DESIRS (Dichroïsme Et Spectroscopie par Interaction avec le Rayonnement Synchrotron) is an undulator-based beamline covering the VUV range (5-40 eV) with a unique combination of ultra high resolution (200000 Resolving Power with the monochromator and up to 1000000 with a Fourier-transform spectrometer), spectral purity, flux and fully variable polarization. It provides new opportunities for the study of photon-induced processes via the valence shell on mainly dilute matter, such as cold molecules, molecular complexes, clusters, radicals, isolated nanoparticles, laser-excited species, as well as in the condensed matter. This includes high resolution spectroscopy, molecular dynamics and reactivity, and photoionization dynamics studies. In addition, the availability of calibrated versatile (linear, circular) polarizations of the photon beam is a unique specificity of the beamline allowing the study of molecular chirality and anisotropic properties of matter via different types of dichroism experiments.

The beamline possesses 3 different branches and 2 endstations available for users, connected to a common white-light branch equipped with a gas filter that totally suppresses the high harmonics of the undulator: 

• Fourier-Transform absorption spectroscopy branch (FTS). This branch is equipped with a unique newly designed FT-spectrometer covering the range from 5 to 30 eV with an unprecedented instrumental resolving power of up to 1000000 (that can be lowered down to a few 10000s) for absorption spectroscopy. A sample environment chamber located just upstream includes a LN2-cooled (or Liquid-He) or heated (up to 1000 K) windowless absorption gas cell (100 to 300 K, column density up to 10¹⁷ cm^-2), several windowed cells as well as a free molecular beam (rotational cooling down to 35 K column density up to 10¹⁵ cm^-2). RF and DC discharge cells are also available for the production of transient species such as radicals and ions.

• 2 monochromatized branches (Branch A and B) allowing to perform any types of spectroscopy or dichroism experiments with an optimized resolution/flux compromise ensured by the use of 4 gratings (4300 gr/mm, 2400 gr/mm, 400 gr/mm and 200 gr/mm) (typically 10¹⁰-10¹¹ ph/sec for RP of 100000 up to 10¹³-10¹⁴ ph/sec for a RP of 100), and variable polarization fully calibrated owing to a home-made VUV polarimeter. In the pure circular polarization mode (CPL), the measured absolute circular polarization rate is above 97 % over the whole VUV range.

On branch A is installed a multipurpose molecular beam chamber, SAPHIRS,capable of handling gaseous, liquid and solid samples and equipped with a flash pyrolysis radical source, a MW-discharge flow tube for the production of radicals,  as well as with an aerosol source (atomizer) coupled to an aerodynamic lens. The ionization chamber contains a versatile double imaging electron/ion coincidence (i2PEPICO) spectrometer, DELICIOUS3, able to perform AR-PES on mass-selected (and KER-filtered) samples (electron resolution down to 3% up to 17 eV, mass resolution up to 1500), as well as TPES/TPEPICO with sub-meV resolution. It also permits to measure ion KER on state-selected ions (and more generally electron/ion energy correlations).

Branch B is a free port on which any user experiments can be connected, via a differential pumping stage. The focal point is located 350 mm downstream the last UHV valves, and the beam divergence at the focal point is typically of the order of 5.5 mrad (V) x 3.5 mrad (H). The height of the beam with respect to the ground is 1550 ± 10 mm.

Note that on branch B, the associated experiment CERISES, allowing the study of the reactivity of state-selected ions with neutrals, is available for users. An RF ion trap is also availble for the study of VUV photodynamics on trapped ions (cations and anions) such as biopolymers, metalic clusters, and PAHs.