RELAXATION DYNAMICS OF NEUTRAL IRON OXIDE CLUSTERS USING FEMTOSECOND PUMP-PROBE SPECTROSCOPY

Iron oxides have been used in a number of societally important catalytic processes; however, the molecular-level details behind their reaction mechanisms have been a challenge to observe. Molecular clusters have gained experimental attention due to their ability to model bulk materials, ease of production, and direct application for gaining atomic level insights. Using laser ablation combined with femtosecond pump-probe spectroscopy, the gas-phase dissociation and excitation-relaxation dynamics of neutral iron clusters in the presence of oxygen are observed. Dissociation/fragmentation time of neutral iron oxide clusters from (FeO)$_{n}$ (n = 1-10) decreases with increased molecular size from $>$300 fs to $\sim$150 fs. Clusters deviating from the 1:1 stoichiometry are generally seen to increase in relaxation time, owing to an increased stability induced by a caging effect from atomic oxygen. A notable example of dissociation stability is observed with the addition of O atoms from Fe$_{2}$ $\rightarrow$ Fe$_{2}$O$_{2}$ which decreases in relaxation time from $>$650 fs to $<$250 fs. The molecular-level insights from these cluster studies provides a more comprehensive understanding for the design of future catalysts, leading to iron oxide materials with increased reactivity and decreased impact