Very high spectral resolution obtained with SU5: A vacuum ultraviolet undulator-based beamline at Super-ACO

In this article we wish to report on the commissioning of the SU5 undulator -based beamline, whose primary scientific goal deals with high-resolution s pectroscopy and photon-induced dynamics in the vacuum ultraviolet range on dilute species. In order to achieve such a scientific program, we have conc eived an original optical design, centered around a 6.65 m off-plane Eagle monochromator equipped with two gratings (2400 and 4300 l/mm) illuminated b y an astigmatic prefocusing optical system. The different components of the actual beamline, such as the optical elements, their holders/manipulators and the monochromator are described, with a special emphasis on critical as pects such as the mechanical resolution and stability, the vibrations limit ations, and the thermal stability. Then, a spectral calibration procedure o f the monochromator is described, followed by the presentation of the measu red performances of the beamline in terms of ultimate resolution and flux. Owing to a gas phase ion yield experiment on rare gases, we have been able to measure a raw linewidth of 184 mu eV at 21.61 eV (18s' line of Ne) corre sponding to a resolving power of 117 000 and a raw (respectively, lifetime- deconvoluted) linewidth of 119 mu eV (respectively, 76 mu eV) at 15.82 eV ( 13s' line of Ar) corresponding to a raw resolving power of 133 000 (respect ively, 208 000). The ultimate targeted 10(5) resolving power is therefore o bserved on most of the VUV range, which corresponds to an unprecedented per formance. The measured photon flux in a 1/50 000 bandwidth is in reasonable agreement with the expected values lying in the 10(10)-10(9) photon/s rang e over the VUV range. These very satisfactory performances, with a slit-lim ited resolution down to about 15 mum slits aperture, are due to the special care which has been paid at both the conception and construction stages co ncerning critical issues such as the optical and mechanical design, the vib rations, the driving/encoding system, and the slope errors of the gratings. (C) 2001 American Institute of Physics.