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.

FELICE Highlights

Infrared Spectroscopy of Gas-Phase Polycyclic Aromatic hydrocarbon cation in the 10-50 um Spectral Range

Joost M. Bakker, Britta Redlich, Alexander F.G. van der Meer, and Jos Oomens

The gas-phase infrared spectra of four polycyclic aromatic hydrocarbon (PAH) cations have been recorded in the 10-50 μm (or 1000-200 cm^–1) spectral range via IR multiple photon dissociation (IRMPD) spectroscopy. Ionized PAHs are formed by UV laser ionization in an effusive beam and subsequently irradiated with a single pulse of narrowband tunable infrared light produced by the Free-Electron Laser for IntraCavity Experiments FELICE. The ion population is then analyzed in a time-of-flight mass spectrometer. Upon resonance, dissociation is induced so that IR spectra can be recorded by monitoring either the depleted parent ion intensity or the appearance of fragment ions as a function of the wavelength. The intracavity IR fluence enables the recording of IRMPD spectra of strongly bound PAH cations in the hitherto inaccessible far-IR spectral range. Experimental spectra are presented for the radical cations of anthracene, tetracene, pentacene, and coronene. Spectra calculated with density functional theory at the B3LYP/6-311g(2df,2pd) level reproduce IR frequencies reasonably accurately in this spectral range when a uniform scaling factor of 0.94 over the complete 10-50 μm spectral range is employed. We show that even vibrational modes with a calculated IR intensity lower than 1 km mol^–1 can be observed. For the catacondensed PAH cations we find CH out-of-plane bending vibrations involving four adjacent CH groups within a few wavenumbers of 733 cm^–1, closely matching the 13.6 μm UIR band. For the larger systems, pentacene and coronene, we observe a continuous structureless background absorption above 400 cm^–1 which is attributed to the high density of IR dipole allowed combination modes for these systems. © The American Astronomical Society 2011

The Astrophysical Journal 741, 2011, 74

Structure Determination of Anionic Metal Clusters via Infrared Resonance Enhanced Multiple Photon Electron Detachment Spectroscopy

We report vibrational spectra of anionic metal clusters, measured via electron detachment following resonant absorption of multiple infrared photons. To facilitate the sequential absorption of the required large number of photons, the cluster beam interacts with the infrared radiation inside the cavity of a free electron laser. Far-infrared spectra of the bare metal cluster anion Nb₁₀^– as well as of the Nb₆C^– anion are measured in the 130–1000 cm^–1 range. The structures of these clusters can be unambiguously determined by comparison with calculated spectra of the putative global minima structures, identified by density functional theory calculations.

J. Phys. Chem. Lett. 2, 1720-1724 (2011)

Time-resolved Holography with Photoelectrons

Y. Huismans, A. Rouzée, A. Gijsbertsen, J.H. Jungmann, A.S. Smolkowska, P.S.W.M. Logman, F. Lèpine, C. Cauchy, S. Zamith, T. Marchenko, J.M. Bakker, G. Berden, B. Redlich, A.F.G. vand er Meer, H.G. Muller, W. Vermin, K.J. Schafer, M. Spanner, M. Yu. Ivanov, O. Smirnova, D. Bauer, S.V. Popruzhenko and M.J.J. Vrakking

Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses to allow to measure electron motion in real time. We present experiments where metastabile xenon atoms are ionized by intense 7-micrometer laser pulses from a free electron laser. Holographic structures are observed that record underlying electron dynamics on a sub-laser cycle time scale, enabling photoelectron spectroscopy with a time resolution almost two orders of magnitude higher than the duration of the ionizing pulse. © 2010 American Association for the Advancement of Science

Science Express, published online 16 December 2010

Science 311, 61-64 (2011)

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