Wavelength calibration of the C I line at 94.5 nm for comparison with quasar data

With the use of an ultra-narrow-band extreme ultraviolet laser source, tunable near 94 nm, transition wavelengths are determined for lines connecting the 1s²2s²2p^{2 3}P_0,1,2 ground-term levels to the 1s²2s2p^{3 3}S₁ excited level in neutral carbon at an absolute accuracy of 4×10⁻⁸. With the determination of the zero-velocity rest-frame wavelengths these lines can be included in an analysis of a possible temporal variation of the fine-structure constant α from a comparison with quasar data. A value for the C¹²/C¹³ transition isotope shift was also obtained yielding 0.5107(13) cm⁻¹, in average over the three fine-structure lines. The latter measurement will allow to study isotopic evolution in the universe and test models of nuclear processes in stars.