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Abstract

Absolute photoabsorption cross sections of negatively charged tetra-atomic aluminum clusters have been measured for photon energies between 1.8 and 2.7 eV. The experiment used the depletion technique in combination with an electrostatic ion-beam trap, in which Al₄⁻ions produced in a sputter ion source were stored for 90 ms before being subjected to a short laser pulse. Moreover, the competition between one-atom fragmentation and electron emission of the laser-excited Al₄⁻ has been measured. These measurements show that fragmentation dominates electron emission at all photon energies below the electron attachment energy of ∼2.2 eV, even though the fragmentation energy is expected to be 10%–20% higher than the electron attachment energy. These findings, when taken together with the delayed-electron and fragmentation yields observed in a previous measurement [O. Aviv et al., Phys. Rev. A 83, 023201 (2011)], can be well explained within the statistical phase-space theory for unimolecular decays assuming the Al4− ions to be rotationally hot. The analysis permits the determination of the adiabatic electron detachment energy of Al4− to be Ead=(2.18±0.02) eV and the one-atom fragmentation energy to be D0=(2.34±0.05) eV. Moreover, two direct s-wave ionization channels are observed with threshold energies of (2.18±0.02) eV and (2.45±0.02) eV.