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...

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We have measured the predicted isomeric transition between the two energetically lowest lying isomers of S6 and C2 symmetry of (CH3OH)6. The clusters are size selected by momentum transfer in collisions with He, and identified by their infrared spectra. The transition occurs at a nozzle temperature of 368 K corresponding to a cluster temperature of 23 K.

Vibrational predissociation spectra of hydrazine ͑N 2 H 4) n clusters have been measured from the dimer to the tetramer using a linetunable, isotopically substituted CO 2-laser in order to fill the frequency gap between 990 and 1010 cm Ϫ1. The clusters are size selected in a scattering experiment with helium atoms. The large blue shifts of the asymmetric NH 2 wag mode at 937 cm Ϫ1 are completely interpreted by calculations based on a recently determined systematic model potential. The gross shifts of 60 cm Ϫ1 for the dimer, 80 cm Ϫ1 for the trimer, and 110 cm Ϫ1 for the larger clusters are explained by the different structures: Cyclic arrangements with two hydrogen bonds per molecule for the dimer, rings with one hydrogen bond per molecule for the trimer, and three-dimensional structures for the larger ones. The peaks in the spectra are caused by characteristic vibrations to which more than one isomer contributes.

We have measured the isomeric transition between the energetically lowest lying isomers of S6 and C2-symmetry of (CH3OH)6. The clusters are size-selected by deflection in collisions with He, and the isomers are identified by their infrared spectra of the OH-stretching vibration. The measurements are carried out at three source temperatures 253, 300 and 373 K which correspond to the cluster temperatures 93, 106 and 135 K. The latter ones are estimated by a relaxation model that accounts for the cluster formation and the energy released by the condensation. The transition takes place at a cluster temperature of about 102 K which is in agreement with the Molecular Dynamics simulation of such a transition at about 117 K using a realistic model potential.