Effect of μ₄-O Sites on the Ultrafast Dynamics of Neutral Magnetic Copper Oxide Clusters

The ultrafast dynamics of subnanometer neutral copper oxide clusters is examined with pump-probe spectroscopy. Upon absorption of an ultraviolet (400 nm) photon, all clusters exhibit a subpicosecond lifetime that we attribute to carrier relaxation. We calculate the ground-state structures and spin configurations of the neutral clusters by using density functional theory to identify structural features that facilitate relaxation. By comparing results across 35 clusters, ranging between Cu₃O₃ and Cu₁₆O₈, we find evidence that strong ferromagnetic coupling influences the relaxation dynamics of copper oxide clusters. The total spin magnetic moments increment with addition/subtraction of each Cu atom away from the (Cu₂O)_n stoichiometry and reach as high as 5μ_B in the larger clusters examined. The excited-state lifetimes increase almost linearly by ∼40 fs per additional Cu atom on the subpicosecond time scale. In each series, the formation of μ₄-O atoms associated with terminal Cu atoms consistently deviates from the trendline by increasing excited-state relaxation rates.