Effect of Nitrogen Adsorption on the Mid-Infrared Spectrum of Water Clusters
ABSTRACT Experimental Fourier-transform infrared spectra and DFT calculated infrared spectra are compared to investigate the effect of adsorbed nitrogen on the OH-stretch band complex of water clusters. Using a collisional cooling experiment, pure as well as partially and completely N(2)-covered water clusters consisting of 20-200 water molecules have been generated in thermal equilibrium in the aerosol phase within the temperature range of 5-80 K. Computational IR-spectra simulations have been performed for discrete pure and N(2)-covered water clusters including 10, 15, 20, and 30 water molecules. The adsorbed N(2) molecules especially affect the three-coordinated water molecules at the cluster surface which could be observed as a blue shift of the companion O-H band at 2900 cm(-1) and a red shift of the dangling O-H band at 3700 cm(-1) by about 20 cm(-1) in both cases. The most striking effect of the N(2) adsorbate is an intensity increase of the dangling O-H band by a factor of 3-5. Furthermore, the onset temperature of nitrogen adsorption at the water cluster surface was experimentally found to be roughly 30 K for cluster sizes of about 100 water molecules. Experimental and computational results are in good agreement. The presented results are based on and support the work of V. Buch, J. P. Devlin, and co-workers (e.g., J. Phys. Chem. B, 1997; J. Phys. Chem. A, 2003; Int. Rev. Phys. Chem., 2004).
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ABSTRACT: Amorphous solid water (ASW) is one of the most widely studied molecular systems because of its importance in the physics and chemistry of the interstellar medium and the upper layers of the Earth's atmosphere. Although the global structure of this material, i.e. the bulk and the surface, is well characterised, we are far from having an overall understanding of the changes induced upon chemical or physical perturbation. More specifically, the behaviour of the surface and the immediate sublayers upon mid-infrared irradiation must be understood due to its direct effect on the adsorption capacities of the ASW surface. Small molecules can accrete or form at the surface, adsorbed on the dangling OH groups of surface water molecules. This behaviour allows further reactivity which, in turn, could lead to more complex molecular systems. We have already demonstrated that selective IR irradiations of surface water molecules induce a modification of the surface and the production of a new monomer species which bonds to the surface via its two electronic doublets. However, we did not probe the structure of the dangling bands, namely their homogeneity or inhomogeneity. The structure and orientation of these surface molecules are closely linked to the way the surface can relax its vibrational energy. In this work, we have focussed our attention on the two dH dangling bonds, carrying out a series of selective irradiations which reveal the inhomogeneity of these surface modes. We have also studied the effects of irradiation duration on the surface reorientation, determining that the maximum photoinduced isomerisation yield is ∼15%.Physical Chemistry Chemical Physics 03/2015; 17(14). DOI:10.1039/c5cp00662g · 4.20 Impact Factor
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ABSTRACT: We have examined the adsorption of the weakly bound species N2, O2, CO, and Kr on the (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 "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, "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.The Journal of Chemical Physics 11/2014; 141(18):18C515. DOI:10.1063/1.4896226 · 3.12 Impact Factor