Elaine A. Seddon

Electronic and surface properties of PbS nanoparticles exhibiting efficient multiple exciton generation

Ultrafast transient absorption measurements have been used to study multiple exciton generation in solutions of PbS nanoparticles vigorously stirred to avoid the effects of photocharging. The threshold and slope efficiency of multiple exciton generation are found to be 2.5 ± 0.2 ×E(g) and 0.34 ± 0.08, respectively. Photoemission measurements as a function of nanoparticle size and ageing show that the position of the valence band maximum is pinned by surface effects, and that a thick layer of surface oxide is rapidly formed at the nanoparticle surfaces on exposure to air.

4GLS: a new type of fourth generation light source facility

Consideration is now being given in the UK to the provision of an advanced facility at lower energy to complement the DIAMOND x-ray light source. The proposed solution, 4GLS, is a superconducting energy recovery linac (ERL) with an output energy around 600 MeV, delivering both CW beam currents up to 100 mA and alternatively high charge bunches for FEL applications. Production and manipulation of short electron bunches (fs) is a vital part of the source specification. In addition to beam lines from undulator sources in the ERL recovery path there will be three FELs: two will be oscillator types in the infrared and VUV respectively, and the third will be a high gain system for XUV output. The project is outlined, together with its status and the R&D challenges posed. A funded prototype based on a 50 MeV ERL is also described.

4GLS: a fourth-generation light source that for the biomedical scientist is more than a laser and more than a storage ring

The fourth generation light source (4GLS) is a new synchrotron radiation facility proposed for the United Kingdom. It is based on an energy recovery linac, and will consist of a suite of instruments providing radiation from the soft x-ray to the far infrared. In addition to undulator sources, three free electron lasers (FELs) are proposed. Two cavity-based lasers will provide infrared and vacuum-ultraviolet (VUV) radiation, and one FEL will use the self-amplification of spontaneous emission (SASE) phenomenon to produce extreme-ultraviolet (XUV) radiation. The combination of sources will provide unprecedented wavelength coverage, power, and timing structure. The 4GLS facility is expected to have great potential for many areas of research in the biomedical sciences. We discuss some potential biomedical applications of 4GLS FELs in, for example, the areas of macromolecular conformation dynamics, imaging, and radiation damage, and show where the unique properties of this combination of sources will benefit these areas of research.

Electronic structure and surface reactivity of La1-xSrxCoO3

Resonant photoemission performed at the SRS Daresbury is used to investigate the temperature- and doping-dependent spin transitions in single crystal samples of the La1-xSrxCoO3 system (x=0, x=0.1). For LaCoO3, the measurements taken over a wide temperature range are interpreted in the light of current models for the spin transitions. The Sr-doped compound is found to be in the same spin state at all temperatures. The temperature dependence of low binding energy features primarily associated with Co in the low spin state is investigated in detail. In particular, the intensity and onset energy of the Co3p→3d resonance associated with these features is measured at small binding energy intervals. This is used to locate the separate contributions of the low spin and higher spin states to these signals. In addition, the surface reactivity of the LaCoO3 (111) surface is investigated using H2O as a probe molecule, and the results are used to interpret changes which may occur in the spectra as a function of time in UHV.

First results from the Daresbury Compton backscattering x-ray source (COBALD)

Polarized X-ray pulses at 0.6 Å have been generated via head-on collision of a laser pulse from the high-field laser facility at Daresbury with a 30 MeV electron bunch in the ALICE energy recovery linear accelerator. The angular distribution of the backscattered X rays was obtained in single-shot using a scintillation screen. The temporal profile of the X ray yield as a function of the time delay between the laser pulse and electron bunch was measured and agreed well with that expected from the collision point dependence of the laser-electron beam longitudinal overlap.

Inverse Compton Backscattering Source driven by the multi 10 TW-Laser installed at Daresbury

Inverse Compton is a promising method to implement a high brightness, ultra-short, energy tunable X-ray source at accelerator facilities. We have developed an inverse Co Compton ba mpton backscattering X-ray source driven by the multi 10 TW-L backscattering Laser installed at Daresbury (COBALD). Hard X-rays, with spectral peak ranging from 15 to 30 keV, depending on the scattering geometry, will be generated through the interaction of a laser pulse with an electron bunch delivered by the energy recovery linear accelerator prototype (ERLP) at Daresbury. X-ray pulses containing 9×107 photons per pulse will be created from head on collisions, with a pulse duration comparable to that of the incoming electron bunch. For transverse collisions 8×106 photons per pulse will be generated, where the laser pulse transit time defines the X-ray pulse duration. The peak spectral brightness is predicted to be ~ 1021 photons / s / mm2 / mrad2 / 0.1% ΔE/E, which is comparable to fourth generation synchrotron light sources.

4GLS: the UK’s fourth generation light source

4GLS is a suite of accelerator-based light sources planned to provide state-of-the-art radiation in the low energy photon regime. Superconducting energy recovery linac (ERL) technology will be utilised in combination with a variety of free electron lasers (IR to XUV), undulators and bending magnets. The 4GLS undulators will generate spontaneous high flux, high brightness radiation, of variable polarisation from 3 - 800 eV, optimised in the lower harmonics up to about 200 eV. Viable radiation at energies up to several keV may be provided from multipole wiggler magnet radiation. The ERL technology of 4GLS will allow shorter bunches and higher peak photon fluxes than possible from storage ring sources. It will also give users the added bonuses of pulse structure flexibility and effectively an infinite beam lifetime. VUV and XUV FELs will be used to generate short pulses (in the fs regime) of extreme ultraviolet light that is broadly tuneable and more than a million times more intense than the equivalent spontaneous undulator radiation. A strong feature of the scientific programme planned for 4GLS is dynamics experiments in a wide range of fields. Pump probe experiments will allow the study of chemical reactions and short-lived intermediates on the timescale of bond breaking and bond making, even for very dilute species. The high intensity of the FEL radiation will allow very high resolution in imaging applications. Funding for the first three years of the 4GLS project was announced by the UK Government in April 2003. This includes the research and development work necessary to produce a design study report, with the construction of an ERL-prototype. Additional funds have recently been awarded that will enable a study of the production of ultra-short pulsed X-rays from the ERL-prototype via Thomson scattering. It is anticipated that the full 4GLS facility will be available to users in 2011.