At aqueous mineral oxide interfaces, the presence of ions and surface charge perturbs the H-bond network, reorganizing the water structure and affecting the fundamental properties of the interfacial solvent. Measuring the vibrational lifetime of a molecular probe that is sensitive to the local solvent environment is a powerful tool for studying the local solvent structure and solute/solvent interactions. Previous investigations of the vibrational dynamics of these probe molecules yielded insights into bulk solvent structure and properties, including the energy relaxation mechanism, and so we hypothesized that interface specific vibrational dynamics measurements would yield similar insights into the interfacial solvent organization. In this work, the ultrafast vibrational dynamics of the Stark active nitrile stretch of SCN- at the α-Al2O3(0001)/H2O and α-Al2O3(0001)/D2O interfaces was studied by IR pump-vSFG probe spectroscopy, showing three times faster vibrational relaxation for interfacial D2O (~7 ps) compared to bulk D2O (~21 ps). The vibrational lifetime at the α-Al2O3(0001)/H2O interface (~3 ps) is faster, on a similar timescale to dynamics in bulk H2O where there is effective coupling of the solvent combination band (water bending + librational modes) and nitrile stretch of SCN- providing an efficient pathway for intermolecular vibrational energy transfer. Ab-initio simulations of SCN- at the interface show that there is some increase in the vibrational density of states in the low-frequency region of the O-D stretch, resulting in greater overlap between SCN- and D2O vibrational modes. This work provides information regarding the vibrational relaxation pathways and coupling between solute and solvent vibrational modes which is essential for understanding fundamental condensed phase phenomena.

Probing the interfacial solvent environment by measuring the vibrational lifetime of SCN- at the α-Al2O3(0001)-aqueous interface

ACS Spring Meeting 2021

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