PHOTODISSOCIATION AND SIZE ANALYSIS OF (HBR)(N) CLUSTERS

Hydrogen bromide clusters that are generated in an adiabatic expansion with Ar are size-selected by scattering from a He or a Ne beam and a subsequent separation by a velocity selector. The complete fragmentati on pattern for electron impact ionization of clusters n = 3, 5, 7, 10, and 15 is obtained for electron energies of 70 eV. Up to cluster size s of n = 15 strong fragmentation is observed. Based on these results t he cluster sizes are measured as a function of different source condit ions. For the same experimental conditions the (HBr)(n) clusters are p hotoionized at 243.1 nm. A time-of-flight mass spectrometer with low e xtraction fields is used to obtain the translational energy distributi ons of hydrogen atoms. Several distinct features are observed which se nsitively depend on the cluster size. At small cluster sizes a tail ap pears towards lower energies in the fast hydrogen kinetic energy distr ibution which is caused by few collisions with the cluster molecules. Peaks at higher energies are attributed to internally excited HBr mole cules which gain their energy in collisions with photofragmented PI at oms. Small clusters exhibit pure rotational excitation with averaged J = 7, while the larger ones show vibrational excitations to upsilon = 1 with J = 8 and upsilon = 2. Finally, we observe a narrow H component peaking near zero velocity and exhibiting an isotropic angular distri bution. This effect appears only at larger cluster sizes and is attrib uted to a complete caging of the photofragments.