Frances M. Quinn

The Mobile Luminescence End-Station, MoLES: a new public facility at Daresbury Synchrotron

A new mobile end-station is described for use on multiple beamlines at the Daresbury synchrotron radiation source (overall excitation range 5 eV to 70 keV) that allows for the detection and dispersion of photoluminescence from solid-state samples in the emission range 190-1000 nm (1.2-6.5 eV). The system is fully self-contained and includes sample-cooling facilities for the temperature range 8-330 K using a closed-cycle refrigerator, thus eliminating the need for liquid cryogens. The system also includes solid-state laser sources for use with a variety of pump-probe-type experiments, and an Ar(+) surface-cleaning facility. In order to demonstrate the various capabilities of the system, the results of a variety of experiments are summarized, carried out over the excitation range 5-5000 eV on beamlines 3.2, MPW6.1 and 4.2. These include the optical detection of XAS of L-edge structure in natural minerals and archaeological ceramics, band-gap determinations of wide-band-gap silicates, and pump-probe studies of quartz.

Luminescence excitation characteristics of Ca, Na and K-aluminosilicates (feldspars) in the stimulation range 5–40 eV: determination of the band-gap energies

The first comprehensive survey of band-edge features in the ternary group of naturally occurring aluminosilicates (the feldspars) is presented. Synchrotron-based luminescence excitation of KAlSi3O8, NaAlSi3O8 and CaAl2Si2O8 allows the measurement of the evolution of the band-gap across the system, which, at 8 K, is found to vary from 7.86 eV in NaAlSi3O8 to 7.7 eV and 7.62 eV in KAlSi3O8 and CaAl2Si2O8, respectively: the band-gap energies are typically 0.1 eV smaller at 300 K. In comparison with measurements made on natural and synthetic hydrothermal α-quartz, where both the direct and indirect band-gap structures are distinctly observable, no significant post-edge band structure is discernable in the feldspars. In Ca-rich material, the luminescence is attenuated by more than two orders of magnitude for excitation energies up to 3 eV above the band-gap, partly supporting the proposition (derived from previous cyclotron resonance experiments) that the bands are simply parabolic and that the materials may be direct band-gap insulators. The luminescence excitation experiments also allow an initial survey to be made of sub-band-gap features in the materials, including low mobility, temperature sensitive band-tail states and mid-gap defects.

A W:B4C multilayer phase retarder for broadband polarization analysis of soft x-ray radiation

A W: B4C multilayer phase retarder has been designed and characterized which shows a nearly constant phase retardance between 640 and 850 eV photon energies when operated near the Bragg condition. This freestanding transmission multilayer was used successfully to determine, for the first time, the full polarization vector at soft x-ray energies above 600 eV, which was not possible before due to the lack of suitable optical elements. Thus, quantitative polarimetry is now possible at the 2p edges of the magnetic substances Fe, Co, and Ni for the benefit of magnetic circular dichroism spectroscopy employing circularly polarized synchrotron radiation.

Measuring modulated luminescence using non-modulated stimulation: ramping the sample period

Conventional methods of recording linearly modulated (LM) optically stimulated luminescence (OSL) require control over either the exciting light intensity, or the ability to pulse the source. For many light sources (e.g. constant-power CW lasers, arc lamps and synchrotrons) this can be problematic. Directly analogous results to LM-OSL can, however, be achieved with non-modulated excitation sources, by ramping the sample period (RSP) of luminescence detection. RSP-OSL has the distinct advantage over LM-OSL in that, since the excitation remains at full power, data accumulation times (that can be considerable) can be reduced by typically 50%. RSP methods are universally applicable and can be employed, for example, where the excitation source is constant heat, rather than light: here, iso-thermal decay of phosphorescence becomes recorded as a sequence of peaks, corresponding to de-trapping of charge from different defect levels, and is particularly useful for analysing shallow-trap effects. RSP methods are also useful in providing significant compaction of data sets, where signal analysis is required of overlapping systems having a wide range of decay kinetics.

Luminescence excitation characteristics of Ca-, Na- and K-aluminosilicates (feldspars), in the stimulation range 20–500 eV: optical detection of XAS

We demonstrate that the visible/UV luminescence from common feldspar crystals (NaAlSi3O8, KAlSi3O8 and CaAl2Si2O8) can be used to detect detailed L-edge and associated near-edge absorption structure of the main constituent atoms (Ca, K, Na, Al, Si), when exciting in the energy range 20–500 eV. Comparisons of the spectral features are drawn with similar measurements made on the associated materials SiO2, Al2O3 and CaCO3. The potential for using optically detected x-ray absorption spectroscopy as a method for identifying the luminescent components of mixed mineral samples is considered.

