A two-platinum-atom molecule captures a photon, initiating a vibration. This vibrational state enables a change in the electron spin of the molecule, leading to a simultaneous change in its electronic states—a process termed inter-system crossing. Acknowledgment: Argonne National Laboratory
Technological advances in ultrafast lasers and X-ray methods have disclosed the interconnection between electronic and nuclear dynamics within molecules.
Around a hundred years ago, physicists Max Born and J. Robert Oppenheimer postulated a theory regarding the behavior of quantum mechanics in molecular systems. This theory, known as the Born-Oppenheimer approximation, suggests that within a molecule, the motions of nuclei and electrons are autonomous and can be analyzed separately.
While this model is often sufficient, its boundaries are currently under investigation. A recent research collaboration has unveiled that this approximation fails under extremely rapid timescales, revealing a previously unacknowledged connection between the motion of nuclei and electrons. This groundbreaking revelation has implications for numerous applications, including solar energy conversion, power generation, quantum information science, among others.
Researchers from the U.S. Department of Energy’s Argonne National Laboratory, Northwestern University, North Carolina State University, and the University of Washington collectively announced their findings in two closely related articles published in the journals Nature and Angewandte Chemie International Edition.
Shahnawaz Rafiq, a research associate at Northwestern University and the primary author of the paper in Nature, stated, “Our research brings to light the interconnected dynamics of electron spin and nuclear vibrational states on exceedingly fast timescales. The independence of these properties can no longer be assumed; rather, they intermingle and influence electronic behavior in intricate ways.”
A specific effect known as the spin-vibronic effect is observed when nuclear motions within a molecule exert influence over its electron motions. These motions of the nucleus—whether inherent or induced by external factors like light—can then alter the electron behavior and consequently change the molecule’s quantum mechanical spin, a property linked to magnetism.
Inter-system crossing, a particular mechanism in which an energized molecule changes its electronic state by altering its electron spin, is vital for various chemical reactions, including those in photovoltaic systems, photocatalysis, and even bioluminescence. Enabling this mechanism necessitates particular conditions and energy disparities between the electronic states involved.
For decades, it was theorized that the spin-vibronic effect could be involved in inter-system crossing, but conclusive evidence had been elusive. This is due to the extremely rapid timescales over which changes in electronic, vibrational, and spin states occur.
Lin Chen, an Argonne Distinguished Fellow and professor of chemistry at Northwestern University, explained, “We employed ultrafast laser technology, utilizing pulses as brief as seven femtoseconds, to monitor the motion of nuclei and electrons in real-time. This revealed how the spin-vibronic effect could be a driving force in inter-system crossing. Grasping this interaction could offer new avenues for manipulating and utilizing the electronic and spin characteristics of molecular systems.”
The team’s research involved studying four specially-designed molecular systems, conceived by Felix Castellano, a professor at North Carolina State University. These systems were constructed to be analogous but featured controlled structural variations, enabling a comprehensive understanding of the phenomenon.
Castellano commented, “By designing geometric variations in these molecular systems, we were able to observe interactions between electronic excited states under different energy conditions. This offers new perspectives for material design to optimize inter-system crossing.”
The study confirmed that induced by vibrational movements, the spin-vibronic effect changes the energy distribution within the molecules, thereby increasing the likelihood and speed of inter-system crossing. The research further identified significant transitional electronic states that play a crucial role in the functioning of the spin-vibronic effect.
The findings were supported by quantum dynamics calculations from Xiaosong Li, a professor of chemistry at the University of Washington. Li noted, “The experiments aligned beautifully with our theoretical expectations, displaying clear and exquisite chemistry in real-time.”
The deeper understanding achieved through these experiments offers promising avenues for designing molecules to harness this complex quantum mechanical interrelationship. Potential applications include more efficient solar cells, enhanced electronic displays, and light-based medical treatments.
References:
“Spin–Vibronic Coherence Drives Singlet–Triplet Conversion” by Shahnawaz Rafiq, Nicholas P. Weingartz, Sarah Kromer, Felix N. Castellano, and Lin X. Chen, 19 July 2023, Nature.
