Orientational Dynamics of the Hydrogen-bonded OH of Interfacial Water*

We report on the orientational dynamics of the bonded OH (bOH) of interfacial water, i.e., the OH groups in which the H atom participates in a hydrogen bond (HB). A vibrationally resonant infrared-visible sum-frequency signal provides a surface sensitive probe of the dynamics following excitation by a vibrationally resonant infrared pump. Using p-polarized probe beams, we measure the evolution of the pump-induced orientational anisotropy following p- and s-polarized pump excitation.
Previous work showed that the dangling OH (dOH) groups (i.e., OH groups in which the H atom does not participate in a HB) reorient much faster than do bulk bOH[1,2]. Measuring the dynamics of interfacial bOH is challenging, though, on account of strong intermolecular resonant vibrational energy transfer (RVET). Using isotopic dilutions down to 1 part H₂O in 19 parts D₂O, we minimize RVET and show that interfacial bOH reorient on the same timescale as do bulk bOH. These dynamics are robust to interfacial ion concentrations and suggest that the interfacial hydrogen bond network becomes bulk-like within a single molecular distance from the surface.
References:
[1] S.H. Xiao et al, JACS 138, 5551 (2016).
[2] C.S. Hsieh et al, PNAS 110, 18780 (2013).