In 2003 the design and layout phase for a major extension of the FELIX facility with a project named FELICE was started; FELICE stands for Free Electron Laser for Intra-Cavity Experiments. This project involves the construction of a third beam line which can be operated interleaved with one of the two existing beam lines at a maximum repetition rate of 10 Hz for each line and therefore is in fact doubling the amount of beam time available to the users. The purpose of FELICE is to provide significantly higher infrared energies for low-absorption, gas-phase experiments. The increase as compared to FELIX is a factor of up to fifty in the spectral range from 3 to 100 µm.

The gas-phase experiments to be performed on FELICE will be done in two specialized intra-cavity setups - a molecular beam apparatus and an FTICR mass spectrometer. Currently the versatile intra-cavity molecular beam apparatus is operational. The instrument is equipped with a quadrupole ion trap, linear and reflectron type time-of-flight detectors. At present, an effusive beam source, a pulsed valve as well as a Smalley type laser ablation source are available. The modular design of the instrument offers users the possibility to bring their own sources. Radiation from FELICE can be combined with one of the facility's other laser sources such as a ns dye laser, an excimer laser and a ns mid-IR OPO laser.

Figure: Artist's view of the folded resonantor of the FELICE cavity with the two intra-cavity experiments.

Layout of FELIX/FELICE beamlines

The accelerator consists of a thermionic triode gun, which is modulated at 1 GHz, 25 or 16.7 MHz, producing 200pC bunches in a few µs long train and three normal-conducting S-band structures, each giving an energy boost of up to 20 MeV at the nominal current of 200 mA. By switching the approprate dipoles, FELICE can run interleaved with either FEL1 or FEL2.

First results

On Friday 4 May 2007 first light was produced by the new beam line FELICE (Free Electron Laser for Intra-Cavity Experiments) at our Free Electron Laser Facility FELIX. This was achieved in a basic configuration of the laser in a straight cavity at a wavelength of about 8 µm.
On Saturday 11 August 2007 a next step was taken in the commissioning of the new beam line FELICE (Free Electron Laser for Intra-Cavity Experiments) at our Free Electron Laser Facility FELIX. In the meantime the final resonator configuration for one of the two intra-cavity experimental setups, the molecular beam apparatus, has been realized. The drawing shows the 4-mirror cavity housing this intra-cavity setup. Lasing was obtained at a wavelength of 12 and 17 microns. In addition, interleaved operation of the FELIX and FELICE beamline, which allows to run two experiments simultaneously on alternating shots of the electron beam at a repetition rate of 10 Hz, was demonstrated for the first time in June 2007.

Tuesday 25 September 2007: the first successful intracavity experiments were carried out at the new FELICE molecular beam experiment. Isolated neutral C₆₀⁺ molecules were irradiated with one macro pulse of infrared light at 19.2 µm. In contrast to previous experiments with FELIX where virtually only C₆₀⁺ was observed, this time a broad distribution of ionized species: ranging from C₆₀⁺ to C₄₀⁺  was obtained, a direct consequence of the much higher irradiation intensity available in FELICE. The experiments can help to improve our understanding of the competition between multiple energy release pathways in super-excited molecules and clusters.