Electronic Structure and Photochemistry of Molecular and Cluster Anions via Tandem Time-of-Flight Mass Spectroscopy and Photoelectron Imaging
AuthorHabteyes, Terefe Getaneh
Committee ChairSanov, Andrei
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractMolecular and cluster anions have been investigated using a newly built tandem time-of-flight mass spectrometer combined with photoelectron imaging system. Solvation particularly hydration is shown not only to stabilize metastable anions such as CO₂⁻ in their ground state and impede autodetachment but also to alter the dynamics in the excited states. For instance, the 355 nm photoelectron image of mass-selected CO₂⁻(H₂O)(m) evolves from anisotropic to isotropic as m increases indicating excited state decay via electron autodetachment. Dissociation channels open at m=2 at 266 nm, resulting in O−(H₂O)m-k and CO₂⁻(H₂O)(m-k) products, the later becoming dominant as m increases. The photoelectron imaging of (CS₂)₂⁻ has revealed the coexistence of four electronic isomers: CS₂⁻•CS₂ [C(s)(₂A′)] and three covalent C₂S₄⁻ [C₂ᵥ(²B₁), D(2h)(²B(3g)), and D(2d)( ²A₁)] structures. Water-mediated intermolecular interactions have been shown to facilitate the formation of the global minimum C₂ᵥ(²B₁) structure rather than the less stable local minima C(s)(₂A′) and D(2d)(²A₁) structures that are favored in the dry source condition. In the (CS2)(n)⁻, n ≥ 3 and (CS₂)₂⁻ (H₂O)(m), m > 0 clusters, the population of the C₂ᵥ(²B₁) structure diminishes drastically due to more favorable solvent interactions with the CS2 − monomercore. Photoexcitation of the (CS₂)₂⁻ also results in the formation of CS₂⁻ and C₂S₂⁻ at 532 nm, and C₂S₂⁻, CS₂⁻, CS₃⁻, S₂⁻, and S⁻ at 355 and 266 nm. The relative yields of C₂S₂⁻ is significantly higher when (CS₂)₂⁻ is formed under wet source condition suggesting C₂ᵥ(²B₁) structure as the origin of C₂S₂⁻. An abrupt decrease in the relative yield of C₂S₂⁻ is observed upon adding CS₂ or H₂O to (CS₂)₂⁻. The CS₂⁻ based clusters are the likely origin of the S− photoproduct, while CS₃⁻ is formed through the secondary S⁻+CS₂ reaction. Novel anions (CS₂O₂⁻ and CS₃O⁻) are observed in the CS₂+O₂+e⁻ reaction. The photoelectron imaging and photodissociation results of these and other anionic products are presented. In addition, CS₂⁻•O₂ ion-neutral complex is formed depending on the conditions in the ion source. Despite the positive electron affinity of O₂, no clear signature of O₂⁻•CS₂ ion-neutral complex is seen in the photoelectron image. CO₃⁻ ion is also formed abundantly as a result of CS₂+CO₂+O₂+e⁻ reaction.