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Non-Adiabatic Reaction Mechanism Identified at Conical Intersection
(Professor Kim(center) and Ph.D. candidates Kyung Chul Woo (left) and Kang Do Hyung) Research team led by Professor Sang Kyu Kim at KAIST Department of Chemistry observed two distinct reaction pathways that occur at conical intersection where two different adiabatic potential energy surfaces cross at the same nuclear configuration. Professor Kim previously identified the existence and molecular structure of conical intersection in 2010. In this following study, the team accurately measured reaction rates of two totally different reaction pathways activated only at conical intersection where the seminal Born-Oppenheimer approximation breaks down. This study led by Kyung Chul Woo (1st author) and Do Hyung Kang, both Ph.D. candidates at KAIST, was published in Journal of the American Chemical Society in November 7th, 2017. Chemical reaction induced by light occurs in excited electronic states where the reaction outcome is often destined by coupling among different electronic states mediated by nuclear motions during chemical reaction. Such a coupling is most critical and important at the conical intersection as nonadiabtic surface-hopping is most probable at situation where the Born-Oppenheimer approximation fails. Professor Kim used spectroscopic methods in 2010 to experimentally observe conical intersection of polyatomic molecule. And yet, it was not possible to disentangle complex dynamic processes with frequency-domain study only. The research team used pico-second time-resolution kinetic energy resolved mass spectrometry to identify two possible distinct reaction pathways in both energy and time domains.,. The research team demonstrated that the reactive flux prepared at the conical intersection is bifurcated into adiabatic or non-adiabatic reaction pathways. These two pathways are quite distinct in terms of reaction rates, energy releases, and product branching ratios. This is the first study to capture the moment of bifurcation dynamics at the conical intersection for complex polyatomic molecular system. The study could contribute to conceptual improvement in understanding complicated nonadiabatic dynamics in general. Professor Kim said, “Basic science research is essential in understanding and wisely using the nature. New technological advances cannot be made without the advancement in basic science.” He continued, “I hope this study could lead to growth in many young academic talents in basic sciences.” (Figure 1. Reaction graph starting from reaction intersection that divides into adiabatic reaction pathway (red) and non-adiabatic pathway (blue))
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