Spectroscopy and Ultrafast Vibrational Dynamics of Strongly Hydrogen Bonded OH Species at the α-Al2O3(112̅0)/H2O Interface

Frequency and time-resolved vibrational sum frequency generation (vSFG) are used to investigate the behavior of water at the α-Al2O3(1120) surface. In addition to the typical water OH peaks (∼3200 and ∼3400 cm–1), the α-Al2O3(1120)/H2O interface shows an additional red-shifted feature at ∼3000 cm–1. Addition of ions (0.1 M NaCl) largely attenuates the water OH peaks but has little effect on the 3000 cm–1 peak. The 3000 cm–1 feature is assigned to the O–H stretch of surface aluminol groups and/or interfacial water molecules that are strongly hydrogen bonded to the alumina surface. Density functional theory calculations were performed to test this assignment, revealing the presence of both associated and dissociated H2O configurations (chemisorbed surface OH group) with frequencies at 3155 and 3190 cm–1, respectively, at a hydrated α-Al2O3(1120) surface. IR pump–vSFG probe measurements reveal that the interfacial OH species show very fast (<200 fs; bulk waterlike) vibrational relaxation dynamics, which i...