The realization of superfluidity in a dilute gas of fermionic atoms, analogous to superconductivity in metals, represents a long-standing goal of ultracold gas research. In such a fermionic superfluid, it should be possible to adjust the interaction strength and tune the system continuously between two limits: a Bardeen–Cooper–Schrieffer (BCS)-type superfluid (involving correlated atom pairs in momentum space) and a Bose–Einstein condensate (BEC), in which spatially local pairs of atoms are bound together. This crossover between BCS-type superfluidity and the BEC limit has long been of theoretical interest, motivated in part by the discovery of high-temperature superconductors. In atomic Fermi gas experiments superfluidity has not yet been demonstrated; however, long-lived molecules consisting of locally paired fermions have been reversibly created. Here we report the direct observation of a molecular Bose–Einstein condensate created solely by adjusting the interaction strength in an ultracold Fermi gas of atoms. This state of matter represents one extreme of the predicted BCS–BEC continuum.

Emergence of a molecular Bose-Einstein condensate from a Fermi gas

A bend-magnet vacuum ultraviolet (VUV) beamline, intended for chemical dynamics studies, was constructed and brought into operation at the Swiss Light Source (SLS) of the Paul Scherrer Institut. The beamline delivers synchrotron radiation in the 5–30 eV photon energy range with a photon flux of 10 11  photons/s at 10 eV and 10 12  photons/s at 20 eV with a resolving power of 2500. The resolving power increases to 10 4 at the cost of photon flux. An in-house designed rare gas filter is used to suppress higher harmonic radiation by a factor of >10 4 , yielding purely monochromatic light in the energy range of 5–21.6 eV. The filter is compact, easy to align, requires a total pumping power of less than 645 l/s and consumes only 3 normal l/h of filter gas. It is located at the end of the beamline, right in front of the experimental endstation. It is usually operated at a higher pressure than the endstation, which offers the additional benefit of protecting the beamline vacuum from sample contamination.

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Vacuum ultraviolet beamline at the Swiss Light Source for chemical dynamics studies

Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.

Chemical Dynamics, Molecular Energetics, and Kinetics at the Synchrotron

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We report on the current status and performance of the toroidal grating monochromator beamline at the Brazilian Synchrotron Light Laboratory (Laboratorio Nacional de Luz Sincrotron). This beamline provides photons in the vacuum ultraviolet and soft x-ray regions from 12to330eV with three interchangeable gratings. We report on the improvement, which allows the possibility of choosing the light polarization degree from linear to almost circular. Here, we also describe the development of a new apparatus, namely, the mirror-inserted harmonic attenuator and calibrating-device with a long length (MIRHACLLE). All beamlines based on diffraction gratings suffer from the problem of high harmonics contaminations to some extent. The MIRHACLLE provides a way to efficiently suppress high harmonics from 25% to 1ppm in a grazing incidence bending magnet beamline. Its principle of operation relays on the absorption of the high energy photons in a gas phase region. It allows negligible high harmonics contamination for phot...

A synchrotron beamline for delivering high purity vacuum ultraviolet photons.

Description and performance of an electron-ion coincidence TOF spectrometer used at the Brazilian synchrotron facility LNLS

We investigate the angular distribution of photoionization fragments at low photon energies (12-40 eV) in an open shell atom, by synchrotron radiation recoil ion momentum spectroscopy in a laser cooled and trapped sample. For cesium atoms, for which relativistic effects play an important role and the ion recoil is relatively small, we could determine large and rapid changes of the asymmetry parameter beta from two, observed for s electrons outside resonances and far from the Cooper minimum. They can be explained by relativistic effects and interchannel coupling arising from final state configuration mixing.

Relativistic and Interchannel Coupling Effects in Photoionization Angular Distributions By Synchrotron Spectrocopy of Laser Cooled Atoms

We report recent improvements and performance on the Toroidal Grating Monochromator (TGM) beamline at the Laboratorio Nacional de Luz Sincrotron – LNLS. Compared to normal incidence monochromators, (NIMs), TGMs provide substantially wider energy range, less resolving power and very poor higher harmonic rejection (HHR). A new concept, MIRHACLLE [1], developed at LNLS, allows reaching more than five orders of harmonic rejection (NIM gives 10%). Previously, we were not able to reach below 12 eV using MIRHACLLE. Here we report improvements extending the lower limit to 7.3 eV. Furthermore, HHR is maintained down to the lower limit using a special gas mixture, the upper limit given by 330 eV is kept.

https://iopscience.iop.org/article/10.1088/1742-6596/425/12/122003/pdf

Improvement on a new concept of beamline delivering high purity VUV photons starting at 7.3 eV

We describe a new time-of-flight (TOF) spectrometer for recoil ion momentum determination The ionization is produced by synchrotron radiation and the sample is laser cooled The main purpose is to measure the recoil suffered by an ion after the ejection of the electron, and to obtain the respective photoelectrons angular distribution Flight trajectories and expected detection efficiencies are discussed Results of TOF spectra for cesium ions from a laser cooled sample are presented

P. T. Fonseca

Papers

Description and performance of an electron-ion coincidence TOF spectrometer used at the Brazilian synchrotron facility LNLS

A synchrotron beamline for delivering high purity vacuum ultraviolet photons.

Relativistic and Interchannel Coupling Effects in Photoionization Angular Distributions By Synchrotron Spectrocopy of Laser Cooled Atoms

Improvement on a new concept of beamline delivering high purity VUV photons starting at 7.3 eV

A time-of-flight spectrometer for synchrotron radiation-based recoil ion momentum spectroscopy in laser cooled atoms