Almost a hundred years ago, scientists Max Born and J. Robert Oppenheimer proposed the concept of quantum mechanics in the nucleus, which consists of a complex system of nuclei and electrons. The Born-Oppenheimer theory assumes that the motions of nuclei and electrons in molecules are independent of each other and can be controlled separately. This model works most of the time, but scientists are testing its limits. Recently, a group of scientists showed the destruction of this theory at a very fast time, revealing (Experiment Reveals Link between Vibrational and Electronic Dynamics) a close relationship between the evolution of nuclei and electrons.
The discovery could affect the design of molecules useful for solar energy conversion, energy, quantum information science and more. The team, including scientists from the US Department of Energy’s (DOE) Argonne National Laboratory, Northwestern University, North Carolina State University and the University of Washington, recently published their findings in two related articles on Nature and Angewandte Chemie International Edition.
“Our work reveals the relationship between electron spin dynamics and vibrational dynamics of nuclei in molecules at ultra-fast time scales,” said Shahnawaz Rafiq, a research scientist at Northwestern University and first author of the Nature paper.
“These things cannot be treated on their own – they connect and affect the electrical system in a very complex way.” The so-called spin-vibronic effect occurs when changes in the motion of a molecule’s nuclei affect the motion of its electrons.
When the nuclei in a molecule vibrate – either because of their internal energy or because of an external stimulus, such as light – these vibrations can affect the movement of their electrons, which can change the spin of a molecule, a quantum property related to magnetism. In a process called inter-system crossing, an excited molecule or atom changes its electronic state by changing the direction of its electron spin.
Transition between systems plays an important role in many chemical processes, including those in photovoltaic devices, photocatalysis, and even bioluminescent animals. In order for this transition to be possible, it requires a special condition and energy difference between the electronic states involved.
Since the 1960s, scientists have thought that the spin-vibronic effect can work and pass between systems, but the observation of the event has proven to be difficult, since it involves measuring changes in electronic, vibrational, and vibrational states in very fast systems. time scales.
“We use ultrashort laser pulses – up to seven femtoseconds, or seven million seven billionths of a second – to follow the movement of nuclei and electrons directly, showing how the spin-vibronic effect can drive intersystem crossing,” said Lin Chen, Argonne Distinguished Fellow, professor of chemistry at Northwestern University and author.
“Understanding the relationship between spin-vibronic effects and inter-system crossing can lead to new ways to control the electronic and spin properties of molecules.”
The team studied a unique four-cell system developed by Felix Castellano, a professor at North Carolina State University and co-sponsor of both studies. Each of the systems is similar to the other, but they have differences that are controlled and popular in their structure.
This allowed the team to get the effect between the crossover system slightly different from the vibrational effect to get a more complete picture of the interaction. “The geometric changes we made in these systems led to the transition points between excited states interacting with slightly different energies and under different conditions,” Castellano said. said.
“It provides insight into the alignment and design to improve this traverse.”
Driven by vibrational motion, the spin-vibronic effect on molecules changes the nature of energy in molecules, increasing the chance and rate of transition between systems. The team also discovered that intermediate electronic states are important in the operation of the spin-vibronic effect.
Xiaosong Li, a chemistry professor at the University of Washington and a researcher at the DOE’s Pacific Northwest National Laboratory predicted and confirmed the results. “These experiments showed a very clear and beautiful chemistry that directly matches our predictions,” said Li, author of the study published in Angewandte Chemie International Edition.
The insights demonstrated by the experiments represent a step forward in the design of cells that can use this powerful quantum mechanical coupling. This could be especially useful for solar cells, better electronic displays, and even medical treatments based on light interaction.
Source: Argonne National Laboratory
