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Time-resolved high-harmonic spectroscopy of ultrafast photo-isomerization dynamics
We report the first study of time-resolved high-harmonic spectroscopy
(TR-HHS) of a bond-making chemical reaction. 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. By associating the change of the harmonic yield with the
changes of the ionization energy and vibrational frequency of the molecule due
to the isomerization, we find that the electronic excited state of CHD created
via two-photon absorption of 3.1 eV photons relaxes almost completely within 80
fs to the electronic ground state of CHD with vibrational excitation.
Subsequently, the molecule isomerizes abruptly to HT, i.e., ring-opening
occurs, around 400 fs after the excitation. The present results demonstrate
that TR-HHS, which can track both the electronic and the nuclear dynamics, is a
powerful tool for unveiling ultrafast photo-chemical reactions.Comment: 25 pages, 7 figure
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