Facilities for spin‐resolved photoemission at the SRS

Electron polarimetry at Daresbury is carried out using either a conventional high‐energy Mott polarimeter or a conical retarding potential Mott polarimeter (microMott). The high‐energy Mott polarimeter is a fixed feature of Station 1.2, the microMott polarimeter can be used on Stations 1.1, 5U.1, 3.3, or 6.1. This combination of beamlines, spectrometers, and polarimeters provides the research community with the capability of performing spin‐resolved photoemission from both gases and solids over the photon energy range 6 to 1500 eV. First results from Ni(110) on Station 1.2 are discussed along with a preliminary test of the microMott polarimeter.

Higher-order suppression in diffraction-grating monochromators using thin films

Although a continuously tuneable source of photons is a very desirable feature of synchrotron radiation it has one main drawback: the contamination of the photon beam by higher-order diffracted light. Several elements have absorption edges which lie between 10 and 200 eV, a range prone to high second- and third-order content in XUV monochromators. They can, therefore, be used as transmission filters to reduce this higher-order content. This paper describes the use of thin filters to reduce the higher-order content in diffraction-grating monochromators. Their suppression efficiency, transmission and ageing have been characterized using photoelectron spectroscopy and compared with calculated values. The effect of oxide contamination on their performance has been assessed. Filters are now installed on eight XUV beamlines and have been in routine use for several years.

A FAST AND FLEXIBLE MULTICHANNEL ELECTRON DETECTOR WITH PARALLEL READOUT FOR PHOTOELECTRON SPECTROSCOPY

To satisfy end user requirements for higher throughput and reliability in photoelectron spectroscopy, a new multichannel electron detector with discrete electronics has been designed and commissioned at Daresbury Laboratory. Count rate performance has been enhanced by the use of low resistance microchannel plates which amplify the electron pulses incident on the anode array. The low resistance microchannel plates are linear to 2.5 × 104 s−1 mm−2 for a bias voltage of 900 V per plate, providing more than an order of magnitude improvement in count rate performance over high resistance microchannel plates, microchannel plate outgassing in the ultra-high vacuum environment limits the scale of this improvement. A novel anode design maximizes the collection efficiency, while minimising crosstalk between channels (< 0.6% after discrimination) and provides up to 40 parallel channels on a 1 mm pitch. The collection efficiency is increased from 50% to 70% (for the present geometry) by applying a 50 V bias between the guard rail and electrodes on the anode. Each data collection channel comprises a fast current amplifier and discriminator, capable of a throughput of 2.5 × 107 s−1, and a 24 bit scaler. The integral non-linearity for flat-field illumination is better than 10% with no correction. The fast multichannel detection system gives a throughput enhancement of 10–20 on single-channel detection systems. It has also proved to be significantly better than previous multichannel detection systems with discrete electronics due to its high throughput, modular design and flexible structure.

Development of high-throughput flexible multichannel electron detectors at Daresbury Laboratory.

Continuing demands from Synchrotron Radiation Source (SRS) end-users for higher throughput and improved reliability in photoelectron spectroscopy experiments have driven an intensive development programme for new multichannel electron detectors. The development philosophy focuses on high throughput to match present and future source intensity, flexible structures to allow increased mobility of designs and modular design for easy maintenance and repair. Developments include parallel readout electronics and innovative detector heads for the hemispherical deflection analysers currently in use on the SRS. Novel anode arrays have been implemented in the detector heads and extensive microchannel plate (MCP) characterization has been undertaken to source the MCPs most suited to this application. The present multichannel detection systems provide a significant enhancement to single-channel detection systems. They have also surpassed previous multichannel detection systems due to their high throughput, flexible structure and modular design. Information on these developments and experimental results obtained at Daresbury Laboratory are presented.

Electron Polarimetry on the Synchrotron Radiation Source (SRS)

Two polarised electron experimental facilities have been developed for use on the SRS. The first is a dedicated beamline and station, Station 1.2, which incorporates a conventional high-energy Mott polarimeter for spin analysis of electrons photoemitted from surfaces under UHV conditions. The second is an electron energy and angle resolving instrument which incorporates a conical retarding potential Mott polarimeter (the “micro-Mott” polarimeter) for use in a variety of configurations on a number of beamlines. These facilities have been the subject of several preliminary reports.1–3