Fourier transform infrared spectroscopy and temperature programmed desorption of water thin films on the magnesium oxide(100) surface

Temperature Programmed Desorption (TPD) and Fourier Transform Infrared (FTIR) Spectroscopy were used to study the adsorption of water thin films on MgO(100) surfaces. The surfaces under consideration were prepared by cleaving in dry nitrogen followed by a high temperature anneal in oxygen. TPD was used to determine the binding energy of adsorbed water and to calibrate the dosage. FTIR spectroscopy provided information concerning the geometry of adsorbed water molecules in thin films.;A new ultrahigh vacuum chamber was designed and built. TPD and FTIR experiments were performed initially with a surface holder capable of cooling the MgO(l00) surface to ∼120 K. Later, a surface holder was designed that is capable of cooling the surface to ∼90 K.;The TPD spectra showed two desorption peaks. The peak centered at 240 K was assigned to the water monolayer and corresponds to a binding energy 0.75 +/- 0.2 eV. The desorption peak at 155--170 K was assigned as the multilayer peak and reflects the hydrogen bonding energy in ice. No indication of dissociative adsorption of water was observed.;FTIR spectroscopic studies were used to examine the effect of the surface on water thin film growth and the orientation of the adsorbed water molecules. Water films dosed at ∼120 K grew as amorphous solid water. Water films on the MgO(100) surface that were annealed to 150 K and above underwent an irreversible phase transition to cubic ice. Annealing to 185 K desorbed the multilayer leaving a strongly bound water monolayer. The water monolayer stretching peak was broad and red shifted relative to the gas phase stretching vibrations, indicating hydrogen bonding. No dangling bonds or dissociated surface hydroxyl groups were observed during the polarized FTIR studies of the water monolayer. The results are consistent with a water monolayer that is essentially parallel to the MgO(100) surface. Thus, the MgO(100) surface has a profound impact on the geometry of the water monolayer, but the multilayer ice geometry is dependent on surface temperature.