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Determining Bond Strengths and Dissociation Dynamics of Diatomic Metal-Containing Ions by Photofragment Imaging
Author ORCID Identifier
Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Inorganic Chemistry | Physical Chemistry
Studies of simple metal ion – ligand complexes have primarily focused on understanding their roles in activating C-H and C-C bonds. However, data are often lacking on the fundamental properties of these species, which can have unusual bond orders and cluttered electronic structures with many states of multi-reference character, complicating their treatment in theoretical studies. Experimental work determining high-precision bond energies, ground state identities and excited state dynamics of a wider variety of metal-containing ions is needed to establish a robust set of well-characterized benchmark molecules. This work describes studies of the energetics and dynamics of several MX+ species, NiO+, NiS+ and MgI+, by photofragment ion imaging, photodissociation spectroscopy and theory. These systems each have properties that make them challenging for spectroscopic methods alone; however, imaging their fragments provides key context to their spectra, enabling in-depth analysis. NiO+ shows weak absorption near its bond dissociation energy and a broad and featureless photodissociation spectrum. However, imaging its photoproducts reveals that, at a much higher photolysis energy of ~29000 cm-1, a dramatic shift occurs in the preferred dissociation pathway from formation of ground state Ni+ + O to electronically excited Ni+* + O products. Image anisotropy and the results of MRCI calculations suggest NiO+ photodissociates via a parallel transition above the Ni+* threshold, and via overlapping parallel and perpendicular transitions to several excited states at lower energy below the Ni+* threshold. In contrast to NiO+, NiS+ absorbs strongly at its bond dissociation energy (~20000 cm-1), resulting in a highly structured photodissociation spectrum. Analysis of the spectrum is complicated by the plethora of predicted transitions in the region, the presence of suspected hot bands, and uncertainty in the identity of the ground state. In this case, observation of parallel anisotropy in the images of Ni+ (2D) photofragments is key to assignment of the ground state and the dominant electronic transitions, while the kinetic energy release (KER) distributions of images near the dissociation onset confirms the two lowest-energy peaks in the spectrum are due to hot ions. Finally, MgI+ is the most spectroscopically challenging of the three molecules, despite absorbing strongly near both its ground and excited state product thresholds. In fact, absorption via the most intense transition (from 33000 to 40000 cm-1) does not result in dissociation to the expected ground electronic state Mg+ + I products, likely due to rapid, efficient fluorescence. Instead, despite being energetically forbidden, excited charge transfer (Mg + I+) photoproducts are observed. The KER distributions of I+ fragment images show that the observed fragment channels result from resonance enhanced two-photon dissociation. This is the first reported direct observation of REPD of a molecule in which this photolysis process was not already known to occur. Finally, the KER distributions give high-precision bond dissociation energies: D0 (MgI+) = 203.0 ± 1.8 kJ/mol, D0 (NiS+) = 240.3 ± 1.4 kJ/mol, and D0 (NiO+) = 244.6 ± 2.4 kJ/mol.
"Determining Bond Strengths and Dissociation Dynamics of Diatomic Metal-Containing Ions by Photofragment Imaging" (2023). Doctoral Dissertations. 2765.
Available for download on Tuesday, August 01, 2023