IONIZATION INDUCED FRAGMENTATION OF SIZE-SELECTED NEUTRAL SODIUM CLUSTERS

The fragmentation behavior for different ionization methods of size se lected neutral sodium clusters n less than or equal to 5 after ionizat ion in a time scale from picoseconds up to tens of microseconds is the subject of this work. Size selection by scattering the cluster beam f rom 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 frag mentation of the neutral clusters depends on the different types of io nization and moreover on the excess energy that is deposited into the cluster during the ionization process. The fragmentation behavior of t he sodium trimer follows clearly the energetics. No fragmentation with energetically well-defined 290 nm photons from a laser (0.38 eV exces s energy), slight fragmentation with 265 nm laser photons (0.78 eV exc ess energy), severe fragmentation caused by the photons of the broad b and spectrum from the Xe are lamp with lambda greater than or equal to 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 size s, mostly ih agreement with the energetics that result from calculated and experimental ionic and calculated neutral dissociation energies c ombined with experimental ionization potentials. An exception is the t etramer fragmentation using the Xe are lamp with the preferential prod uct ion Na-2(+). In the measured angular distributions, the fragmentat ion processes Na-n --> Na-2(+) (7 less than or equal to n less than or equal to 9) and Na-8 --> Na-5(+) are observed.