Adsorption and dissociation of water on LaB ₆(100) investigated by surface vibrational spectroscopy

The chemisorption of water (H ₂O and D ₂O) on a LaB ₆(100) surface was studied with reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS). The clean surface was exposed to H ₂O and D ₂O at temperatures from 90 K to room temperature, and spectra were acquired after heating to temperatures as high as 1200 K. It was found that water molecularly adsorbs on the surface at 90 K as a monomer at low coverages and as amorphous solid water at higher coverages. Water adsorbs dissociatively at room temperature to produce surface hydroxyl species as indicated by OH/OD stretch peaks at 3676/2701 cm ^-1. Room temperature adsorption also reveals low frequency loss features in HREEL spectra near 300 cm ^-1 that are quite similar to results obtained following the dissociative adsorption of O ₂. In the latter case, the loss features were attributed to the LaO stretch of O atoms bridge-bonded between two La atoms. In the case of dissociative adsorption of H ₂O, the low frequency loss features could be due to either the LaO vibrations of adsorbed O or of adsorbed OH.