Generation of XUV photons and their application in time-resolved photoelectron spectroscopy

To study chemical dynamics of isolated molecules in the gas phase we have used time resolved photoelectron spectroscopy (TRPES). In the work reported in this thesis we have used both a UV and extreme ultra-violet (XUV) probe to study the photodissociation dynamics of methyl-iodide across the A-band. The thesis also discusses the work carried out to develop a semi-infinite gas cell (SIGC) for the generation of the XUV probe by high harmonic generation (HHG). We performed two TRPES experiments on the A-band of methyl iodide, the first experiment excited using either 269 nm or 255 nm and ionised with a multiphoton probe, the second experiment used four pump wavelengths (279 nm, 269 nm 254 nm and 243 nm) and then ionised using a single XUV probe. The states that make up the A-band of methyl iodide are strongly dissociative in character. In both experiments we observe signal that is consistent with a rapid dissociation at all wavelengths except 254/255 nm. At 254/255 nm we observed surprisingly different dynamics with an extended lifetime and a second feature appearing in the photoelectron spectrum at a delay of around 100 fs associated with a shift in the photoelectron peaks to lower binding energies. The cause of the extended lifetime and more complex dynamics is unclear. We suggest the dynamics may be due to increased excitation onto the 1Q1 state or vibrational excitation but the cause of the observation is still uncertain. To generate a XUV probe for the TRPES experiments we use HHG from argon gas. The possible use of a SIGC was investigated to try and increase the photon flux of the harmonic closest to ∼ 20 eV. The effect of focus position, cell pressure and laser power was investigated

Calibration curves for a Semi-infinite gas cell for high harmonic generation

The gas pressure/laser power and focal position dependence of harmonic flux for a semi-infinite gas cell. Data presented in the University of Southampton Doctoral Thesis &quot;Generation of XUV photons and their application in time-resolved photoelectron spectroscopy&quot;</span

Photodissociation dynamics of methyl iodide across the A-band probed by femtosecond extreme ultraviolet photoelectron spectroscopy

The dissociation dynamics of CH3I at three UV pump wavelengths (279 nm, 254 nm, 243 nm) are measured using an extreme ultraviolet probe in a time-resolved photoelectron spectroscopy experiment. The results are compared with previously published data at a pump wavelength of 269 nm, [2020, Phys. Chem. Chem. Phys., 22, 25695], with complementary photoelectron spectroscopy experiments performed using a multiphoton ionization (MPI) probe [2019, Phys. Chem. Chem. Phys., 21, 11142] and with the recent action spectroscopy measurements of Murillo-Sánchez et al [2020, J. Chem. Phys., 152, 014304]. The measurements at 279 nm and 243 nm show signals that are consistent with rapid dissociation along the C-I bond occurring on timescales that are consistent with previous measurements. The measurements at 254 nm show a significantly longer excited state lifetime with a secondary feature appearing after 100 fs which is indicative of more complex dynamics in the excited state. The time-dependence of the changes are consistent with the previously measured MPI photoelectron spectroscopy measurements of Warne et al, [2019, Phys. Chem. Chem. Phys., 21, 11142]. The consistency of the signal appearance across ionization processes suggests that the extended observation time at 254 nm is not an artefact of the previously used MPI process but is caused by more complex dynamics on the excited state potential.Whether this is caused by complex vibrational dynamics on the dominant 3Q0 state or is due to enhanced population and dynamics on the 1Q1 state remains an open question.</p

Dataset for: Photodissociation dynamics of methyl iodide probed using femtosecond extreme ultraviolet photoelectron spectroscopy, and Photodissociation dynamics of methyl iodide across the A-band probed by femtosecond extreme ultraviolet photoelectron spectroscopy

Dataset relevant to: Warne, Emily M. (2020). Photodissociation dynamics of methyl iodide probed using femtosecond extreme ultraviolet photoelectron spectroscopy. Physical Chemistry Chemical Physics, 22, Physical Chemistry Chemical Physics 25695-25703 DOI: 10.1039/D0CP03478A and the paper Downes-Ward, B (2021). Photodissociation dynamics of methyl iodide across the A-band probed by femtosecond extreme ultraviolet photoelectron spectroscopy. Journal of Physics B, 54, 134004 DOI: 10.1088/1361-6455/ac08f3</span

Dataset for: Photodissociation dynamics of CH3I probed via multiphoton ionisation photoelectron spectroscopy

Dataset relevant to: Warne, Emily M. (2019). Photodissociation dynamics of CH3I probed via multiphoton ionisation photoelectron spectroscopy. Physical Chemistry Chemical Physics 21, 11142-11149. DOI:10.1039/C9CP01477B</span

Multi-channel photodissociation and XUV-induced charge transfer dynamics in strong-field-ionized methyl iodide studied with timeresolved recoil-frame covariance imaging

The photodissociation dynamics of strong-field ionized methyl iodide (\ce{CH3I}) were probed using intense extreme ultraviolet (XUV) radiation produced by the SPring-8 Angstrom Compact free electron LAser (SACLA). Strong-field ionization and subsequent fragmentation of CH3I was initiated by an intense femtosecond infrared (IR) pulse. The ensuing fragmentation and charge-transfer processes following multiple ionization by the XUV pulse at a range of pump-probe delays were followed in a multi-mass ion velocity-map imaging (VMI) experiment. Simultaneous imaging of a wide range of resultant ions allowed for additional insight into the complex dynamics by elucidating correlations between the momenta of different fragment ions using time-resolved recoil-frame covariance imaging analysis. The comprehensive picture of the photodynamics that can be extracted provides promising evidence that the techniques described here could be applied to study ultrafast photochemistry in a range of molecular systems at high count rates using state-of-the-art advanced light sources