Master presentation: Ossian Hemdal Lundgren

Studies of Negative Ions and their Metastable States

Abstract

Negative ions exist in our world, both in our atmosphere and in space, but also in many fundamental chemical processes. As the importance of understanding electron correlation effects has increased the interest in negative ions as a key component in studying the binding energies of their electrons, known as the electron affinity. As negative ions get excited to a higher energy state, where the electric dipole transition back to the ground state is quantum mechanically seen as a forbidden transition, this de-excitation back down to the ground state then has a very low probability to happen. Thus, a long lived excited state for the negative ion is achieved. Such states with long enough lifetimes are called metastable states. In this thesis, the electron affinity of arsenic negative ions are studied using a laser photodetachment threshold spectroscopy research facility called GUNILLA. Lifetimes of metastable states on arsenic, lanthanum and tungsten negative ions are also measured using a research facility consisting of cryo-storage rings together with laser photodetachment called DESIREE. The electron affinity of arsenic negative ion is calculated to be 0.80409(8) eV for the ground state (J = 2) and 0.67592(8) eV for the first excited state (J = 1). Meanwhile the resulting lifetimes of two metastable states, J = 0 and J = 1, is calculated via weighted exponential fittings to be 397.89 s ± 13.90 s and 40.80 s ± 2.20 s respectively. The lifetime of the J = 1 state shows good agreement with the theoretically predicted lifetime, while the lifetime for the J = 0 state is almost half as long as theoretically predicted. A suspected previously unknown excited state, or a cascade of a series of shorter-lived states, near the lanthanum negative ion ³F₄ state has been detected and measured to have a lifetime of 0.200 s ± 0.0002 s and the lifetime of the actual ³F₄ state to be 27.21 s ± 3.99 s. The measured lifetime of the La^{− 3}F₄ state in this thesis also disagrees with theoretically predicted lifetime value. A previously unknown bound excited state of tungsten negative ion has also been detected below 0.385 eV, as previously suspected in earlier electron affinity measurements. The states lifetime is calculated to be 6.86 s±0.01 s. The significant difference between the theoreticaly predicted lifetime of the metastable state J = 0 in arsenic negative ion and for the lanthanum negative ion ³F₄ state suggests that better theoretical models for understanding longer lived metastable states are needed. In the future, it would be interesting to see someone investigate and determine the exact electron affinity of this newly found bound excited state of the tungsten negative ion. But also to investigate if there are any more previously unknown bound excited states of tungsten negative ion.