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

Abstract

A systematic study of the competition between delayed electron and delayed atom emission from photoexcited Al${}_{4}^{\ensuremath{-}}$ 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 $<{E}_{\ensuremath{\lambda}}<$ 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${}_{4}^{\ensuremath{-}}$ 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 $~0.5$ 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${}_{4}^{\ensuremath{-}}$ clusters carrying high rotational angular momenta are discussed as possible candidates. Results obtained for other aluminum clusters Al${}_{n}^{\ensuremath{-}}$ ($5\ensuremath{\leqslant}n\ensuremath{\leqslant}9$) are also reported.