SOLEIL improves VUV absorption spectroscopy with reflective spectrometer design

Saint Aubin, France--Typically, vacuum ultraviolet (VUV) absorption spectroscopy uses either lasers or grating spectrometers on synchrotron sources. But a SOLEIL group, in collaboration with the Institut d’Optique - Laboratoire Charles Fabry (Orsay) has designed, developed, and implemented an absorption spectrometer using a new concept based on based on a wavefront-division VUV interferometer using exclusively reflective surfaces.

The study of ultra-high resolution spectroscopy of small molecule compounds in the gas phase, of astrophysical or atmospheric interest (interstellar medium, the atmosphere of planets, cosmology), using absorption spectroscopy, provides the "signature" of the system studied--its nature, density, temperature or isotopic composition. Moreover, if the photons used are in the far-UV (known as VUV), they are likely to induce a rich photochemistry through photodissociation. The corresponding spectroscopic data then provides additional information regarding the photodynamics of small molecular systems which, by comparison with spectroscopic data obtained via instruments embarked aboard satellites such as Hubble, help determine the conditions for survival of certain molecules in the interstellar medium, including carbonaceous matter of prebiotic importance. These subjects require absorption spectra recorded with a high resolving power over a broad spectral range. The basic measurements that describe the molecules of astrophysical interest are deduced from these spectra and the greater the resolving power and the number of absorption lines taken into consideration, the more reliable and accurate these are. In addition, high-resolution experimental data could improve theoretical models, thus leading to a better understanding of the processes governing atmospheric and interstellar chemistry.

To carry out this research program, a new Fourier Transform (TF) spectroscopy instrument was developed and installed on a dedicated branch of the DESIRS beamline at the SOLEIL synchrotron. Fourier transform spectrometry (FTS) is an analytical technique widely used, from the infrared to the UV. The success of this type of instrument is based primarily on the accuracy of the spectral scale, coupled with the potential to achieve very high resolution. Most Fourier transform spectrometers are based on Michelson type interferometers requiring at least one beamsplitter. The main problem for the development of the FTS technique in the VUV range is the difficulty encountered in manufacturing beamsplitters for such short wavelengths.

This instrument, which is the subject of a Nature Photonics article at http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2010.314.html, is based on a wavefront division VUV interferometer using exclusively reflective surfaces. Its development, a collaboration between the Institut d’Optique-Laboratoire Charles Fabry (Orsay) and SOLEIL, is an instrumental achievement that pushes the state-of-the-art opto-mechanics and interferential nanoscale coding, to its limit. Its performance is exceptional since recent measurements have achieved a raw resolving power of 850,000 on the Krypton absorption spectrum. For comparison, broadband experimental techniques such as grating-based spectrometer, can at best achieve a resolving power of about 200,000 in the same energy range and with a much longer acquisition time (up to a factor of 30) than the FT spectrometer, for similar data quality. Moreover, the absolute spectral calibration is about two orders of magnitude as accurate with the FT spectrometer.

SOLEIL is the third-generation French synchrotron, a research centre directed by the CNRS (French National Centre for Scientific Research) and the CEA (French Atomic Energy Commission). Its purpose is to accelerate bunches of electrons until they radiate extremely bright light covering a very wide range of wavelengths: from infrared to x-ray, with ultraviolet in between. The characteristics of this light (intensity, focus, stability, polarization) allow scientists to observe matter down to the atomic level and perform experiments in the interests of fundamental, applied, and industrial research.

SOURCE: SOLEIL Synchrotron; www.synchrotron-soleil.fr