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.
Highlights
Body Measuring and understanding ultrafast dynamics in matter has always been a prime research goal
Recent developments of ultrashort-pulse light sources have allowed us to study electronic dynamics at the attosecond and angstrom scales, and progress has been made via high-harmonic generation (HHG), which provides attosecond pulses in the extreme ultraviolet (EUV) and enables the development of time-resolved techniques in the attosecond regime[1,2,3,4]
On account of the strong nonlinearity of the ionization process, high-harmonic spectroscopy (HHS) of molecules can sensitively and selectively probe the highest occupied molecular orbitals (HOMOs), which are of prime importance for understanding chemical reactions
Summary
Body Measuring and understanding ultrafast dynamics in matter has always been a prime research goal. TR-HHS has been used to study the vibrational dynamics of SF617 and N2O418 and photo-dissociation dynamics of Br219, NO220, CH3I, and CF3I21 These experiments have shown that the HHG process is sensitive to both the valence electronic and the vibrational states of the molecules. The TR-HHS of more complicated reactions like the concerted breakage and formation of bonds initiated by photoexcitation remains a challenge This limited application mainly stems from the difficulty of observing high-harmonic signals from photoreactive organic molecules. Since these molecules are in a condensed phase at room temperature, due to their polarity and large molecular weights, their vapor pressure is not high enough to observe high-harmonic signals[8] We overcome this difficulty and demonstrate TR-HHS of ultrafast photoisomerization dynamics of 1,3-cyclohexadiene (CHD), C6H8. This is a significant advancement, which makes TR-HHS a more versatile and practical tool for studying ultrafast chemical reactions
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