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ESUX Report Summary

Project ID: 321319
Funded under: FP7-IDEAS-ERC
Country: Sweden

Final Report Summary - ESUX (Electron Spectroscopy using Ultra Brilliant X-rays - a program for the advancement of state-of-the-art instrumentation and science)

In a photoelectron spectroscopy experiment a sample is illuminated by electromagnetic radiation, most often X-rays, which leads to the emission of electrons. By measuring the energy and direction of emission of these electrons, detailed information about the investigated systems can be derived. In the ESUX project we have constructed new and very efficient instruments for such studies. This is the ARTOF (Angle Resolved Time of Flight) spectrometer. For many types of investigations the ARTOF instrument is between 100 and 1000 times more sensitive than conventional electron spectrometers. We have spent large efforts to improve our scientific instruments, in particular the ARTOF spectrometers. We have also explored in what scientific areas the performance of the instruments give particular advantages.

The spectrometers use an unconventional time-of-flight technique for the analysis of the electrons. This, however, requires a pulsed X-ray source with sufficiently short X-ray pulses. For this reason we use the BESSY II Synchrotron Radiation (SR) facility at HZB, which provides X-rays very suitable for these investigations. Normally, the SR pulses come at a rate of 500 MHz. The separation between the pulses is then too short for the operation of the spectrometer. The SR facility can also operate in a mode where the frequency is only 1.25 MHz, close to ideal for the energy analysis. This mode, however, gives much too low X-ray flux for normal SR users and is available at most three weeks per year. During the ESUX project, techniques have therefore been developed which can single out bunches at 1.25 MHz from the 500 MHz pulse train. With this successful development we can now use the ARTOF systems during all regular SR weeks.

Many important systems, e.g. materials for molecular electronics and biological materials, are very X-ray sensitive and are therefore very difficult to investigate with electron spectroscopy. With the high transmission of the ARTOF spectrometer the X-ray intensity can be much reduced and we have set up a Low Dose Photoemission (LDP) system directed towards such studies. This is one of the most important application areas of the technique.

Another area where the high transmission of the ARTOF spectrometers is important is for the study of electron electron coincidences. A system dedicated to coincidence measurements has also been commissioned.

The performance of the ARTOF instrument is also very well suited for time resolved measurements. This is important for investigating solar cell materials, which is one of the main research areas for our Division in Uppsala. It is essential to understand the basic process when light from the sun enters the material and excites electrons there. One of the most straightforward ways to follow the time evolution of such excitations is to use photoelectron spectroscopy.

For a solar cell system there are many different processes which occur at very different time scales. The most rapid electronic processes happen on the femtosecond time scales, there are many important processes with time constants in the picosecond range and there are chemical replacement processes which occur on much longer time scales. It is essential to understand all these processes. The ARTOF system is very well suited for such time resolved studies. The spectroscopy system has been equipped with a synchronized laser which can mimic the influence of the sun in a controlled way. With the present set up we can now study time dependent processes in a wide range of timescales, from femtoseconds to minutes. This has been used to perform time resolved studies of light induced processes in perovskite solar cell systems. Many different time scales can be investigated efficiently and rapidly. Furthermore the perovskite systems are very X-ray sensitive and therefore very difficult to study with photoelectron spectroscopy. With our technique the problem of X-ray induced damage can be avoided.

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