Figure 2. a) Plot of water-induced resonance peak area from difference spectra such as in Fig. 1b (red circles) and work function change (blue squares) plotted as a function of water exposure for the annealed surface. The solid line is a fit of the work function change to the Helmholtz model. The saturation of the work function change and the maximum of the resonance intensity correspond to the approximately 1 ML coverage. b) The difference spectra recorded at different temperatures after exposing the reduced TiO2 surface to 3.7 L of H2O. Based on published TPD spectra, 1ML coverage is attained at 180 K and associatively chemisorbed water is desorbed at 300 K. The maximum intensity at 180K confirms that the wet electron resonance has a maximum intensity at 1 ML coverage.