VUV photoemission beamline description

Insertion device

The VUV has been designed primarily for surface and solid state experiments involving high resolution photoemission. The light source is an undulator with a range of 20 to 750 eV, while the minimum energy at a ring energy of 2.4 GeV is about 25 eV. The undulator for the VUV beamline consists of 36 periods, divided in three sections. With minimum gap (K=5.3) the high energy part of the spectrum is like that of a wiggler, i.e. continuous and fairly smooth. The light available here has reduced brightness and a lower degree of polarisation compared with an undulator.

Flux

Between 100 and 900 eV, the measured flux on the sample varies from 1.4 x 10¹³ to 5 x 10¹⁰ photons/sec/0.1%bw/200mA, in a spot of maximum size 0.5 mm. Test experiments at a second generation light source and on this beamline indicated at least two orders of magnitude higher count rates at higher resolution.
The monochromator is provided with baffles to reduce stray light and filters will be installed to reduce higher order radiation. Measurements of the higher order contribution are in progress and indicate that the contamination is a few %.

BCS

The beamline is controlled by the Beamline Control System (BCS) except for the monochromator (including the entrance optics and the exit slit position) which is under the experimental station software control.  

Monocromator

The Kirkpatrick-Baez entrance optics focus the light after which it enters the entrance slit of the monochromator. The monochromator is a Spherical Grating Monochromator with five interchangeable gratings and pre- and post-focusing optics. The set of five gratings provides a resolving power of 10,000 over the whole energetic range. The light emerges from the movable exit slit and is then refocused by a post-focusing mirror onto the sample. For lower photon energies (< ca.130 eV), the angle of deflection of the light at the grating is increased from 7° to 20° by means of an additional pair of plane mirrors, one of which is fixed and the other removable.

The resolution has been measured using gas phase absorption at a number of energies. The resolving power is estimated to be as follows:

All spectra were taken using the first order of diffraction and the appropriate harmonic of the undulator. The accuracy with which the resolution can be determined is limited by the natural widths of the absorption lines because, except for He, the lines are much broader than the resolution.