Journal
JOURNAL OF PHYSICAL CHEMISTRY A
Volume 119, Issue 33, Pages 8832-8845Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpca.5b05672
Keywords
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Funding
- National Science Foundation [CBET-1336105]
- Defense Threat Reduction Agency (DTRA) [HDTRA1-14-1-0008]
- European Union [FP7-PEOPLE-2013-CIG-NEWLIGHT]
- Leverhulme Trust [RPG-2013-365]
- Directorate For Engineering
- Div Of Chem, Bioeng, Env, & Transp Sys [1336105] Funding Source: National Science Foundation
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All stages of the electrocyclic ring-opening of 1,3-cyclohexadiene (CHD) were observed by time-resolved photoionization-photoelectron spectroscopy. Spectra of the 1B state, previously unobserved using time-resolved methods, were obtained upon optical excitation using ultrashort laser pulses at 4.60 or 4.65 eV, followed by ionization with pulses at 3.81, 3.85, and 4.10 eV, revealing a 1B lifetime of 30 fs. In an experiment using 3.07 eV probe photons and a 4.69 eV pump, we observed a time-sequenced progression of Rydberg states that includes s, p, and d states of the series n = 3 to 6. The sequentiality of the Rydberg signals points to an ionization mechanism that captures the molecule on different points along the reaction path in 2A. A dynamic fit of the Rydberg signals, coupled with MS-CASPT2 calculations, reveals that as the wavepacket moves down the potential energy surface it acquires kinetic energy at a rate of 28 eV/ps before reaching the conical intersection to the 1A ground state. During the reaction, the terminal carbon atoms separate at a speed of 16 angstrom/ps. A deconvolution of the Rydberg signals from a broad feature assigned to structurally disperse 1,3,5-hexatriene (HT) shows the formation of the open-chain hexatriene structure with an onset 142 fs after the initial absorption of a pump photon. The experimental observations are discussed in the context of recent ultrafast X-ray scattering experiments and theoretical quantum dynamics simulations.
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