Mass-selected photodissociation studies of AlPbn+clusters (n =7–16): Evidence for the extraordinary stability of AlPb10+and AlPb12+

We report fragmentation pathways and dissociation energies of AlPb${}_{n}^{+}$ ($n=7$--16) clusters. The clusters are produced with pulsed laser vaporization and studied in a supersonic molecular beam setup. They are mass selected and photodissociated with 532 and 355 nm laser light. Photofragments are thereafter mass separated in a tandem reflectron time-of-flight mass spectrometer. Bare Pb${}_{n}^{+}$ ($n$ $=$ 8--16) clusters preferentially evaporate Pb atoms, with the exception of Pb${}_{15}^{+}$ that fragments by loss of a Pb${}_{2}$ dimer to form the stable Pb${}_{13}^{+}$ cluster. The smallest AlPb${}_{n}^{+}$ ($n$ $=$ 7--11) clusters also show mainly atomic Pb evaporation, whereas the favored fragmentation pathway of the larger clusters ($n$ $=$ 12--16) involves Pb${}_{2}$ and Pb${}_{3}$ fragments. AlPb${}_{10}^{+}$ and AlPb${}_{12}^{+}$ are the most intense fragments of several larger cluster sizes, demonstrating the high stability of these two sizes. Dissociation energies corresponding to the most facile fragmentation channel of AlPb${}_{n}^{+}$ ($n$ $=$ 11--15) are bracketed from the measured laser fluence dependencies of the fragment intensities using constraints imposed by unimolecular reaction rates.