Ultrafast Dynamics of Strongly Correlated Metal Oxide Clusters

A local rearrangement of 3d-electrons plays a crucial role in many properties of correlated transition metal oxide materials and supplies important information on their reactivity and magnetic interactions. Point defects, such as oxygen vacancies and their associated electrons, can lead to emergent phenomena that influence the lifetime of photoexcited states and therefore influence chemical reactivity. The ultrafast electronic relaxation dynamics of neutral metal oxide clusters were investigated with femtosecond pump-probe spectroscopy and supported by theoretical calculations to show that their excited state lifetimes are strongly dependent on the nature of the electronic transition.^1–3We employ the atomic precision and tunability of gas phase clusters to stress that changes to the character of the orbital of the photoexcited electron leads to vastly different relaxation dynamics. I will present our recent work on sub-nanometer clusters, where absorption of a UV (400 nm) photon initiates several relaxation processes. The simple picture of sequential oxidation of metal atoms reveals a linear tunability to the contributions of each relaxation component to the total transient signal. In particular, in chromium oxides a ~30 fs transient signal fraction grows linearly with oxidation, matching the amount of O to Cr charge transfer character of the photoexcitation and highlighting a gradual transition between semiconducting and metallic behavior at the molecular level. In contrast, the photoexcited lifetimesof nickel oxide exhibit a unique reliance on the nature of the atomic orbital contributions, providing new insights to the analogous band edge excitation dynamics of strongly correlated bulk-scale NiO material. Short lived dynamics in stoichiometric (NiO)_nclusters are attributed to excitation between Ni-3d and Ni-4s orbitals, where their strong exchange coupling produces metallic-like electron-electron scattering. Oxygen vacancies introduce 3d→4p transitions, which increases the lifetimes of the sub-picosecond transient by 20-60% and enables the formation of long-lived (lifetimes &gt; 2.5 ps) states.<br/><br/>(1) Garcia, J. M.; Heald, L. F.; Shaffer, R. E.; Sayres, S. G. Oscillation in Excited State Lifetimes with Size of Sub-Nanometer Neutral (TiO₂)_nClusters Observed with Ultrafast Pump–Probe Spectroscopy. <i>J. Phys. Chem. Lett.</i><b>2021</b>, <i>12</i>, 4098–4103.<br/>(2) Garcia, J. M.; Sayres, S. G. Increased Excited State Metallicity in Neutral Cr₂O_nClusters (n &lt; 5) upon Sequential Oxidation. <i>J. Am. Chem. Soc.</i><b>2021</b>, <i>143</i>(38), 15572–15575.<br/>(3) Garcia, J. M.; Sayres, S. G. Orbital-Dependent Photodynamics of Strongly Correlated Clusters. <i>Under Review</i>. <b>2021</b>.