講演情報
[17p-W9_326-10]First Observation of Time-Resolved Emergence of Photochemical and Thermal Ring-Opening Dynamics in 1,3-Cyclohexadiene
〇Bateer Lu1, Sekikawa Taro1 (1.Hokkaido Univ.)
キーワード：
Ultrafast spectroscopy、Ultrafast phenomena、Time-resolved photoelectron spectroscopy
Ultrafast spectroscopy has largely focused on excited-state dynamics, whereas the dynamics after electronic relaxation—essential for capturing the full course of chemical reactions—remain far less explored. High-harmonic pulses provide a unique opportunity to probe such post-relaxation dynamics because their high photon energies enable access to ground-state molecules and to lower-lying molecular orbitals that are highly sensitive to molecular structure. Here, using time-resolved photoelectron spectroscopy (TRPES), we investigate the post-relaxation dynamics of 1,3-cyclohexadiene (CHD) and reveal, for the first time, the coexistence of a prompt photochemical ring-opening pathway and a delayed thermal ring-opening pathway.
CHD is pumped to the 1B state using 4.65 eV pulses and probed using 29.45 eV high-harmonic pulses. Spectral features are assigned by comparing with the calculated molecular orbital energies of ground-state CHD and HT. In the HOMO region, bleaching of the CHD signal is observed during the excited-state dynamics and is followed by a shift to higher ionization energies in the 30–200 fs time window, a signature of cZc-HT formation. At later times (500–1000 fs), a second CHD bleaching exhibits a similar spectral evolution to that observed at 30–200 fs, indicating a second formation of cZc-HT. Consistent evolutions are also observed in lower-lying molecular orbitals, in accordance with the HOMO dynamics. This delayed ring-opening signal is observed for the first time, enabled by probing ground-state molecules with high photon-energy pulses. We attribute the second HOMO bleaching (500–1000 fs) to thermal ring opening of CHD molecules that have returned to the ground state through a conical intersection. These vibrationally hot CHD molecules subsequently undergo ring opening on the ground-state potential energy surface. This assignment is supported by time-resolved high-harmonic spectroscopy (TR-HHS), which shows a signal change around 650 fs.
CHD is pumped to the 1B state using 4.65 eV pulses and probed using 29.45 eV high-harmonic pulses. Spectral features are assigned by comparing with the calculated molecular orbital energies of ground-state CHD and HT. In the HOMO region, bleaching of the CHD signal is observed during the excited-state dynamics and is followed by a shift to higher ionization energies in the 30–200 fs time window, a signature of cZc-HT formation. At later times (500–1000 fs), a second CHD bleaching exhibits a similar spectral evolution to that observed at 30–200 fs, indicating a second formation of cZc-HT. Consistent evolutions are also observed in lower-lying molecular orbitals, in accordance with the HOMO dynamics. This delayed ring-opening signal is observed for the first time, enabled by probing ground-state molecules with high photon-energy pulses. We attribute the second HOMO bleaching (500–1000 fs) to thermal ring opening of CHD molecules that have returned to the ground state through a conical intersection. These vibrationally hot CHD molecules subsequently undergo ring opening on the ground-state potential energy surface. This assignment is supported by time-resolved high-harmonic spectroscopy (TR-HHS), which shows a signal change around 650 fs.