DOI: 10.1038/s41586-023-06233-y
“Elucidating Excited-State Paths on Potential Energy Surfaces with Real-Time Atomic Resolution” by Denis Leshchev, Andrew J. S. Valentine, Pyosang Kim, Alexis W. Mills, Subhangi Roy, Arnab Chakraborty, Elisa Biasin, Kristoffer Haldrup, Darren J. Hsu, Matthew S. Kirschner, Dolev Rimmerman, Matthieu Chollet, J. Michael Glownia, Tim B. van Driel, Felix N. Castellano, Xiaosong Li, and Lin X. Chen, 28 April 2023, Angewandte Chemie International Edition.
DOI: 10.1002/anie.202304615
Financial support for both studies was provided by the Department of Energy’s Office of Science, with partial support from the National Science Foundation for the study in Nature. The experiments in the Angewandte Chemie International Edition were executed at the Linac Coherent Light Source at the Department of Energy’s SLAC National Accelerator Laboratory. Additional authors on both papers include a wide array of experts from various institutions.
Table of Contents
Frequently Asked Questions (FAQs) about Born-Oppenheimer Approximation
What is the Born-Oppenheimer Approximation?
The Born-Oppenheimer Approximation is a hypothesis developed nearly a century ago by physicists Max Born and J. Robert Oppenheimer. It postulates that in a molecular system, the movements of nuclei and electrons occur independently and can be treated separately.
Who conducted the recent experiments challenging the Born-Oppenheimer Approximation?
The experiments were conducted by a team of scientists from the U.S. Department of Energy’s Argonne National Laboratory, Northwestern University, North Carolina State University, and the University of Washington. They published their findings in two related papers in Nature and Angewandte Chemie International Edition.
What did the scientists discover?
The scientists discovered that the Born-Oppenheimer Approximation breaks down on very fast time scales. They found a close relationship between the dynamics of nuclei and electrons, revealing that these can’t be treated independently. This could have implications for various fields including solar energy conversion, energy production, and quantum information science.
What is inter-system crossing?
Inter-system crossing is a phenomenon wherein an excited molecule or atom changes its electronic state by flipping its electron spin orientation. It plays an important role in many chemical processes, including those in photovoltaic devices and photocatalysis.
What is the spin-vibronic effect?
The spin-vibronic effect is a phenomenon in which the vibrations of the nuclei within a molecule affect the motion of its electrons. This, in turn, can change the molecule’s spin, a quantum mechanical property related to magnetism.
How did the team conduct their experiments?
The team used ultrafast laser pulses, with durations down to seven femtoseconds, to track the motion of nuclei and electrons in real time. This allowed them to observe how the spin-vibronic effect can drive inter-system crossing.
What potential applications could this discovery have?
Understanding the interplay between the spin-vibronic effect and inter-system crossing could potentially lead to new ways to control and exploit the electronic and spin properties of molecules. Applications could range from solar cells and electronic displays to medical treatments relying on light-matter interactions.
Who supported the research?
The research was supported by the U.S. Department of Energy’s Office of Science. The study published in Nature was also partially supported by the National Science Foundation.
Where were the experiments conducted?
Experiments mentioned in the Angewandte Chemie International Edition were conducted at the Linac Coherent Light Source at the U.S. Department of Energy’s SLAC National Accelerator Laboratory.
What does this discovery mean for the field of quantum mechanics?
The discovery represents a significant step forward in understanding the quantum mechanical relationships within molecules. It challenges long-standing assumptions and could pave the way for advancements in various scientific and technological domains.
More about Born-Oppenheimer Approximation
- Born-Oppenheimer Approximation Overview
- Argonne National Laboratory
- Nature Journal
- Angewandte Chemie International Edition
- U.S. Department of Energy’s Office of Science
- Northwestern University
- North Carolina State University
- University of Washington
- Inter-system crossing in Chemical Processes
- Quantum Information Science
- Linac Coherent Light Source
- National Science Foundation
- SLAC National Accelerator Laboratory
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