Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, September, 2020
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| Format: | Thesis |
| Language: | eng |
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Massachusetts Institute of Technology
2021
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| Online Access: | https://hdl.handle.net/1721.1/130591 |
| _version_ | 1826194797074841600 |
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| author | Hull, Alexander W.(Alexander William) |
| author2 | Robert W. Field. |
| author_facet | Robert W. Field. Hull, Alexander W.(Alexander William) |
| author_sort | Hull, Alexander W.(Alexander William) |
| collection | MIT |
| description | Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, September, 2020 |
| first_indexed | 2024-09-23T10:02:16Z |
| format | Thesis |
| id | mit-1721.1/130591 |
| institution | Massachusetts Institute of Technology |
| language | eng |
| last_indexed | 2024-09-23T10:02:16Z |
| publishDate | 2021 |
| publisher | Massachusetts Institute of Technology |
| record_format | dspace |
| spelling | mit-1721.1/1305912021-05-15T03:28:26Z Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation Hull, Alexander W.(Alexander William) Robert W. Field. Massachusetts Institute of Technology. Department of Chemistry. Massachusetts Institute of Technology. Department of Chemistry Chemistry. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, September, 2020 Cataloged from the official PDF of thesis. Page 242 blank. Includes bibliographical references (pages 235-241). The Great Oxygenation Event, the introduction of O2 into the Earth's atmosphere approximately 2.5 billion years ago, is a critical milestone in the development of life on Earth. The exact timing of this event is thought to be correlated with the disappearance of Archean sulfur isotopic anomalies, called Sulfur Mass -Independent Fractionation (S-MIF), in the rock record. This anomalous fractionation pattern can be described, generally, as an enrichment in the three rare isotopes: S-33 (0.75%), S-34 (4.25%) S-36 (0.01%), relative to the most abundant isotopologue S-32 (0.75%), However, the mechanism for the generation of S-MIF in a reducing atmosphere is still unknown. I use the B-X UV transition (~31,000-36,000 cm⁻¹) in S₂ as a proxy for study of excited state collisional transfer as a possible mechanism for S-MIF. The short-lifetime B state (natural lifetime: 32 ns) state-mixes extensively with a longer-lifetime B" state (4200 ns). Furthermore, the most abundant isotopologue of S₂, ³²S-³²S has only half the number of rotational states compared to its asymmetric counterparts. In this work, I replicated Green and Western's effective Hamiltonian for the X, B, and B" states with additional considerations for mass-dependent vibrational level shifts and nuclear permutation effects. I hypothesize that the collisional transfer between the B and B" states occurs differently for different isotopologues. This difference results in a different average excited state lifetimes, which, in turn, affects the relative rate at which they undergo chemical reactions and enter the rock record. Here, spectroscopic B/B" perturbations act as doorways through which population can exchange between the B and B" states. My model incorporates absorption, fluorescence, and predissociation, as described by Green and Western. It also includes the Gelbart-Freed model for electronically inelastic collisions and Brunner and Pritchard model for rotationally inelastic collisions. I calculate the amount of each isotopologue that enters the rock record by time-dependently solving a master equation kinetic model. Results show that, generally, each region where a B vibronic state crosses a B" vibronic state behaves differently. However, the interactions display some systematic behavior. Because of the energy level patterns, lighter isotopologues are generally favored over the heavier ones (i.e. 32-36 < 32-34 < 32-33 < 32-32) in the absence of predissociation. When predissociation is present the trend reverses, but remains Mass-Dependent, and cannot explain the S-MIF signature in the rock record. The most important conclusion, however: the interactions with the smaller B/B" state-mixing showed the larger isotope effects. To continue my analysis, therefore, I considered the same B/B" system where the perturbation matrix elements are 1% of their original Green and Western values, i.e. a "weak perturbation model". Here, I develop a statistical doorway model, which posits that doorway locations are somewhat random, and that the asymmetric isotopologues converge to a limiting, ensemble behavior at lower pressures than do symmetric species that are missing half of their rotational states. Results show that this statistical isotope effect is relevant to the weak perturbation model, and may help explain the anomalous isotope patterns in the rock record. Further analysis shows that non-statistical effects may also play a critical role. These include transfer between B and B" states with very small state-mixing (as little as 0.005% ) and non-statistical doorway sampling. I conclude that a model that combines statistical and non-statistical isotope effects may explain Archean S-MIF. by Alexander W. Hull. Ph. D. Ph.D. Massachusetts Institute of Technology, Department of Chemistry 2021-05-14T16:25:46Z 2021-05-14T16:25:46Z 2020 2020 Thesis https://hdl.handle.net/1721.1/130591 1249684740 eng MIT theses may be protected by copyright. Please reuse MIT thesis content according to the MIT Libraries Permissions Policy, which is available through the URL provided. http://dspace.mit.edu/handle/1721.1/7582 242 pages application/pdf Massachusetts Institute of Technology |
| spellingShingle | Chemistry. Hull, Alexander W.(Alexander William) Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation |
| title | Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation |
| title_full | Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation |
| title_fullStr | Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation |
| title_full_unstemmed | Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation |
| title_short | Collisional transfer between excited electronic states as a mechanism for sulfur mass-independent fractionation |
| title_sort | collisional transfer between excited electronic states as a mechanism for sulfur mass independent fractionation |
| topic | Chemistry. |
| url | https://hdl.handle.net/1721.1/130591 |
| work_keys_str_mv | AT hullalexanderwalexanderwilliam collisionaltransferbetweenexcitedelectronicstatesasamechanismforsulfurmassindependentfractionation |