Characterising the photodissociation dynamics of few-atom systems via X-ray Coulomb explosion imaging

Thesis

Coulomb explosion imaging has shown promise as a technique to study ultrafast chemical reactions. When a molecule undergoes a Coulomb explosion, it fragments into multiple charged ions, with their geometry fixed in the configuration at the time of the explosion. Analysis of the positions and momenta of these resultant ionic fragments allows determination of the molecular structure prior to explosion. By varying the time between initiation of a reaction, and when it is Coulomb exploded, the evolution of molecular dynamics can be followed.

This thesis demonstrates Coulomb explosion imaging’s capability to characterise a wide variety of different dissociation properties. This is accomplished by following the photodissociation dynamics of three molecules. In each case, photoexcitation and dissociation is initiated via an ultraviolet pulse, with either an infrared or X-ray pulse initiating the Coulomb explosion.

CH₂BrI is used to show that comparison of the experimental data to that predicted by a classical over-the-barrier model can determine that at 200 nm the primary dissociation occurs via a three-, rather than two-body mechanism. Additionally, within this three-body mechanism, which of the resultant fragments are charged is identified.

The C₆H₅I data is used to demonstrate that Coulomb explosion imaging can isolate and identify the signals produced from two overlapping dissociation channels. These two channels are found to produce the same molecular cofragments, but each residing in a different electronic state. Comparison to an over-the-barrier model again enables the identification of this electronic state.

The dissociation of CS₂ is used to show Coulomb explosion imaging’s sensitivity to nuclear motion on a transient excited state. By utilising a combination of simple modelling, and covariance analysis, the degree of bending undergone by the molecule prior to dissociation is identified.

These explored dissociations demonstrate that CEI is a widely applicable tool for the characterisation of ultrafast chemical reactions.

Authors: Unwin, J

• DOI: 10.5287/ora-r5revaexa
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: CS2; C6H5I; coulomb explosion imaging; CH2BrI; photodissociation
• Subjects: Ultrafast Photochemistry