Ultrafast lasers and X-rays have been used to uncover the relationship between electronic and nuclear dynamics in molecules, challenging a long-standing assumption in quantum mechanics. The research, conducted by scientists from the U.S. Department of Energy’s Argonne National Laboratory, Northwestern University, North Carolina State University, and the University of Washington, could have implications for various fields including solar energy conversion, energy production, and quantum information science.
The team’s findings were recently published in two related papers in Nature and Applied Chemistry International Edition. The research demonstrates the breakdown of the Born-Oppenheimer approximation, a model developed by physicists Max Born and J. Robert Oppenheimer nearly a century ago. This model postulates that the movements of nuclei and electrons within a molecule can be treated separately. However, the team’s work shows that the dynamics of electron spin and nuclear vibrations in molecules occur simultaneously and can affect electronic dynamics in complex ways.
One of the key phenomena observed in the study is the spin-vibronic effect, where changes in the motion of nuclei within a molecule impact the motion of its electrons. This effect can lead to a process called inter-system crossing, where an excited molecule or atom changes its electronic state by flipping its electron spin orientation. Inter-system crossing is crucial in various chemical processes, including those in photovoltaic devices and photocatalysis.
Observing the spin-vibronic effect directly has been challenging due to the need to measure changes in electronic, vibrational, and spin states on extremely fast time scales. The team overcame this challenge by using ultrashort laser pulses to track the motion of nuclei and electrons in real time, down to a time scale of seven femtoseconds.
The team studied four unique molecular systems, each designed with controlled structural differences, to examine different inter-system crossing effects and vibrational dynamics. By inducing vibrational motion, the researchers observed how the spin-vibronic effect altered the energy landscape within the molecules, increasing the probability and rate of inter-system crossing. They also discovered key intermediate electronic states that were crucial to the operation of the spin-vibronic effect.
The results of the study have significant implications for the design of molecules that can exploit this quantum mechanical relationship. This could lead to advancements in the development of solar cells, electronic displays, and medical treatments that rely on light-matter interactions.
The research was supported by the U.S. Department of Energy’s Office of Science and the National Science Foundation. Experiments for the study published in Applied Chemistry International Edition were conducted at the Linac Coherent Light Source at DOE’s SLAC National Accelerator Laboratory.
Overall, the findings provide valuable insights into the coupling between electronic and nuclear dynamics in molecules and could pave the way for new approaches to controlling and utilizing the electronic and spin properties of molecules.