Emergence of molecular vibrations and the evolution to covalent bonds observed in this research. (VIDEO)
Caption
The movement of three gold atoms measured in this study is shown on the upper left. At 0 fs, the sample was excited by UV light. The table on the upper right shows the measured values of the molecular structure during the photochemical reaction. The horizontal and vertical axes of the 2D graph on the lower left are the distances between gold atoms A-B and B-C, respectively. The points represented by S0 and T1' correspond to the molecular structures before the photochemical reaction and after linear transformation during the reaction process, respectively. The molecular structure changed from S0 to T1' along the red line known as a "reaction path". The 3D graph shown in the lower right was made by adding an energy axis to the 2D graph. The curved surface drawn in the 3D graph is known as a "potential energy surface (PES)". The photochemical reaction is described by the movement of the corresponding position in the 3D graph, and the reaction path can be expected to connect the valleys of the PES. Before UV light excitation, the corresponding position is at the bottom of the PES of S0. When a molecule is excited by UV light, the energy of the molecule is increased, but the molecular structure is still maintained. Therefore, the corresponding position moves vertically in the 3D graph. Then, the molecule structure begins to change with a decrease of its energy, and the corresponding position slides down the PES of T1' along the valley. Finally, the molecular structure becomes linear, and the corresponding position reaches the bottom of the PES of T1'. Until now, to calculate the reaction path, the structural change during a chemical reaction has been discussed. However, in this research, we are able to determine the reaction path without relying on a calculation.
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