Isomerisation Puzzles viewed with High Resolution Photoelectron Imaging

Abstract

Anion photoelectron spectroscopy has proven to be a versatile technique for studying transient neutral species, with very fast dynamics, as it obtains information from both the parent negative ion and the reactant neutral molecule. This technique is particularly useful for the study of chemical reactions where the transition state possess a stable anion. The development of velocity map imaging has greatly improved the spectral resolution and collector efficiency that may be achieved in charged particle detection, to allow for unprecedented detail in the recorded molecular spectra. In this work, a High Resolution Photoelectron Imaging (HR-PEI) spectrometer is used to investigate the nature of isomerisation. A benchmark study of NO2- photodetachment reveals rotationally resolved structure, with a rapid change in the electron anisotropy behaviour near threshold used to decode the character of the highest occupied molecular orbital (HOMO). Measurements close to photodetachment threshold display additional high kinetic energy electron structure that cannot be associated with the ONO C2v isomer. These additional fast electrons have the spectral signature of the Cs peroxy isomer NOO, with spectral analysis providing the first experimental evidence to confirm that the NOO isomer exists as a bound molecule. High resolution studies of similar atmospherically relevant molecules NO, O, and O2 have been used for calibration, and determine new detail about these species. A second section of this work tackles the long-standing problem of the vinylidene - acetylene isomerisation. The 1,2-hydrogen migration is a common mechanism in more complex organic reactions, making this an attractive area of research as a prototypical study. However, the dynamics of the vinylidene isomer are still poorly understood, in part due to conflicting experimental results. Photoelectron spectra of the vinylidene anion, presented in this work, reveal the presence of forbidden non-totally symmetric vibrational modes associated with a discrete jump in the anisotropy parameter. This is a consequence of Herzberg-Teller coupling between electronic states, and is essential for isomerisation to occur. Spectral signatures possessing an admixture of both vinylidene and highly excited acetylene local-bender character are discovered, providing a map of the doorway vibrational states through which the isomerization proceeds. Similar molecules dicarbon and ethynyl are also studied, where the complex coupling mechanisms between the ground and first excited state of ethynyl provide more insight into the Herzberg-Teller interaction. The nature of dicarbon bonding is examined, with the anisotropy of C2- photodetachment determining the presence of a double bond, despite recent theoretical suggestions that a quadruple bond may be present. Show more