Time-resolved high-harmonic spectroscopy of ultrafast photoisomerization dynamics

Abstract

We report the first time-resolved high-harmonic spectroscopy (TR-HHS) study of a chemical bond rearrangement. We investigate the transient change of the high-harmonic signal from 1,3-cyclohexadiene (CHD), which undergoes ring-opening and isomerizes to 1,3,5-hexatriene (HT) upon photoexcitation. We associated the harmonic yield variation with the changes in the molecule's electronic state and vibrational frequencies, which are caused by isomerization. This showed us that the electronic excited state of CHD created through two-photon absorption of 3.1 eV photons relaxes almost completely within 100 fs to the electronic ground state of CHD with vibrational excitation. Subsequently, the molecule isomerizes to HT (i.e., ring-opening occurs, around 400 fs after the excitation). The present results demonstrate that TR-HHS, which can track both electronic and nuclear dynamics, is a powerful tool for studying ultrafast photochemical reactions.

A deep UV trigger for ground-state ring-opening dynamics of 1,3-cyclohexadiene

“…The CHD* decays in 208 ± 11 fs out of the originally excited electronic state, in excellent agreement with prior lifetime measurements ( 11 , 13 ). The decay leads back to the ring-closed ground-state CHD with a quantum yield of 76 ± 3%.…”

“…They interpreted photoelectron spectra taken with extreme UV high harmonics probe pulses as showing rapid conversion from the Rydberg state to 2A, followed by a dynamical evolution of the ring structure on the 2A surface, eventually causing ring opening on a 500- to 800-fs time scale via a CI, leading to HT with a minimum quantum yield of 0.46 ( 12 ). Subsequent experiments where the target CHD molecules themselves produce the high harmonics appeared supportive of the earlier findings ( 13 ). The dominance of ring opening in the high harmonics experiments is inconsistent with Bühler’s photoelectron spectra, yet the harmonics experiments are encumbered by substantial noise and the uncertainty generally associated with two-photon excitation.…”

“…These authors also reported the first indication that at shorter wavelengths, CHD might not undergo the same ring-opening reaction as in the 1B electronic state, but instead return to the ground electronic state in the ring-closed form. However, two studies with two-photon excitation using 400-nm pulses by Sekikawa and co-workers ( 12 , 13 ) challenged this finding. They interpreted photoelectron spectra taken with extreme UV high harmonics probe pulses as showing rapid conversion from the Rydberg state to 2A, followed by a dynamical evolution of the ring structure on the 2A surface, eventually causing ring opening on a 500- to 800-fs time scale via a CI, leading to HT with a minimum quantum yield of 0.46 ( 12 ).…”

Ultrafast Photoinduced Dynamics of 1,3-Cyclohexadiene Using XMS-CASPT2 Surface Hopping

“…[16][17][18] Apart from the conventional one-photon-driven process, the recent timeresolved photoelectron and high harmonic spectroscopic studies used a two-photon initial excitation to one of the lowest Rydberg states, followed by longer-time decay dynamics, suggesting the existence of alternative pathway(s) yet to be explained. 35,36 Following a one-photon absorption at a wavelength around 260−270 nm, the S 1 state depopulation time constant estimation has been highly consistent across different experiments, usually amounting to 130−140 fs (commonly subdivided into the 1 1 B and 2 1 A lifetimes, each of which is more broadly estimated to last 30 − 70 fs and 60 − 80 fs correspondingly). [5][6][7][8][9][10][11] The time for product formation is also estimated rather broadly in the range 142 − 230 fs.…”

Structural or population dynamics: what is revealed by the time-resolved photoelectron spectroscopy of 1,3-cyclohexadiene? A study with an ensemble density functional theory method

“…A similar estimate for the ring opening kinetics, i.e., the product formation occurs at ca. 400-500 fs, was obtained by Iikubo et al 42 using the TRPES (29.5 eV probe) in connection with the two-photon excitation and by Kaneshima et al 43 using the time-resolved high-harmonic spectroscopy.…”