Structural dynamics of photochemical reactions probed by time-resolved photoelectron spectroscopy using high harmonic pulses

Abstract

Femtosecond ring-opening dynamics of 1,3-cyclohexadiene (CHD) in gas phase upon two-photon excitation at 400 nm (=3.1 eV) was investigated by time-resolved photoelectron spectroscopy using 42 nm (=29.5 eV) high harmonic photons probing the dynamics of the lower-lying occupied molecular orbitals (MOs), which are the fingerprints of the molecular structure. After 500 fs, the photoelectron intensity of the MO constituting the C[double bond, length as m-dash]C sigma bond (σ) of CHD was enhanced, while that of the MO forming the C-C sigma bond (σ) of CHD was decreased. The changes in the photoelectron spectra suggest that the ring of CHD opens to form a 1,3,5-hexatriene (HT) after 500 fs. The dynamics of the σ and σ bands between 200 and 500 fs reflects the ring deformation to a conical intersection between the 2A and 1A potential energy surfaces prior to the ring-opening reaction.

Time-resolved high-harmonic spectroscopy of ultrafast photoisomerization dynamics

“…If the CHD had stayed in the electronic excited state, the harmonic yield would have changed. In the previous TR-PES measurement, the relaxation time of the 3p x -Rydberg state of CHD was 37 ± 13 fs upon 2PA [28], which is consistent with the present result. Therefore, we conclude that the contribution of the excited state of CHD was obscured by the coherent artefact around the zero delay and that the CHD returned to its electronic ground state within 80 fs.…”

“…Hence, the observed modulations should be attributable to the vibrational modes stimulated by the ultrafast internal conversion from the excited state whose equilibrium geometry is different from that of the ground state [51,52]. In the previous TR-PES measurement, the relaxation time of the 3p x -Rydberg state of CHD was 37 ± 13 fs upon 2PA [28]. This rapid relaxation via internal conversion coherently excites low-frequency vibrational modes whose periods are longer than the excited state life time [51].…”

“…As a prototypical example of an electrocyclic reaction, this reaction is critical for understanding a large number of organic reactions, including the photo-induced formation of provitamin D [24,25]. It is being actively studied by novel ultrafast spectroscopic techniques such as x-ray fragmentation [26] and x-ray scattering [27] with femtosecond x-ray pulses that are obtained from a free-electron laser, photoelectron spectroscopy with single-order harmonic pulses [28,29], and near edge x-ray absorption fine structure at the carbon Kabsorption edge with soft-x-ray high harmonics [30].…”

“…However, we observed that the ring opens at around 500 fs upon excitation from the HOMO to the 3p x -Rydberg state via two-photon absorption (2PA) using 3.1 eV photons, which is much slower than that observed in case of 1PA. In this experiment, we employed time-resolved photoelectron spectroscopy (TR-PES) using an isolated single-order harmonic at 29.5 eV [28] that was selected using a time-delay compensated monochromator [41][42][43][44]. Thus, another motivation of the present study is to confirm the delayed ring-opening of CHD upon 2PA by a spectroscopic method other than TR-PES; therefore, in this study, we employed TR-HHS.…”

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

“…There is also an ongoing discussion regarding what dynamics occurs when the molecule is pumped at 200 nm rather than 267 nm. 46,47 Finally, and perhaps most importantly, CHD has been used extensively to demonstrate new techniques and concepts. This includes coherent control, both experiment 43,48,49 and theory, 7,50,51 and a range of new ultrafast imaging techniques 52 such as electron diffraction, [53][54][55][56] X-ray scattering, 44,45 Coulomb explosion, 57 and near-edge X-ray absorption fine structure (NEXAFS).…”