Fourier-transform spectrometer has highest vacuum-UV resolving power

Saint-Aubin, France--French researchers have designed, developed and implemented a high-resolution vacuum-UV (VUV) Fourier-transform (FT) spectrometer based on all-reflective optics and a synchrotron as the light source. A group at the SOLEIL synchrotron, in collaboration with the Institut d'Optique--Laboratoire Charles Fabry (Orsay), has designed, developed, and implemented the absorption spectrometer.¹

High-resolution VUV absorption spectroscopy is a unique tool for understanding the electronic structure of atoms and molecules, with applications notably in astrophysics, cosmology, and atmospheric sciences. So far, this technique has been applied using either lasers or grating spectrometers on synchrotron sources. But none of these methods can simultaneously provide access to high resolution, wide tunability and accurate wavenumber scale.

The FT spectroscopy instrument was developed and installed on a dedicated branch of the DESIRS beamline at the SOLEIL synchrotron. 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 FT spectrometers are based on Michelson type interferometers requiring at least one beamsplitter -- but beamsplitters are difficult to make for such short wavelengths.

Accurate spectral calibration

The new spectrometer is based on a wavefront-division VUV interferometer using exclusively reflective surfaces. Its development pushes the state-of-the-art in opto-mechanics and interferential nanoscale coding to its limit. Measurements show a raw resolving power (RP) of 850,000 on the krypton (Kr) absorption spectrum. This RP, recorded in a region near the first ionization threshold of Kr at 88 nm, is accessible over a wide spectral range. For comparison, broadband experimental techniques such as grating-based spectrometers can at best achieve an RP 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 more accurate with the FT spectrometer.

Operational since 2008 on the DESIRS beamline, this instrument, combined with an absorption cell and a dedicated molecular beam, has already hosted several international groups who have been able to exploit its unique capabilities, notably for the study of interstellar photochemistry and the stability of fundamental constants over cosmological time.

REFERENCE:

1. N. de Oliveira et al., Nature Photonics, DOI: 10.1038/NPHOTON.2010.314, (2011).