Spectroscopy and Ultrafast Vibrational Dynamics of Strongly Hydrogen Bonded OH Species at the α-Al2O3(11(2)over-bar0)/H2O Interface

Frequency and time-resolved vibrational sum frequency generation (vSFG) are used to investigate the behavior of water at the alpha-Al2O3 (11 (2) over bar0) surface. In addition to the typical water OH peaks (similar to 3200 and similar to 3400 cm(-1) ), the alpha-Al2O3 (11 (2) over bar0)/H2O interface shows an additional red-shifted feature at similar to 3000 cm(-1) . Addition of ions (0.1 M NaCl) largely attenuates the water OH peaks but has little e ff ect 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 alpha-Al2O3 (11 (2) over bar0) surface. IR pump - vSFG probe measurements reveal that the interfacial OH species show very fast (<200 fs; bulk waterlike) vibrational relaxation dynamics, which is insensitive to surface charge and ionic strength, thus suggesting that the interfacial OH species at the alpha-Al2O3 (11<(2)over bar>0)/H2O interface are in a highly ordered and strongly hydrogen-bonded environments. The observed fast vibrational relaxation of the interfacial OH species could be due to strong coupling between the 3000 cm(-1) species and the interfacial water OH groups (3175 and 3450 cm(-1) ) via strong hydrogen bonds, dipole - dipole interaction between several interfacial OH groups (Fo rster type energy transfer), and/or ultrafast photoinduced proton transfer