Water and ion dynamics at interfaces

This thesis explores the ultrafast dynamics of interfacial water using time-resolved vibrational sum-frequency generation (TR-vSFG) spectroscopy, with a focus on triggering and monitoring transient electric fields at the air-water and water-water interfaces.
First, the effect of temperature on the vibrational lifetimes of hydrogen-bonded and free OD stretching modes in D₂O is studied. The results show that the hydrogen-bonded OD stretching mode relaxes independently of temperature, while the free OD relaxes faster at higher temperatures. These results highlight the similarity between the interfacial and bulk relaxation dynamics of the hydrogen-bonded OD stretching mode. Additionally, we show that intramolecular energy transfer dominates the reorientation relaxation mechanism for free OD.
Next, TR-vSFG is used as a contactless T-jump method to study electric double layer (EDL) dynamics at the air-DCl/water interface. The interfacial hydrated proton population is found to be thermally modulated, and the EDL rearrangement occurs within tens to hundreds of picoseconds. Simulations combined with analytical modeling using a modified Poisson-Nernst-Planck equation confirm that electrostatic interactions govern these dynamics. This aligns with the Debye-Falkenhagen theory, even at high ion concentrations.
Finally, we investigate the electrolyte's electrical response to the IR-induced shock wave following the T-jump. The measured TR-vSFG signal reveals interference between two SFG signals generated at the air-water and traveling water-water interface, respectively. Strong chemical specificity is observed, and its causes are discussed.
These findings deepen our understanding of ultrafast aqueous interfacial phenomena and offer pathways for tuning interfacial ionic behavior relevant to technologies such as electrochemical capacitors and ion-based transistors.