Index Introduction The Research at Rijnhuizen Results in 2008 Education, Training, Outreach and Public Information Output Appendix website Rijnhuizen 3.4 | The IR user facility FELIX, expanded with FELICE Coordinator: Dr. A.F.G. van der Meer Funding: FOM Programme 58 Objectives: To operate the IR user facility FELIX in order to offer the international science community access to a very bright, tuneable mid- and far-IR source. To expand the facility FELIX with FELICE, a new beamline for intra-cavity experiments with a 7 T FTICR and/or a molecular beam set-up. Major funding for an upgrade of the facility was received from NWO in 2002 for a project with the acronym FELICE (Free Electron Laser for Intra-Cavity Experiments), which involves the construction of a new FEL beamline dedicated to gas-phase, intra-cavity experiments in the wavelength range from 3 to 100 µm. Figure 3.12 shows the two existing beamlines (FEL 1 and 2) as well as a new beamline with the FELICE undulator located under the ceiling of the vault. The cavity of FELICE crosses the ceiling and includes on each end a 2-m long horizontal part in which the experimental setups are placed. Figure 3.12: Picture of the FELIX and FELICE free electron laser showing the three beamlines in the accelerator vault. After the demonstration of first lasing of the FELICE beamline in summer 2007, phase I was completed this year. The intensities reached are according to specifications and the beamline operates reliably and reproducibly. In the present configuration, FELICE is driven by the two accelerating modules of the old beamlines, which means that a wavelength range from 5 – 40 microns can be covered. The intra-cavity power available to the experiment is typically a factor of 50 to 100 higher than the intensity provided by FELIX at the user stations and can reach values as high as 2 mJ per micropulse. Interleaved operation of the old beamlines (FEL 1 or 2) and the FELICE beamline has become routine operation, effectively doubling the available beamtime for users. Notwithstanding serious problems with some of the new waveguide components, the planned upgrade of the high-power RF system has been completed. This resulted in a greatly enhanced flexibility in the interleaved operation, to the extent that presently almost any combination of machine settings concerning energy range and micropulse repetition rate is possible. The new RF system is also capable of powering a third accelerating section present in one of the FELICE beamlines. Once commissioned, this beamline will allow extension of the spectral range to 3 - 40 µm. With the implementation of a retractable waveguide structure in the resonator foreseen for 2009, FELICE will reach the design value of 3 - 100 µm. The two intra-cavity experiments - a molecular beam machine and an FTICR mass spectrometer - will be used for infrared spectroscopy of gas-phase species ranging from small clusters to large biomolecules. They are placed in the new building on top of the existing accelerator vault. At present, the molecular beam setup is in place and the FTICR mass spectrometer is under development. In 2008 several upgrades were made to the molecular beam setup, ranging from a laser ablation cluster source to an ion trap and an imaging detector. Following the first test experiments on the ionisation of C60, the internal energy dependence of the IR-REMPI process was studied. Since the relative width of an energy distribution created by multiple photon absorption is inversely proportional to the square root of the number of photons absorbed, well-defined internal energy ensembles can be prepared. However, when integrated over the complete spatial profile of FELICE, these distributions are smeared out over the interaction volume. Therefore, an electrostatic lens ensemble allowing for the spatial mapping of ions created by FELICE on a detector was implemented. Ions created in the centre of the laser beam are thus separated from those made at the edge of the laser beam, thereby partially recovering the narrowness of the internal energy distributions. With this setup the fragment distribution resulting from irradiation of C60 molecules has been measured. In Figure 3.13 a spatially resolved time-of-flight spectrum is shown. It is clearly visible that at longer delay times, corresponding to higher masses, the distribution is more displaced from the laser beam axis and that the smallest fragment, C40+, can only be observed in the centre of the laser beam, so at the highest intensities. The first experimental campaigns on FELICE already indicated that a broad user community can benefit from this new installation. In the first year, experiments on trapped ions, i.e. C60+, on metal-carbide and metal clusters and on strong-field ionisation have been performed. Figure 3.13: Spatially resolved time-of-flight spectrum of the ion distribution after irradiation of neutral C60 with FELICE at 520 cm-1. In order to allow access to the vault during the day for the construction and building of the new beamline, the number of shifts allocated to users on the FELIX installation had to be reduced in 2008 to 80% of the nominal value. As a consequence, the number of beam hours was only slightly over 2600 hours. More than 47% of the beam time went to research groups from other EU countries and about 26% to research groups from non-European countries. In addition, more than 400 beam hours have been produced for experiments making use of the FELICE beamline, 33% of which have been used by the in-house user group and 67% by external users. 