The effect of sequential oxidation and composition on the structural and electronic properties of gas-phase transition-lanthanide bimetallic clusters.

Abstract

This thesis presents experimental and theoretical work performed on various rhodium-holmium (Rh-Ho) and gold-prascedymium (Au-Pr) bimetallic clusters and their oxide counterparts. More specifically, structural and /or electronic properties for these clusters are ascertained from investigating how their adiabatic ionisation energies (IEs) are affected by: (a) the sequential addition of oxygen atoms onto the base bimetallic cluster within each series, or (b) the composition of the bimetallic clusters i.e. the transition:lanthanide metal atom ratio within each cluster. The clusters were experimentally generated via dual laser ablation and detected using time-of-flight mass spectrometry (TOF-MS) coupled with threshold laser ionisation. Upon successful formation and detection, the experimental adiabatic lEs of these clusters were determined using Photoionisation Efficiency (PIE) spectroscopy. In regards to aspect (a) of this thesis listed above, it was observed that, the sequential addition of individual oxygen atoms onto bare Rh-Ho and Au-Pr clusters either caused: (i) a significant change in or (ii) had little-to-no effect on the experimental adiabatic IE. For clusters that displayed the former behaviour, the addition of the first oxygen atom was observed to significantly decrease the adiabatic IE relative to that of the bare bimetallic cluster within that series. The addition of a second oxygen atom onto the monoxide counterpart was observed to significantly increase the adiabatic IE back to a value similar to that of the bare bimetallic cluster. In regards to aspect (b) of this thesis listed above, it was observed that: (i) the substitution of a transition metal atom for a lanthanide metal atom generally lowers the experimental adiabatic IE of each cluster, and (ii) the sequential addition of transition metal atoms onto a cluster generally increases the experimental adiabatic IE of each cluster. In order to gain more insight into the nature of the observed experimental adiabatic IE trends mentioned above, Density Functional Theory (DFT) Investigations were performed on the neutral and cationic species for the RhHo₂On [n subscript](n = 0-2), Rh₂Ho₂Om [m subscript](m = 0-2) and the Au3-kPrk [3-k subscript & k subscript](k = 0-3) clusters. From these, the lowest energy neutral and cationic geometries (in addition to other properties such as atomic charges and normal modes of vibration) were determined and subsequently, the theoretical adiabatic IEs of each cluster were calculated. When compared within each series, the experimental and theoretical adiabatic IE trends as a function of: (i) sequential addition of oxygen atoms in the RhHo₂On [n subscript] (n = 0-2) and Rh₂Ho₂Om [m subscript] (m = 0-2) cluster series and (ii) substitution of a gold atom for a praseodymium atom in the Au3-kPrk [3-k subscript & k subscript](k = 0-3) cluster series, both displayed similar behaviour. From this, specific ionisation transitions between neutral and cationic structures were able to be assigned and thus, structural and electronic information about each cluster was able to be inferred. In addition to the DFT investigations, Franck-Condon Factor (FCF) calculations were performed in order to simulate the Zero Electron Kinetic Energy (ZEKE) and PIE spectra for each cluster in the RhHo₂On [n subscript], (n = 0-2), Rh₂Ho₂Om [m subscript] (m = 0-2) and Au3-kPrk [3-k subscript & k subscript](k = 0-3) series. The purposes of these additional calculations were to: (i) identify the most likely transition from two or more competing candidates that occurs upon ionisation for each cluster, and (ii) apply slight corrections to the experimental adiabatic IEs obtained from the PIE spectra in order to account for thermal tailing resulting from vibrational hot band transitions at 300 K.