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Abstract:
A novel technique for absolute wavelength determination in high-precision crystal x-ray spectroscopy
recently introduced has been upgraded reaching unprecedented accuracies. The method combines vis-
ible laser beams with the Bond method, where Bragg angles (
θ
and
−
θ
) are determined without any x-
ray reference lines. Using flat crystals this technique makes absolute x-ray wavelength measurements
feasible even at low x-ray fluxes. The upgraded spectrometer has been used in combination with first
experiments on the 1
s
2
p
1
P
1
→
1
s
21
S
0
w
-line in He-like argon. By resolving a minute curvature
of the x-ray lines the accuracy reaches there the best ever reported value of 1
.
5ppm. The result is
sensitive to predicted second-order QED contributions at the level of two-electron screening and two-
photon radiative diagrams and will allow for the first time to benchmark predicted binding energies for
He-like ions at this level of precision.