3.4.1 User experiments Conformational preference of a peptide A collaboration between the in-house group and external user groups from the University of Oxford has led to the identification the conformational preference of an amyloidogenic peptide. Amyloidoses, such as Alzheimer’s disease and type II diabetes, are a class of ‘protein misfolding’ diseases characterized by the formation of insoluble amyloid fibrils from normal proteins. The cause and nature of fibril formation is subject of intense research efforts and it is of clear importance to understand the structures of amyloid fibrils and the factors influencing their formation at a molecular level. The 306VQIVYK311 sequence in the tau peptide is essential for the formation of intracellular amyloid fibrils related to Alzheimer’s disease. The inherent conformational preferences of the capped peptide were characterized in the gas phase. IR/UV double-resonance spectroscopy of the peptide isolated in a cold molecular beam was used to probe the conformation of the neutral peptide (Figure 3.14 left). Moreover, the influence of protonation was investigated at 298 K by characterizing the protonated peptide ion with IRMPD spectroscopy in the fingerprint and amide I/II band region in an FTICR mass spectrometer (Figure 3.14 right). The neutral peptide was shown to adopt a beta-hairpin-like conformation with two loosely extended peptide chains, demonstrating the preference of the sequence for extended beta-strand-like structures. In the protonated peptide this extended conformation is disrupted and a transition to a random-coil-like structure is induced. Figure 3.14: Infrared spectra measured of the capped protein, Ac-VQIVYK-NHMe, in the 3-micrometer range in a cold molecular beam (left) and of the protonated peptide in the fingerprint regime in an FTICR mass spectrometer. The corresponding preferential confomations – extended beta-strand-like structure and random coil-like structure – are also shown. Structure of neutral gold clusters An example of a collaboration between the in-house group and external user groups from the Fritz-Haber Institut der Max-Planck-Gesellschaft in Berlin and the NRC Steacie Institute for Molecular Sciences in Ottawa is the search for the structure of small neutral gold clusters. Recently, gold clusters have attracted much attention, mainly due to the remarkable catalytic properties of small gold particles. Additionally, with increasing size, gold clusters undergo a fascinating structural evolution. However, most of the knowledge on their structures comes from studies on charged clusters and so far there have been no direct experimental probes for the geometries of neutral gold clusters in the gas phase. In this study the vibrational spectra of neutral gold clusters have been measured by means of photodissociation of their complexes with rare gas atoms. Multiple photon dissociation spectra are recorded in the range of the structure-specific vibrational fundamentals of the gold clusters, i.e. their finger-print range. Photodissociation signals are obtained from as low as 47 cm-1 to about 220 cm-1. For example, a two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. As shown in Figure 3.15, the lowering of the symmetry when a corner-atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19. These larger neutral gold clusters adopt structures very similar to the ones determined for the corresponding anionic species, in contrast to e.g. Au7, where the neutral clusters are distinctly different from the charged ones. Figure 3.15: Infrared photodissociation spectra of the neutral Au20 and Au19 cluster (blue line) compared to the calculated infrared spectra (green line) for the pyramidal structures. Condensed matter physics (CMP) is another field of research for which FELIX presents a very valuable tool. The UK user programme in CMP, largely based on a contract between the British research council EPSRC and FELIX, presently comprises work from user groups in several UK universities and receives about 20% of the beam time being divided over different experimental programmes. Below we present some projects from this field. Spin-galvanic effect in quantum wells In previous FELIX studies, an interesting effect known as the spin-galvanic effect was investigated, in which spin polarization can spontaneously produce a current when in a semiconductor crystal of low symmetry. The optically induced spin-galvanic effect in zero magnetic field was demonstrated and it was shown that the wavelength dependence of the effect has a shape characteristic of the spin-galvanic effect. The results have enabled improvement in the knowledge of the energy splitting of the spin states in zero magnetic field for GaAs based structures, and represent a first step towards generation of spin polarisation using (unpolarised) currents. Recently, spin-current experiments have been performed using more novel material systems like GaN and HgTe semiconductor quantum well structures. Lifetimes of excited states of shallow donor and acceptors in silicon Hydrogen-like transitions in donors such as phosphorus and arsenic in silicon are of considerable technological interest. Using non-linear optical techniques, the relaxation and dephasing of the excited states of the shallow centers (P, As, Sb) in silicon have been investigated in detail. For the 1s-2p0 transition in phosphorus-doped silicon, a lifetime was measured that is close to the inverse of the linewidth measured in isotopically pure silicon. This implies that the dominant decoherence mechanism for excited states is lifetime broadening, just as for atoms in ion traps. These results are important because they show that coherent control and manipulation of atomic-like quantum levels – key to many well known schemes for quantum computing – may be feasible in the most common semiconductor.