Observing electronic relaxation pathways towards isomerization

Significance 

Time-resolved high-harmonic interferometry

In essence, chemical reactions are the result of the coupled dynamics of valence electrons and nuclei. Consequently, for one to fully comprehend these reactions, techniques that allow for monitoring of the dynamics of electrons and nuclei simultaneously in their own time scale are required. At present, techniques used to monitor the dynamics of chemical reactions employ the attosecond to femtosecond approach. Nonetheless, a recently developed approach has taken over as the leading candidate for such simultaneous monitoring of electron and nuclear dynamics. This novel technique, termed time-resolved high-harmonic spectroscopy (TR-HHS), employs the high harmonic generation (HHG) to probe the electronic state of the generating medium. Preceding publications have shown that the HHG process is sensitive to both the valence electronic and vibrational states of molecules. Since chemical reactions are a result of the coupled dynamics of valence electrons and nuclei, the ability to simultaneously monitor the electronic and nuclear dynamics of molecules makes time-resolved high-harmonic spectroscopy an ideal tool for probing ultrafast chemical reactions.

The ionization potential difference between photoexcited and ground-state molecules results in a phase difference between their high harmonics, which causes high harmonic interference. Unfortunately, this field remains widely unexplored. In fact, technologies that examine the dynamics of coupled valence electrons and nuclei are largely underdeveloped. To address this, Hokkaido University researchers: Dr. Keisuke Kaneshima, Mr. Yuki Ninota and Professor Taro Sekikawa, used the TR-HHS to study the ultrafast ring-opening dynamics of a hydrocarbon ring molecule, 1,3-cyclohexadiene (CHD), C6H8, which isomerizes to 1,3,5-hexatriene (HT) upon photoexcitation. Their work is currently published in the research Journal of the Optical Society of America B.

As a model example of an electrocyclic reaction, the photoinduced dynamics of CHD is critical for understanding a large number of organic reactions. Bearing this in mind, the research team studied the ring-opening isomerization dynamics of CHD by state-of-the-art time-resolved spectroscopic techniques.

The authors reported that the electronic relaxation dynamics of the photoexcited CHD and retrieved the relaxation pathway via high harmonic interference, which occurs on the attosecond time scale. In addition, they noticed that the molecular vibrations that were unique to the isomers of CHD as high harmonic yield modulations. This in turn revealed the time required for the isomerization reaction.

In summary, the new study demonstrated the use of time-resolved high-harmonic spectroscopy as a powerful tool for studying ultrafast photochemical reactions. Credit to its high sensitivity to both electronic and nuclear dynamics, the electronic relaxation dynamics and the following ring-opening dynamics of photoexcited 1,3-cyclohexadiene were revealed. Remarkably, the modulations on the harmonic yields revealed how the molecular vibrations change and when the ring-opening occurs. In a statement to Advances in Engineering, Professor Taro Sekikawa highlighted that their observations will open up the possibility of complete characterization of ultrafast molecular dynamics, by combining TR-HHS with molecular alignment/orientation techniques.

Observing electronic relaxation pathways towards isomerization - Advances in Engineering
(a)Ring-opening of 1,3-cyclohexadiene (CHD) to cZc-1,3,5-hexatirene (cZc-HT). (b) Interference between the harmonics emitted from the excited molecules and those emitted from the unexcited molecules. (c) Experimentally observed transient changes in the yields of the 13th−21st harmonics.

Reference

Keisuke Kaneshima, Yuki Ninota, Taro Sekikawa. Dynamic interference of the high harmonics from photo-isomerizing 1,3-cyclohexadiene. Journal of the Optical Society of America B: Volume 38, No. 2.

Go To Journal of the Optical Society of America

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