Turning things downside up: Adsorbate induced water flipping on Pt(111)

Abstract

We have examined the adsorption of the weakly bound species N2, O2, CO, and Kr on the (\sqrt {37} \times \sqrt {37})R25.3\circ(37×37)R25.3? water monolayer on Pt(111) using a combination of molecular beam dosing, infrared reflection absorption spectroscopy, and temperature programmed desorption. In contrast to multilayer crystalline ice, the adsorbate-free water monolayer is characterized by a lack of dangling OH bonds protruding into the vacuum (H-up). Instead, the non-hydrogen-bonded OH groups are oriented downward (H-down) to maximize their interaction with the underlying Pt(111) substrate. Adsorption of Kr and O2 have little effect on the structure and vibrational spectrum of the "\sqrt {37}37" water monolayer while adsorption of both N2, and CO are effective in "flipping" H-down water molecules into an H-up configuration. This "flipping" occurs readily upon adsorption at temperatures as low as 20 K and the water monolayer transforms back to the H-down, "\sqrt {37}37" structure upon adsorbate desorption above 35 K, indicating small energy differences and barriers between the H-down and H-up configurations. The results suggest that converting water in the first layer from H-down to H-up is mediated by the electrostatic interactions between the water and the adsorbates.