Time-resolved studies of photoinduced dynamics using fast imaging sensors

Thesis

In this thesis, the three-dimensional, multi-mass, and covariance imaging capabilities of the Pixel Imaging Mass Spectrometry (PImMS) camera has been demonstrated in a range of velocity-map imaging (VMI) experiments.

The development of an experimental configuration for VMI experiments allows for three-dimensional imaging with high velocity resolution along the time-of-flight axis. This was demonstrated by recording the full three-dimensional velocity distributions of CO and S photofragments, following the photodissociation of OCS, with the PImMS camera. The resultant velocity distributions are sufficiently well resolved to express as an expansion of spherical harmonics. This was further emphasised through the investigation of a system with no axis of cylindrical symmetry and therefore unsuitable for simple conventional VMI experiments.

The ultraviolet (UV) photoexcitation dynamics (λ = 267 nm) of 2(5H)-thiophenone were studied by ultrafast time-resolved Coulomb explosion imaging. The Coulomb explosion of the system was probed separately via strong-field and inner-shell ionisation. The velocity-dependent ion channels arising from an intense non-resonant infrared (IR) (λ = 801 nm) probe provide information on the photoexcitation dynamics of thiophenone. The channels derived from Coulomb explosions are supported through covariance analysis. The photoexcitation scheme and delaydependent covariance analysis are tentatively attributed to a prompt ring-opening of thiophenone, within 300 fs. The UV photoexcitation of thiophenone probed via XUV ionisation revealed many ion correlations were revealed through time-of-flight covariance analysis, supported by angular covariance. The velocity-dependent ion yields are notably different to the UV-IR which is unsurprising given the nature of ionisation.

Coulomb explosion imaging experiments of OCS were performed induced by an intense IR pulse at varying laser intensities. The rich and complicated Coulomb explosion dynamics were determined through various covariance analyses. Through these analyses, minor channels and branching ratios were identified.

Authors: Mason, R

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: velocity map imaging; chemical dynamics; multi mass imaging; covariance analysis
• Subjects: Chemistry, Physical and theoretical