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: 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
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ABSTRACT: Amorphous solid water (ASW) is one of the most widely studied molecular systems. Ubiquitous in the interstellar medium (ISM) and potentially present in the upper layers of the Earth’s atmosphere, ASW plays a major role in heterogeneous physical chemistry. Small molecules form or accrete at the ice surface, bonding to water molecules with an OH bond projecting from the surface, so-called “dangling bonds”. These dangling OH are of crucial importance in the quest to identify and quantify surface reactions. Water ices in the ISM or Earth’s atmosphere undergo constant processing by thermal and irradiation effects, which can significantly affect both the bulk and surface structures and therefore the catalytic properties of the surface. In this work we have studied thermal and irradiation processing of ASW and determine that there is a photochemical processing pathway of the ice surface which is clearly distinct from purely thermal effects. Selective IR irradiations of each of the surface water modes led to the observation of a “hole-burning” at the irradiation frequency, counterbalanced by the production of a new band, identified as a water monomer interacting with the surface. The thermal effects, meanwhile, led to a global decrease of all the dangling modes due to global reorganization of the water ice structure. It is thus obvious that, depending on the processing history of an ice, its catalytic properties will not be affected in the same way. The fact that we observe an IR selective irradiation effect illustrates that some fraction of the vibrational energy, rather than being relaxed through the H-bonded network of the bulk ice, is trapped at the surface; this energy induces a reorganization of the surface structure, forming new trapping sites, and thus generating new catalytic properties.The Journal of Physical Chemistry C 07/2014; 118(35):20488. DOI:10.1021/jp506943k · 4.84 Impact Factor