The vibrational OH stretch spectra have been measured for size-selected pure water clusters ͑H 2 O͒ n , in the size range n 8 10. Comparison between experiment and calculations suggests that the spectra originate from a small number of "microcrystalline" structures, based on the cubic octamer. The n 8 spectra are caused by two isomers of D 2d and S 4 symmetry. The proposed lowest energy nonamer and decamer structures are derived from the octamer by insertion of one and two two-coordinated molecules, respectively, into the cube edges. [S0031-9007 (98)05604-X] PACS numbers: 36.40.MrWater clusters have been the focus of intense interest during the last several years. One of the objectives is to understand how the properties of ͑H 2 O͒ n evolve towards condensed phase behavior. One may ask, for example, at which size a cold cluster starts displaying attributes of a small crystal. The initial stages of the evolution as a function of n have been largely understood. Recently, an elegant series of far-infrared vibration-rotation-tunneling [1,2] and infrared [3] laser spectroscopic studies demonstrated conclusively a cyclic structure for n 3 5, and a transition towards a three-dimensional cage structure at n 6 [4]. Similar conclusions were drawn from the double resonance ion-dip infrared experiments on water clusters connected to a benzene molecule [5]. At present, the challenge is to understand the evolution in the "cage" regime n $ 7. A number of theoretical studies addressed this problem [6-9]. The only pertinent experimental results focus on the OH stretch spectroscopy of larger clusters without size selection [10,11] and n 7, 8 clusters attached to benzene [5,9].The present Letter reports the first measurement of the infrared spectroscopy of the OH stretch mode of pure water clusters in the size range n 8 10. The OH stretch spectra of hydrogen bonded H 2 O, which are redshifted by hundreds of cm 21 with respect to gaseous H 2 O, are known to be strikingly sensitive to hydrogen bond coordination and to bonding geometry [7,12]. Thus they can be used to probe the cluster structure. Moreover, the spectroscopy of clusters can serve as a benchmark for the calibration of flexible force fields for studies of condensed H 2 O and H 2 O surfaces.The experimental method which we apply is a combination of size selection by momentum transfer in a scattering experiment with atoms, with the infrared depletion technique [13,14]. This technique has been developed in our laboratory in Göttingen and mainly applied to cluster sizes n # 6. In the first step the different clusters are dispersed into different angles according to their masses and detected by a mass spectrometer. Then the OH stretch vibrational mode of the water molecules is excited by infrared laser radiation. The detector records the depletion in the cluster signal caused by the clusters which are dissociated by the absorbed radiation.The experimental setup consists of a crossed molecular beam apparatus with an angular dependent detection of the scattered beam with a resol...

The vibrational OH stretch spectra have been measured for the size-selected pure water clusters (H2O)7. In contrast to (H2O)n, n=8–10 clusters, which exhibit three distinct bands corresponding to three distinct types of OH bonds, the heptamer spectrum displays seven bands spanning the range from 2935 to 3720 cm−1. Calculations suggest that the spectra originate from two isomers, derived from the S4 octamer cube by removal of either one double donor or one double acceptor water molecule.

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The oxidation of ammonia to nitrogen or nitric oxide, respectively, was investigated using on the one hand a pol ycrystalline copper foil and on the other hand deposited copper clusters prepared with the inert gas aggregation technique. The behavior in the ammonia oxidation of both model catalysts was studied using in situ near edge X-ray absorption fine structure (NE XAFS) spectroscopy in the soft X-ray range and mass spectrometry. It is shown that the copper foil reacts in a similar way as the copper clusters. Differences appear only with respect to the reaction temperature required, which is lowered for the cluster sample. It can be concluded that results obtained in experiments with polycrystalline copper foil are exemplary for and can be transferred to supported copper catalyst consisting of small copper particles.

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