Absolute measurements of spectral lines of Ar+ ions using collinear and anticollinear geometries were performed. To provide a precise reference for the laser wavelength, iodine saturation spectroscopy was applied. The precision of this reference is effected by observing the beat node between the spectroscopy laser and the corresponding mode of a femtosecond laser frequency comb. Laser‐induced fluorescence allowed to perform precision frequency measurements of an Ar+ transition in collinear and anticollinear geometries simultaneously; then an exact relativistic formula for the absolute transition frequency V0 = VcVa was used. In this geometry the influence of ion source instabilities due to pressure and anode voltage fluctuations was minimized. The result is v0 = 485,573,619.7(3) MHz, which corresponds to Δv/v = 6*10−10. This represents an improvement of two orders of magnitude over the previous NIST published value.

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