Size onset of metallic behavior in neutral aluminum clusters

The excited state lifetimes of neutral (Al)_n clusters up to ∼1 nm in diameter in size, where n ≤ 43, are systematically measured with femtosecond time-resolved mass spectrometry. The onset of metallic behavior is identified as a distinct change in the relaxation behavior initiated with single ultraviolet (400 nm) photon excitation. The experimentally measured excited state lifetimes gradually decrease with size for small molecular scale clusters (n < 10) before becoming indistinguishable for larger clusters (n > 9), where the measurements are comparable to electron-lattice relaxation time of bulk Al (∼300 fs). Particularly intense, or magic, Al_n clusters do not exhibit any significant excited state lifetime behavior. Time-dependent density functional theory quantify the excited state properties and are presented to show that dynamics are strongly tied to the excited state charge carrier distributions and overlap, rather than detailed changes related to changes in the cluster's electronic and geometric structure. The consistency in excited state lifetimes for clusters larger than n = 9 is attributed to the hybridization of the s- and p-orbitals as well as increasing delocalization. Al₃ exhibits unique temporal delay in its transient behavior that is attributed to a transition from triangular ground state to linear structure upon excitation.