PHOTON AND ELECTRON-IMPACT FRAGMENTATION OF SIZE-SELECTED SODIUM CLUSTERS

We investigate the fragmentation behaviour of size selected sodium clu sters Na(n), n less-than-or-equal-to 5, for different ionization metho ds. The experimental technique, size selection by means of scattering the cluster beam from a helium target beam, enables us to find a uniqu e correlation between the detected cluster ions and their neutral prec ursors. 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 nm, w hereas for the pulsed, frequency doubled dye laser the wavelength is t uned to 265 and 290 nm. Electron impact ionization has been examined w ith an electron energy of 25 eV. The ionization by electron impact wit h the large excess energy results in a total fragmentation of the sodi um trimer. In the case of photoionization the fragmentation probabilit ies depend sensitively on the excess energy.