Science, Technology and Engineering

Centre for Materials and Surface Science

Toroidal Spectrometer Project - Technical Overview

The system was assembled at La Trobe University in Melbourne, Australia and transported by Lufthansa to Berlin.

The system is supported by a unique motorized and electronically controlled frame, which allows easy and quick alignment at the beamline. Phosphor screens with cross-hairs may be placed in position at either the entrance or exit edges of the toroidal analyser to permit accurate placement of the spectrometer relative to the photon beam. This procedure is currently being upgraded using additional CCD cameras so as to make alignment automatic in future. Since the photon beam must pass through the spectrometer input slit without touching, the alignment process is critical on certain beamlines and must be monitored in case of beam position changes.

The vacuum system and all motion is computer controlled by software written in Labview (TM). Laser alignment of the sample manipulator is used to guarantee that the sample is placed exactly in the center of the analyzer. A CCD camera is used to monitor the sample when in the measurement position and to verify manipulator movements during data acquisition.

The system consists of the spectrometer chamber, the transfer cube, a preparation chamber and a loadlock. The loadlock allows us to insert different samples while the spectrometer is on a beamline. The preparation chamber is equipped with a number of devices for sample preparation:

• Noble gas ion gun
• Needle Evaporators
• Sample heating (up to 1400°C)
• MBE tpe evaporation cells
• Reverse view LEED system with CCD camera.

After preparation the sample is transferred to the "cube" above the main spectrometer chamber, where it is inserted into the specimen holder at the manipulator. This operation is motorized and controlled by a hand-held PDA. Standard locations for the sample and the 5-axis manipulator make transferring samples almost automatic: only the final insertion of the sample into the manipulator holder needs to be guided manually. The sample is then imaged by a CCD camera at cube level so that its precise dimensions can be obtained and a point on the surface chosen as the measurement point. With this information, the computer can drive the sample into the correct measurement position and can perform azi rotations when required keeping the source point constant.

A sample storage region is located between preparation and analyser chambers to minimise the need to access the loadlock during beamtime.

Showing the ease with which the system can be moved between beamlines. Reducing the pressure in the tyres fixes the lower part of the alignment frame (white) on the floor after which the aligment procedure can be commenced.

An extensive suite of softwae has been developed in LabView to both control the spectrometer and display incoming data essentially in real time. Standardised routines are available for Energy distribution plots (EDC) for Constant Initial State spectra (CIS) and Constant Final State spectra (CFS). The energy window shows data covering a region approximately 8% of the current pass energy set for the spectrometer. In software, this region is divided into a number of radial slices each of which represents a "new" experiment being run in parallel with the others.

In some cases (e.g. XPD) is may be sensible to integrate data across the energy window at each emission angle so that emission from an entire core level is recorded. This clearly requires that a pass energy large enough for the core line to be accommodated within the window be chosen.

In other circumstances (e.g. Fermi mapping using azi scans) the energy window would be arranged to collect the profile of the Fermi distribution. The location of the point of inflection can then be determined accurately at each polar angle by considering the intensity distribution at each angle as a mini EDC. Not only does this remove any small deviations in energy as a function of angle but clearly demonstrates the advantage of being able to choose the Fermi energy after rather than before data acquisition.

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