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Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Ricardo B. Metz
Chemistry | Physical Chemistry
Metal-containing ions have been the subject of much research due to their roles in catalytic activation and small cluster chemistries. However, they can be difficult to study both experimentally and theoretically, and new approaches are needed. The goal of the research described here is to characterize the electronic structures and thermodynamics of metal-containing ions using gas-phase spectroscopy experiments performed on a powerful new instrument. Presented in the following chapters are the details of a recently built velocity map imaging mass spectrometer that is capable of imaging the photofragments of trap-cooled (≥7K) ions produced in a versatile ion source. This instrument has been used to study the predissociation of N2O+ produced by electric discharge as well as the photodissociation of Al2+ and MnO+ formed by laser ablation. The experimental resolution is currently limited by the diameter of the collimating iris to a value of Δv / v = 7.6 %. Photofragment images of N2O+ show that when the predissociative state is changed from 2Σ+(200) to 2Σ+(300) the dominant product channel shifts from spin-forbidden ground state, N (4S) + NO+(v=5), to a spin-allowed pathway, N*(2D) + NO+. The first photofragment images of Al2+ confirm the existence of a directly dissociative parallel transition (2Σ+u ← 2Σ+g) near 23,250 cm-1 that yields products with a large amount of kinetic energy. The D0 of ground state Al2+ (2Σ+g) obtained from these images is 138 ± 5 kJ/mol. At higher energy, above 40,400 cm-1, transitions are observed to a predissociative excited state, which calculations identify as the G 2Σ+u state. The photodissociation spectrum of the G 2Σ+u ← X 2Σ+g transition in Al2+ gives an average vibrational spacing of 170 cm-1 for the G 2Σ+u state and ν0 =172 cm-1 for the ground state (X 2Σ+g). Photofragment images from the G 2Σ+u ← X 2Σ+g transition indicate that once the Al (4P) + Al+ (1S) product channel is energetically accessible, it dominates the lower energy pathways despite being spin-forbidden. These images also yield a more precise D0= 136.6 ± 1.8 kJ/mol, highlighting the improved resolution achieved from imaging at near-threshold energies. For MnO+, the photodissociation spectroscopy and images from 21,300 – 33,900 cm-1 confirm the theoretical ground state (5Π) and give its bond dissociation energy (D0= 242 ± 5 kJ/mol). The spin-orbit constant of the dominant optically accessible excited state (5Π*) in the region is also measured (A=22 cm-1). Photodissociation from this state is observed to proceed faster than the rotational period and result exclusively in ground state Mn+ (7S) + O (3P) products. At energies above 30,000 cm-1, the Mn+* (5S) + O (3P) channel becomes the preferred dissociation pathway. Overall, imaging the photofragments of metal-containing ions can help elucidate their electronic structures and photodissociation dynamics, providing valuable experimental benchmarks for these often computationally challenging species.
Johnston, Michael D. Jr., "Photofragment Imaging Fast Ion Beams" (2018). Doctoral Dissertations. 1251.