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The vibrational OH-stretch spectra of large water clusters were measured by photofragment spectroscopy after the absorption of pulsed tunable infrared radiation in the frequency range from 3000 to 3800 cm -1 . The mean size of the clusters from 〈n〉 ) 20 to 1960 was measured by threshold photoionization of the water clusters doped with sodium atoms. The largest abundance of the fragments was that of water hexamers. The fragment intensities are measured for different excitation energies and different cluster temperatures as function of the cluster size. For the selected sizes 〈n〉 ) 48, 111, 631, and 1960 complete OH-stretch spectra have been measured. The comparison with calculations revealed that the method is mainly sensitive to the outer cluster surface which has for all sizes an amorphous structure dominated by 3-coordinated and to a lesser extent also by 4-coordinated molecules. The intensity of the hexamer fragments goes through a maximum at n ) 70 and drops to n ) 300 where it levels off with a different slope. This behavior is attributed to the number of available connected 3-coordinated water molecules and the influence of the emerging 4-coordinated molecules in these clusters.

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In a continuous neat supersonic expansion ammonia clusters are generated and doped with sodium atoms in a pickup cell. Thus clusters of the form Na(NH(3))(n) are produced that are photoionized by a tunable dye laser system. The ions are mass analyzed in a reflectron time-of-flight mass spectrometer, and the wavelength dependent ion signals serve for the determination of the ionization potentials (IP) of the different clusters in the size range 10< or =n< or =1500. Aside from a plateau for 10< or =n< or =17 and smaller steps at n=24, 35, and 59 on the average a continuous decrease of the IP with cluster size is observed. The IPs in this size range are linear with (n+1)(-13) and extrapolate to IP(n=infinity)=1.66+/-0.01 eV. The slope is consistent with a dielectric continuum model of the solvated electron and the dielectric constant of the solid. The extrapolated IPs are compared with results obtained for negative ammonia cluster ions and metallic solutions in liquid ammonia. Differences are explained by the presence of counterions and their various distances from the solvated electron.

The vibrational OH stretch spectra have been measured for Na(H2O)n clusters in the size range from n = 8 to 60. The complete size selection is achieved by coupling the UV radiation of a dye laser below the ionization threshold with the tunable IR radiation of an optical parametric oscillator. The spectra are dominated by intensity peaks around 3400 cm(-1) which we attribute to an increased transition dipole moment of delocalized electrons in this type of doped cluster. Aside from the positions of free (3715 cm(-1)) and double donor (3560 cm(-1)) bonds which are known from pure water clusters, specific transitions are observed at 3640 cm(-1) and in the range of the single donor bonds between 3000 and 3200 cm(-1).

In a pickup arrangement, the reactive scattering of sodium atoms and dimers with water clusters (H2O) m , m ≤ 140, is investigated. The water cluster beam is generated in a pure supersonic expansion and crosses an atmosphere of sodium vapor. The reaction products are detected by photoionization at a wavelength of 360 nm (3.45 eV) and mass analyzed in a reflectron time-of-flight mass spectrometer. Doping the H2O clusters with Na atoms results in the formation of solvated sodium atoms Na(H2O) m , representing the distribution of the pure water clusters. At higher sodium pressures also Na2 dimers are produced, which then react in a second step with the solvated Na atoms to products of the type Na(NaOH)2(H2O) m and, to a lesser extent, also to Na(NaOH)4(H2O) m . The size range follows that of the precursors with a pronounced maximum of intensities between m = 6 and 14 in the former case.

In a crossed molecular beam experiment, the reactive scattering of sodium clusters Na n (n E 21) with water clusters (H 2 O) m (m E 40) is investigated. By measuring the angular and the velocity distributions of the scattered reaction products, direct information on the reaction mechanisms is obtained. The sodium clusters are generated in a supersonic expansion of sodium vapor from an oven with a refilling system with argon carrier gas. The products are detected by photoionization at wavelengths of 308 and 355 nm and mass analyzed in a time-of-flight mass spectrometer (TOF-MS). In addition, a fast chopper with a pseudorandom sequence modulating the sodium cluster beam allows us to determine at the same time the velocity distributions of the products. The scattering of Na clusters with H 2 O clusters shows only one series of reaction products, that is, solvated sodium atoms Na(H 2 O) m (m E 32) with maximum intensities at m ) 4, 5, 6. For all products, the measured angular and velocity distributions exhibit the formation of a complex that is stabilized by isotropic evaporation of water molecules. The detected products have low translational energy. Products containing NaOH have not been observed.