NOT AVAILABLE

STATION 4.2

Description

• NIXSW: Normal Incidence X-ray standing wavefield
• NEXAFS: Near edge x-ray absorption fine structure.
• SEXAFS: Surface x-ray absorption spectroscopy
• XPS: X-ray photoelectron spectroscopy

News

Detector Commissioned
A solid state fluorescence yield detector will be commissioned in within the next allocation period.

Functional Description

This is a UHV station comprising a beamline with a double crystal monochromator and associated control and data acquisition systems. The beamline, which is primarily intended for routine measurements of surface X-ray absorption spectroscopy, standing X-ray wave field and photoelectron diffraction, was commissioned for and maintained by Liverpool University. The photon energy range covered is 1800-10000 eV, using Ge(111), InSb(111) or Si(311) crystal pairs. The beamline employs a chromatic pre-mirror system, which allows the high-energy cut-off to be selected in situ.

A dedicated UHV Surface Science chamber is permanently mounted on the beamline. The station consists of a PSP 5-channeltron hemispherical electron energy analyser, a dual anode X-ray source for conventional XPS which is useful for off-line work also, rear view LEED optics and the usual sample cleaning techniques.

Samples can be mounted for either e-beam heating or direct heating, with in-situ sample transfer as standard.

A fluorescence yield detector is also available and will be commissioned shortly.

Technical Description

This is a double crystal monochromator beamline with the main elements as follows:

• Water cooled entrance baffles.
• Plane mirror of size 800mm x 160mm with surfaces of uncoated quartz, platinum and chromium.
• Cylindrical mirror of size 800mm x 80mm with two equal size coatings of platinum and chromium. the mirror mount incorporates an in-situ bending mechanism to change the meridional radius to vary the stigmatic focus of the beam.
• Baffles and filters (C and Be)
• Double crystal monochromator with a Bragg angular range of 14° to 76°.
  A piezo drive has been installed to servo the monochromator crystals on the Bragg peak during measurement.
• Io foils and filter foil rack with Ti, V and Al foils.
• A dedicated UHV Surface science station.

Development

Liquid nitrogen cooling for direct heating sample holder has been implemented.

Azimuthal rotation for direct heating sample holder.

The station software has been reconfigured for NIXSW measurements.

The beamline has been recently reconfigured, bringing the monochromator closer to the station. This has significantly improved beam stability.

Benchmarks

(Using Ge(111) crystals)

The transmission curve shows a photon flux maximum of 3 x 1011 photons/s/100mA stored current, in agreement with expected performance. Crystal glitches appear at positions predicted theoretically, indicating the good quality and alignment of the crystals.

Rocking curve FWHM indicate a resolution of less than 1 eV at 2450 eV and less than 3 eV at 4000 eV photon energy.

Similar measurements using InSb(111) and Si(311) crystals have also been made and the results can be found on the station homepage.

The station has been successfully used to measure Fluorescence yield SEXAFS from Ni(110)x(2x2)S. By taking advantage of the chromatic pre-mirror system, it has been possible to extend the measurement to a photon energy range of 650 eV.

A NEXAFS study of thiophenecarboxylate on Cu(110) indicates that the resolution of the beamline is less than 1 eV at a photon energy of 2470 eV.

The station has recently been used primarily for NIXSW measurements. Systems studied include Si(111), Cu(111), Au(111), Cu(100).

Typical experiment turn round time

Two days are generally required to mount samples, sources and bake the chamber to obtain UHV conditions before the station can take the X-ray beam. Experiments are normally allocated 2-3 week period which will include 2-3 days set up time without X-ray beam.
During set-up and off-line, the station can be used with the dual anode X-ray source for XPS measurements.