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 spectral purity, flux and fully variable polarization.
DESIRS provides new opportunities for the study of photon-induced processes via the valence shell on mainly isolated gas phase samples, such as cold molecules, radicals, laser-excited species, biomolecules, large ionic biopolymers, clusters and nanoparticles, as well as on condensed matter. This includes high resolution spectroscopy, molecular dynamics and reactivity, and photoionization dynamics studies. In addition, the availability of calibrated versatile (linear, circular) polarisations 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.
A full description of the beamline’s conception and performances is available here.
Note also that the research carried out on DESIRS takes place within the framework of the “Gas phase, atmosphere, universe” scientific section of SOLEIL.
The beamline possesses 3 different branches and 2 endstations available for users. The 3 branches are connected to a common white-light branch (Undulator + first optics) equipped with a gas filter that totally suppresses the high harmonics of the undulator (Plan | Publication):
• 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 linewidth down to 0.08 cm-1 for such a multiplex instrument (corresponding to a resolving power of up to 1000000 @ 10 eV). The resolution can be easily adjusted for absorption spectroscopy requiring high S/N and moderate spectral resolution. (Plan | Publication | FTS instruction manual).
A sample environment chamber located just upstream includes:
- a LN2-cooled windowless absorption gas cell (100 to 300 K, column density up to 1017 cm-2)
- a heated windowless gas cell (up to 1000 K)
- several windowed absorption cells of various lengths for absolute cross sections measurement and/or high column densities. One of them is equipped with a DC discharge for the production of radicals.
- a free molecular beam (rotational cooling down to 20 K column density up to 1015 cm-2).
• 2 monochromatized branches (Branch A and B) : to perform any type 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 (linear horizontal, vertical as well as left- and right-handed circular polarization mode (CPL) fully calibrated owing to a VUV polarimeter.
In the pure CPL mode, the absolute polarization rate, as measured at the sample location, is above 97 % over the whole VUV range.