Ionization induced fragmentation of size selected neutral sodium clusters

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.