Nearly a century ago, physicists Max Born and J. Robert Oppenheimer proposed a hypothesis about how quantum mechanics operates in molecules. These molecules are composed of complex systems of atomic 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.
The model works in the vast majority of cases, but scientists are testing its limits. Recently, a team of scientists demonstrated that this assumption is broken on extremely fast time scales, revealing the intimate relationship between the dynamics of atomic nuclei and electrons. The discovery could impact molecular design in areas such as solar energy conversion, energy production, quantum information science and more.
The research team, which includes scientists from the U.S. Department of Energy's Argonne National Laboratory, Northwestern University, North Carolina State University and the University of Washington, recently published two related papers in Nature and Angewandte Chemie International Edition.
"Our work reveals the interplay of electron spin dynamics and atomic nucleus vibrational dynamics in molecules on ultrafast time scales," said Shahnawaz Rafiq, first author of the Nature paper and an associate researcher at Northwestern University. "These properties cannot be treated independently - they mix together to affect electron dynamics in complex ways."
When changes in the motion of the nuclei within a molecule affect the motion of electrons, a phenomenon called spin-vibration effect occurs. When the nuclei within a molecule vibrate due to their inherent energy or external stimuli such as light, these vibrations affect the movement of their electrons, thereby changing the molecule's spin, a quantum mechanical property related to magnetism.
In a process called intersystem crossover, an excited molecule or atom changes its electronic state by flipping the direction of its electron spin. Intersystem crossover plays an important role in many chemical processes, including photovoltaic devices, photocatalysis, and even bioluminescent animals. To achieve this crossover, specific conditions and energy differences between the relevant electronic states are required.
Since the 1960s, scientists have theorized that spin-vibration effects might play a role in crossover between systems, but direct observation of the phenomenon has proven challenging because it involves measuring changes in electronic, vibrational and spin states on extremely fast time scales.
"We used ultrashort laser pulses—as low as seven femtoseconds, or seven billionths of a second—to track the motion of nuclei and electrons in real time, showing how spin-vibration effects drive crossover between systems," said Lin Chen, an Argonne Distinguished Fellow and professor of chemistry at Northwestern University and co-corresponding author of both studies.
Understanding the interplay between spin-vibration effects and crossover between systems may make it possible to find new ways to control and exploit the electronic and spin properties of molecules.
The research team studied four unique molecular systems designed by Felix Castellano, a professor at North Carolina State University and co-corresponding author of both studies. Each system is similar to other systems, but contains known differences in their structure that can be controlled. This allowed the research team to exploit the slightly different cross-over effects and vibrational dynamics between the systems to gain a more complete understanding of the relationship between the two.
"The geometric changes we engineered in these systems caused the crossover point between interacting electronic excited states to change slightly differently at different energies and conditions. This provides implications for tuning and designing materials to enhance this crossover," Castellano said.
Induced by vibrational motion, the spin-vibration effect in molecules changes the energy distribution inside the molecule and increases the probability and rate of crossover between systems. The team also discovered key intermediate electronic states that are inseparable from the operation of the spin oscillator effect.
Xiaosong Li, a professor of chemistry at the University of Washington and a researcher at the Department of Energy's Pacific Northwest National Laboratory, predicted and supported these results through quantum dynamics calculations. "These experiments showed very clear and beautiful chemical reactions in real time, which coincided with our predictions," said Li Xiaosong, one of the authors of the study published in the international edition of Angewandte Chemie.
The insights revealed by the experiments represent a step forward in designing molecules using this powerful quantum mechanical relationship. This could be particularly useful for solar cells, better electronic displays, and even medical treatments that rely on light-matter interactions.