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Publication TypeThesis
Year of Publication2017
AuthorsXu, S
Academic DepartmentPhysics
Number of Pages236
Date Published2017

The properties of many important molecules are only known from condensed phase studies, where the interaction with solvent or other chemical environments masks the intrinsic properties of the solute molecule. Consequently, these intrinsic properties cannot be easily obtained from condensed phase experiments. Spectroscopy of gas-phase molecular ions in vacuo is an attractive alternative to spectroscopic studies in the condensed phase. Ions in vacuo are isolated systems and free from perturbation inflicted by a chemical microenvironment such as a solvent or crystal lattice. Consequently, it becomes possible to examine the intrinsic properties of the ions in vacuo. Furthermore, gas-phase ions can be trapped, cooled and mass-analyzed, which provides researchers with the opportunity for controlling and selecting ion species and conditions. This thesis comprises my work on photodissociation spectroscopy of several series of metal-polypyridine ions in vacuo. These species have been extensively studied in inorganic chemistry because they serve as potential candidates for broad industrial applications and as prototypical systems to study fundamental scientific questions such as the mechanisms of metal-ligand interaction and chemical reaction. This thesis describes the first investigation of such systems in the gas phase using laser spectroscopy in the visible and UV spectral ranges. 

In this work, the gas-phase ions are produced by electrospray ionization. The ions are trapped and buffer-gas cooled in a cryogenic quadrupole ion trap and then undergo mass-selection in a time-of-flight mass spectrometer. The selected ion species are irradiated by a laser pulse with widely tunable photon energy. If photoabsorption leads to dissociation, fragments will be separated and analyzed by a reflectron. The yield of fragment ions is monitored as a function of photon energy and recorded as a photodissociation spectrum. While absorption spectra in solutions of the species studied here are usually broad and featureless, the photodissociation spectra acquired in the gas phase show new resolved features, often even at room temperature. 

Four series of ions were studied and the results are presented in this thesis. The first two are [Cu(bpy)-L]+ and [Ru(bpy)(tpy)-L]2+, where bpy = 2,2’-bipyridine and tpy = 2,2′:6′,2′′-terpyridine, and L represents a variable ligand. The choices for L in the two types of complexes are different, but they both form a series from weakly (e.g. N2) to strongly interacting ligands (e.g. Cl, CH3CN). It is found that the ligand can dramatically shift the molecular orbitals and change the electronic structure of the ion. Another critical element in the ion is the metal. To explore how the metal affects the properties of the ion, the photodissociation spectra of a series of [MII(bpy)3]2+ ions were measured. Here, M represents a transition metal, and in this study, several first-row transition metals and the d6-column metals are included (Mn, Fe, Co, Ni, Cu, Zn, Ru, Os). The experimental data suggest that the [MII(bpy)3]2+ ions can have significantly different excitation energies, excited-state lifetimes and symmetries. Finally, the complex ion can be generated in microsolvated states by condensing solvent molecules onto the bare ion in the cryogenic ion trap. By experimenting on mass-selected ions solvated by a well-defined number of solvent molecules, it is possible to observe the solvation effect in a stepwise fashion, i.e. one solvent molecule at a time. The target ion used in this study is [Ru(bpy)(tpy)-OH2]2+ and it is found that solvation by only four water molecules already contributes more than 75% of the full solvatochromic shift exhibited in its bulk solution. By comparing the geometry and the spectrum of each solvation stage, the evolution of the solvatochromic shift can be linked to the formation of solvation shells. 

Apart from the experimental work, computations based on density functional theory were carried out to provide more ground-state information of the systems. Excited-state calculations were performed using time-dependent density functional theory and are compared with the experimental results. This comparison is used to assign the measured spectra, and to gauge the reliability of the calculations. 

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