Femtosecond dynamics of solvated oxygen anions. II. Nature of dissociation and caging in finite-sized clusters

Ultrafast dissociation and recombination dynamics of (O_2)^−_n, n=3–10 was studied using femtosecond,time-resolvedphotoelectron spectroscopy. The observed transients of nascent fragment anions, following 800 nm fs pulse excitation, exhibit a biexponential rise with two distinct time constants. The time constants, which vary with the number of solvent O_2 molecules, clearly show the solvation effect in two different dissociation pathways. Consistent with the bifurcation picture in the preceding paper, the direct subpicosecond dissociation(τ_1=110-620 fs, depending on n) is governed by electron recombination and kinematics of the half-collision. The second pathway is indirect (τ_2=0.7–8.0 ps, for O^−_6 to O^−_(20)) and controlled by intramolecular vibrational-energy redistribution. In the solvent cage, only O^−_(16), O^−_(18), and O^−_(20) show the reformation of the bond, with the caging time constant decreasing from 4 ps for the first two to 2 ps for the latter. This caging through ion-induced dipole interaction is then followed by vibrational relaxation on the time scale of 12 to 3 ps, for O^−_(16) to O^−_(20). The time scale for the initial direct caging is two to five times slower than that previously observed for diatoms, neutral, or ionic, in van der Waals clusters. We suggest that this initial slower caging is due to the reorientation of O^−_2 and O_2 to acquire a proper geometry for O^−_4 bond reformation. In these finite-sized homogeneous clusters, we compare theory with experiment. We also found a correlation between the vertical detachment energy and n^(−1/3), for n in the range of 2–10, which allow for a connection between the mesoscopic structures and a bulk-type dielectric continuum, with an effective dielectric constant