Felix N. Castellano, and Lin X. Chen, July 19, 2023, nature.
doi: 10.1038/s41586-023-06233-y

“Detection of excited state paths on potential energy surfaces with real-time atomic resolution” by Denis Leshchev, Andrew J. S. Valentine, Byosang Kim, Alexis W. Mills, Subhanji Roy, Arnab Chakraborty, Elsa Pyasen, Christopher Haldrup, and Darren J. Hsu, Matthew S. Kirchner, Dolev Remmerman, Mathieu Chollet, J. Michael Glonea, Tim B. Van Driel, Felix N. Castellano, Xiaosong Li and Lin X. Chen, April 28, 2023, Angewandte Chemie International Edition.
doi: 10.1002/anie.202304615

Both studies were supported by the Department of Energy’s Office of Science. the nature The study was supported in part by the National Science Foundation. Experiments in Angewandte Chemie International Edition They were performed at the Linac Coherent Light Source at the Department of Energy’s SLAC National Accelerator Laboratory. Other authors on nature The study includes Nicholas P. Weingartz and Sarah Cromer. The other authors on the paper published in Angewandte Chemie International Edition Includes Denis Leshchev, Andrew J. S. Valentine, Pyoosang Kim, Alexis W. Mills, Subhanji Roy, Arnab Chakraborty, Elissa Pyasin, Christopher Haldrup, Darren J. Su, Matthew S. Kirchner, Dolev Riemerman, Mathieu Chollet, J. Michael Glonea, and Tim. B. Van Driel.