Fig. 1. (A) MIES spectra and (B) UPS (He I) spectra collected from a low-defect SiO₂/Mo(1 1 2) surface as a function of water exposure at 90 K. The MIES and UPS spectra were acquired simultaneously. The shift toward higher binding energies as a function of water exposure (charging) has been removed from all spectra. Enlarged region of the UPS spectra near at the Fermi edge was used to estimate the degree of shifting. (C) Direct comparison of the MIES and UPS spectra after an exposure of 5.25 L water corresponding to multilayer coverage.

Fig. 2. Intensities of the water induced features in the MIES spectra from Fig. 1 as a function of water exposure. The orbitals of molecular water, which causes the features seen in MIES, are noted at the curves.

Fig. 3. (A) MIES difference spectra and (B) UPS difference spectra of the data shown inFig. 1. Before subtracting the SiO₂-spectrum from the D₂O/SiO₂-spectrum, the SiO₂-spectrum was attenuated taking into account the D₂O-induced damping of the SiO₂-features. In addition, the background was subtracted from the UPS spectra using an exponential approach.

Fig. 4. (A) Water TPD spectra of a low-defect SiO₂/Mo(1 1 2) surface for 0.1, 1.0 and 5 L water exposure at 90 K. (B) Plot of the desorption rate (logarithmic) versus the reciprocal temperature for the TPD data taken after exposure of 5 L water. Such a water exposure corresponds to multilayers of water on SiO₂/Mo(1 1 2).

Fig. 5. HREELS spectra collected from a low-defect SiO₂/Mo(1 1 2) surface as a function of water exposure at 90 K: (a) bare SiO₂ surface, (b) after exposing to 0.5 L, (c) 1.0 L, (d) 2.0 L, (e) 5.0 L and (f) 10 L H₂O. The sensitivity of the ion gauge in the HREELS chamber is comparable to that in the UHV chamber in which the MIES/UPS measurements were performed.

Fig. 6. (A) Initial water adsorption (0.35 L D₂O) at 90 K on two differently prepared SiO₂ thin films: (a) Low-defect SiO₂ film produced by deposition of Si at room temperature followed by a 1150 K anneal; (b) SiO₂ film prepared by annealing at 1050 K with a high density of extended defects at the surface. The MIES and UPS difference spectra are compared. The background was subtracted from the UPS spectra using an exponential approach. (B) Work function change Δ upon water exposure at 90 K for the two differently prepared SiO₂ thin films on Mo(1 1 2): (a) Low-defect SiO₂ film and (b) SiO₂ film with extended defects.

Fig. 7. (A) MIES spectra and (B) UPS spectra collected from a SiO₂/Mo(1 1 2) surface initially covered with 6 L water at 90 K as a function of the anneal temperature. The SiO₂ surface was prepared by annealing in oxygen at 1050 K and posses a high density of extended defects.

Fig. 8. (A) Data-fit to the MIES difference spectrum for initial water adsorption (0.35 L D₂O) at 90 K on low-defect SiO₂: single curves (lower panel), and 3 curve data-fit (upper panel). (B) Data-fit to the MIES difference spectrum for initial water adsorption (0.35 L D₂O) at 90 K on the SiO₂ surface possessing extended defects. In all panels the MIES difference spectrum (5–11.5 eV) is shown for comparison.

Energy positions of the three valence orbitals for water ice as observed with MIES and UPS

Binding energies are in eV with respect to the Fermi level. In all cases the sample temperature during water exposure was 90–100 K.