FELBE - a new facility providing coherent radiation for infrared spectroscopic investigations in actinide research

FELBE is an acronym for the free-electron laser (FEL) at the Electron Linear accelerator with high Brilliance and Low Emittance (ELBE) located at the Forschungszentrum Rossendorf in Dresden, Germany. This FEL is a source of pulsed, coherent light which is continuously tunable over the infrared wavelength range from 5 to 25 µm. This will be enlarged to about 150 µm in late 2006.
At the FELBE facility a laboratory suitable for radiochemistry research was installed. This lab is classified as a controlled zone for investigations of certain radionuclides obeying to all aspects of radiation protection. The maximum activity of the investigated radioactive samples can add up to 105 times of the admissible limit. A glove box provides the possibility to perform experiments on sensitive samples which have to be kept in an inert gas atmosphere.
The time structure of the pulsed FEL beam is related to the frequency of the electron pulses of the accelerator which offers a 13 MHz repetition rate in macropulses of a few 100 μs at up to 25 Hz. A continuous 13 MHz regime is also available as well as single pulse selection down to a frequency of 1 Hz. The energy of the infrared pulses is up to 1 µJ/pulse depending on the wavelength.
The FELBE facility is a member in the Integrated Infrastructure Initiative (I3) on synchrotron and free-electron laser science (IA SFS) within the 6th framework programme of the EU. With this grant, external users of FELBE can be financially supported ("transnational access").
In our first experiment we use the FEL beam for investigation of mineral surfaces by Phototermal Beam Deflection (PTBD) spectroscopy. The PTBD technique is based on the theory of photothermal spectroscopy which describes the conversion of absorbed energy of a light beam incident on a sample into heat by nonradiative de-excitation processes. In typical PTBD experiments the magnitude signal is proportional to the slope of the induced displacement of the sample surface. There is a direct proportionality between the observed signal and the absorption coefficient of the material under investigation which provides a direct access to infrared absorption spectra with a very low detection limit is provided.
Furthermore, the PTBD technique is capable to gain spatial information (microspectrometry) of the sample’s surface since generation and detection of the thermal wave occurs generally in the submillimeter length scale. This may result in a microspectrometric technique for determining the distribution e.g. of sorbed metal species on mineral surfaces.