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

摘要

We have examined the adsorption of the weakly bound species N-2, O-2, CO, and Kr on the (root 37 x root 37)R25.3 degrees 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 O-2 have little effect on the structure and vibrational spectrum of the "root 37" water monolayer while adsorption of both N-2, 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, "root 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 Hup is mediated by the electrostatic interactions between the water and the adsorbates. (C) 2014 AIP Publishing LLC.