XUV and VUV photoabsorption and emission studies in thorium and other high-z laser plasmas

Fundamental investigations into the atomic structure of actinide atoms and ions provide a wealth of information on the physical and chemical behaviour of these elements. In this work, thorium, the second actinide element is investigated. Inner shell photoabsorption features in the 6p and 5d spectra of the free thorium atom and thorium ions produced in a laser plasma plume have been recorded using the space and time resolved Dual Laser Plasma (DLP) spectroscopic technique. Studies focus, in particular, on the investigation of the 6p —» 6d and the 5d —> 5, si' giant dipole resonance features residing in the VUV (15 to 40eV) and XUV (80 to 150eV) inner shell photoabsorption spectra of atomic and ionic thorium. The evolution of both giant dipole resonance profiles with increasing ionisation along the thorium isonuclear sequence Th to Th6+ has been studied. Results indicate that the 6d wavefunction is collapsed in neutral thorium, whereas the 5f wavefunction is to a large extent un-collapsed in neutral thorium but collapses with increasing ionisation along the thorium isonuclear sequence. A picosecond XUV continuum laser plasma light source has been developed in this work and is characterised both spectrally and temporally using time integrated flat field spectroscopy and time resolved streak camera detection. XUV continuum emission pulsewidths of less than 200ps in the 100 to 200eV energy region were recorded from the high-Z metal plasmas of tungsten, gold, lead and samarium. When incorporated into the DLP technique, the picosecond XUV continuum source permits probing of the absorbing plasma plume with superior temporal resolution than previously achieved using XUV laser plasma sources produced using nanosecond laser pulses. The newly developed source was used to the study the collapse of the broad asymmetric 5d -» 5, rf giant dipole resonance feature to discrete Rydberg-like structure along the thorium isonuclear sequence

Short-pulse, extreme-ultraviolet continuum emission from a table-top laser plasma light source

We have observed extreme-ultraviolet (XUV) “line-free” continuum emission from laser plasmas of high atomic number elements using targets irradiated with 248 nm laser pulses of 7 ps duration at a power density of ∼ 1013 W/cm2. Using both dispersive spectroscopy and streak camera detection, the spectral and temporal evolution of XUV continuum emission for several target atomic numbers has been measured on a time scale with an upper limit of several hundred picoseconds limited by amplified spontaneous emission