Improved MgF2etalon plates for Fabry-Perot interferometry in the VUV from 170 to 138 nm

To extend multiple beam interferometry in the VUV range below the limit of fused silica at 167 nm, magnesium fluoride etalon plates have been developed. The coatings are double layer films of Al-MgF 2 as in silica interferometers. These etalon plates give improved performances for the transmission and the recorded finesse. Interferograms are presented for some UV transitions between 160 nm and 138 nm.