Assignment of a surface vibrational mode by chemical means: Modification of the lattice modes of Al2O3 by a surface reaction with H2O

The adsorption of water on a well-characterized Al2O3 film produced by oxidizing an Al(111) surface has been studied using electron energy loss spectroscopy (EELS). Water reacts with this oxide layer to form surface hydroxyl species characterized by a sharp O–H (O–D) stretching vibration at 3720 (2740) cm−1. The assignment of the three-peak vibrational spectrum of Al2O3 to surface and bulk Al–O modes is confirmed by chemical means based on the observation that the 625 cm−1 loss feature assigned to a surface Al–O stretching vibration is preferentially affected upon formation of surface hydroxyl groups. The remaining two bulk modes are uninfluenced by surface hydroxylation. The surface hydroxyl species can be quantitatively removed by electron stimulated desorption (ESD), reversing the spectroscopic changes observed upon exposure of Al2O3 to water, further substantiating the vibrational assignments.