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.

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The fragmentation behavior for different ionization methods of size selected neutral sodium clusters n≤5 after ionization in a time scale from picoseconds up to tens of microseconds is the subject of this work. Size selection by scattering the cluster beam from a He target beam in combination with a quadrupole mass filter and time resolved measurements at specific laboratory angles enables us to determine the neutral precursor masses of the detected ions. The fragmentation of the neutral clusters depends on the different types of ionization and moreover on the excess energy that is deposited into the cluster during the ionization process. The fragmentation behavior of the sodium trimer follows clearly the energetics. No fragmentation with energetically well-defined 290 nm photons from a laser (0.38 eV excess energy), slight fragmentation with 265 nm laser photons (0.78 eV excess energy), severe fragmentation caused by the photons of the broad band spectrum from the Xe arc lamp with λ≥220 nm (up to 1.75 eV excess energy), and finally total fragmentation under electron impact ionization with 25 eV electrons (2–4 eV excess energy). Similar results have been obtained for the other cluster sizes, mostly in agreement with the energetics that result from calculated and experimental ionic and calculated neutral dissociation energies combined with experimental ionization potentials. An exception is the tetramer fragmentation using the Xe arc lamp with the preferential product ion Na+2. In the measured angular distributions, the fragmentation processes Nan→Na+2 (7≤n≤9) and Na8→Na+5 are observed.

A supersonic sodium cluster beam source with a refilling system is described in detail. Due to the external refilling system a new loading of the oven, which is designed for high-temperature application at 800 °C, is accomplished very fast and interrupts the measuring process only for 1–2 h. Additional purification of the alkali metal in the reservoir allows the usage of standard quality sodium samples.

A b s t r a c t . We investigate the fragmentation behaviour of size selected sodium clusters Na~, n < 5, for different ionization methods. The experimental technique, size selection by means of scattering the cluster beam from a helium target beam, enables us to find a unique correlation between the detected cluster ions and their neutral precursors. In the case of photoionization we use the light from a Xe-arc lamp in a broad spectral range down to a wavelength of about 220 rim, whereas for the pulsed, frequency doubled dye laser the wavelength is tuned to 265 and 290 nm. Electron impact ionization has been examined with an electron energy of 25 eV. The ionization by electron impact with the large excess energy results in a total fragmentation of the sodium trimer. In the case of photoionization the fragmentation probabilities depend sensitively on the excess energy.PACS: 36.40+d I n t r o d u c t i o nIn recent years new experimental studies on the interaction of alkali metal clusters with radiation attracted increasing attention [1][2][3]. Aside from different available production methods [3][4][5], the two most prominent detection schemes remain cluster ion formation due to electron impact ionization and photoionization. Especially the former one, in which electron impact energies up to about 100 eV are used, leads to significant fragmentation of the neutral clusters. In contrast, photoionization is generally thought of being the "softer" ionization technique so that, at least, the near threshold ionization is expected to be free of fragmentation. But aside from one experiment with reneutralized mass selected sodium cluster ions [6], detailed experimental information about the fragmentation process of metal clusters is rare. With the recently developed method, the size selection of neutral clusters by means of scattering from a target beam [7,8], we are able to extract quantitative information on the fragmentation probabilities of size selected neutral clusters. Using this method, in the present contribution results are presented on the fragmentation probabilities of sodium clusters Nan, n _< 5, for electron impact ionization at 25 eV as well as photoionization using different light sources. Experiment ~.I. Crossed beam apparatusThe crossed beam apparatus used in the experiments has been described in detail elsewhere [9]. Briefly, the two supersonic nozzle beams, which intersect at an angle of 900 in the triangular shaped scattering chamber, are generated in two slightly different oven chambers which are fixed on the side walls of the scattering chamber. The detector unit is rotated around the scattering center with help of three half cylindrical segments. In addition to the detection scheme described in [9] which consists of an electron bombardment ionizer, an on-axis mounted quadrupole mass filter and a particle multiplier, a second independent detector for photoionization has been incorporated in the actual setup. The quadrupole rod system of this detector is mounted orthogonal to the beam axis and the ph...