AbstractVelocity map imaging has been coupled to the Pixel Imaging Mass Spectrometery cam- era to enable cutting edge multi-mass imaging experiments to be performed following pulsed laser excitation of jet-cooled gas phase molecular samples. These experiments exploit different forms of universal ionisation to make novel measurements of molec- ular dynamics or as an ultrafast diagnostic of molecular structure.
Single photon ionisation using vacuum ultraviolet photons is investigated as a method of universal ionisation and a comparison is made with traditional resonance en- hanced multiphoton ionisation methods which are fragment and state selective. The method is benchmarked on the photolysis of dimethyl sulfide at 227.5 nm, successfully demonstrating the simultaneous detection and imaging of both fragments (methyl and methyl sulfide) and the effects of space charging, dissociative ionisation and photofrag- ment angular alignment are discussed.
The multi-mass imaging capabilities are then utilised in conjunction with single pho- ton ionisation to investigate the excited state dynamics of an asymmetric thioether t-butyl methyl sulfide wherein all four primary fragments are imaged in a single exper- iment. This reveals that the competition between dissociation of the two C–S bonds in the excited state is twice as likely to proceed via fission of the t-butyl–S bond at an excitation wavelength of ∼ 225.0 nm. Further investigation of the potential energy surfaces imply that this is most likely dominated by non-adiabatic coupling between excited states.
Strong field ionisation is then used as the basis for a Coulomb explosion imaging experiment. The dynamics of ground state Coulomb explosions in methyl iodide, trifluoro methyl iodide and 2-bromothiophene are investigated and the recoil frame covariance analysis technique is employed to correlate the ion trajectories and back out the neutral structure. This sets up future experiments to investigate the evolution of molecular structure on excited states.
|Date of Award||6 Dec 2019|
|Supervisor||Michael N R Ashfold (Supervisor)|