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Aqueous solvation from the water perspective

Authors :
Saima Ahmed
Gustavo Ciardi
Ricardo Fernández-Terán
Andrea Pasti
Andrey Shalit
Peter Hamm
University of Zurich
Source :
The Journal of chemical physics. 148(23)
Publication Year :
2018

Abstract

The response of water re-solvating a charge-transfer dye (deprotonated Coumarin 343) after photoexcitation has been measured by means of transient THz spectroscopy. Two steps of increasing THz absorption are observed, a first ∼10 ps step on the time scale of Debye relaxation of bulk water and a much slower step on a 3.9 ns time scale, the latter of which reflecting heating of the bulk solution upon electronic relaxation of the dye molecules from the S1 back into the S0 state. As an additional reference experiment, the hydroxyl vibration of water has been excited directly by a short IR pulse, establishing that the THz signal measures an elevated temperature within ∼1 ps. This result shows that the first step upon dye excitation (10 ps) is not limited by the response time of the THz signal; it rather reflects the reorientation of water molecules in the solvation layer. The apparent discrepancy between the relatively slow reorientation time and the general notion that water is among the fastest solvents with a solvation time in the sub-picosecond regime is discussed. Furthermore, non-equilibrium molecular dynamics simulations have been performed, revealing a close-to-quantitative agreement with experiment, which allows one to disentangle the contribution of heating to the overall THz response from that of water orientation.

Details

ISSN :
10897690
Volume :
148
Issue :
23
Database :
OpenAIRE
Journal :
The Journal of chemical physics
Accession number :
edsair.doi.dedup.....fb9f7718e9b367d5dd616a9768d90c53