Electron-induced dynamics of small molecules of atmospheric and astrochemical relevance

Thesis

Electron ionisation, and electron scattering in general, is a fundamental physical process. Ubiquitous in the known universe, the interaction of electrons with molecules is thus of great interest.

From experimental and theoretical studies, we can obtain great insight into electron ionisation. We have used the binary-encounter-Bethe model to determine theoretical total ionisation cross-sections, which quantify the ionisation probability for a molecule. We also use an electron-molecule crossed beam experiment to obtain partial ionisation cross-sections, which quantify the probability of forming a specific ion.

The electron-molecule crossed beam experiment utilises the velocity-map imaging technique, which allows the scattering distribution of the product ions to be recorded and the velocity of each ion determined. Study of these data grants us an insight into the molecular dynamics involved, and the potential energy landscape of the molecule under study.

In our experiments, we use a multi-mass imaging sensor to record the scattering distribution of all ions produced following electron ionisation. The use of a multi-mass imaging sensor allows us to utilise covariance analysis techniques, which reveal the correlations between the velocity distributions of different ions. We have recently shown that covariance allows us to much more readily study the dissociation of doubly-charged molecules following electron ionisation than was previously possible using our experiment. The recoil-frame covariance technique is particularly useful in this context, allowing the momentum of a signal ion to be determined with reference exclusively to a second, covariant, ion.

Scattering distributions are recorded as two-dimensional projections of the three-dimensional distribution, meaning a reconstruction method, or inversion method, is required to `re-inflate’ the distribution and obtain the velocity distribution. Recoil-frame covariance analysis cannot occur following inversion, meaning the resulting covariance maps must also be inverted. However, previously available inversion methods cannot validly be used. A new inversion method has been developed to allow the successful inversion of covariance maps.

We have used the above techniques to investigate a number of molecules of both atmospheric and astrochemical relevance, namely carbon dioxide, nitrous oxide, carbonyl sulphide, sulphur dioxide, and benzene. We find that fragmentation following electron ionisation is dominated by dissociation from low-lying electronic states of the molecule, and that a large portion of the energy available, i.e. the kinetic energy of the incident electron, is carried away by the ionising and ejected electrons, rather than being transferred to the molecular framework.

Authors: Heathcote, D

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: molecular reaction dynamics; multi-mass imaging; chemistry; electron-molecule scattering; electron ionisation; covariance; ion imaging; physics; velocity-map imaging; binary-encounter-Bethe model; time-of-flight mass spectrometry; abel inversion; electron ionisation cross-sections
• Subjects: Electron ionization; Chemistry, Physical and theoretical; Reaction dynamics; Electron-molecule scattering; Ion imaging