Coincidence imaging spectroscopy of extreme-ultraviolet photoionisation processes in molecules and clusters

Presented here is the design, construction and commissioning of a coincidence imaging spectroscopy endstation coupled to the Harmonium high-harmonic generation (HHG) extreme-ultraviolet photon source at the Lausanne Centre for Ultrafast Science. The endstation is capable of recording photoelectron-photoion coincidences (PEPICO), photoion-photoion coincidences (PIPICO), and photoelectron-photoion-photoion coincidences (PEPIPICO) of photoionised molecular beam constituents using velocity-map imaging onto a delay-line detector ("DLD-VMI").Photoelectrons can be detected on the CCD-VMI with full-width-at-half-maximum (FWHM) resolution ranging from 0.1 eV at 1 eV kinetic energy to 0.4 eV at 15 eV kinetic energy, and on the DLD-VMI from 0.2 eV at 1 eV kinetic energy to 0.7 eV at 15 eV kinetic energy. Detecting photoions with the electrodes held at fixed potentials gave TOF FWHM resolution ranging from 1500 to 4500, and spatial focusing from 150 µm to 380 µm depending on the m/z of the ions and the exact voltage scheme used. Switching the electrode potentials to image both electrons and ions results in large, mass- and charge-dependent perturbations to the ion trajectories, requiring the ion-imaging potentials to be optimised for a particular ion of interest. The TOF resolution, determined for ions between m/z 18 and m/z 92, can vary from 200 to 2800. The spatial resolution for these ions varies from 200 µm to 14000 µm.Initial trial experiments were performed by analysing PEPICO coincidences of the double photoionisation of toluene. The partial double photoionisation spectra of the toluene dication suggest the double ionisation potential is approximately 23.8 eV, which agrees with previous studies. Metastable toluene dications were observed and a metastable fragmentation of the dehydrogenated ion C7H62+ to C2H3+ and C5H3+. Metastable monocations of a toluene fragment, as well as the monomer and dimer were also observed. Numerous ion pairs corresponding to Coulomb explosions of the parent dication were observed, as well as that of a tetramer dication.Partial double photoionisation spectra of 2-propanol estimate the double photoionisation potential at an upper bound value of approximately 28 eV. The exact mechanism of double ionisation is not clear from the electron data. Double photoionisation was also observed for 2-propanol clusters at least as large as the tetramer. Analysis of the ion-ion coincidences from these clusters suggest that the two positive charges created by double ionisation are born on different monomer units. Numerous metastable fragmentation pathways of singly-ionised 2-propanol monomers and clusters were also observed in TOF-TOF coincidence spectra, where the parent ions dissociate into a charged and neutral fragment at various points along the flight path to the detector. A model was developed to determine the relationship between the parent lifetime and the TOF of the detected fragments. However, the assumptions used to create the model were found to result in incorrect lifetime values when compared with electrostatic simulations. Despite this, the model successfully reproduced the shapes of the metastable signatures on the TOF-TOF coincidence spectra and allowed isolation of the coincidences corresponding to these pathways to give a qualitative scheme of the signal variation with lifetime.