Photon and electron impact fragmentation of size selected sodium clusters

Abstract

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...

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