Competition between delayed ionization and fragmentation of laser-excited Al4-

A systematic study of the competition between delayed electron and delayed atom emission from photoexcited Al-4(-) clusters was performed using a bent electrostatic ion beam trap, which allows for simultaneous measurement of both decay channels. The aluminum cluster anions, produced with high internal energy in a sputter source, are stored for 100 ms prior to their excitation by a short laser pulse. Delayed ionization and delayed fragmentation were observed for photon energies 1.55 eV < E-lambda < 2.25 eV. Decay spectra, integrated relative cross sections, and branching ratios were determined as a function of photon energy and storage time. The results show the characteristic pattern expected for a statistical decay ofthe photoexcited Al-4(-) clusters; however, the ratio between fragmentation and ionization is found to increase with decreasing photon energy, even though the threshold energy for fragmentation is calculated to be similar to 0.5 eV higher than the adiabatic electron detachment energy. It is suggested that the tetramers, while cooling down in the trap, may be in long-lived excited configurations with lower effective fragmentation barriers; Al-4(-) clusters carrying high rotational angular momenta are discussed as possible candidates. Results obtained for other aluminum clusters Al-n(-) ( 5 <= n <= 9) are also reported.