High-precision wavelength measurements of the 1s21S-1snp1P resonance lines in He-like nitrogen and oxygen

Spectra of C, N and O have been generated simultaneously by focusing 1 GW laser pulses on targets made of either ammonium hydrogen carbonate or ammonium oxalate. These materials, in the form of compressed powders, were found to give significantly smaller linewidths than more conventional targets such as boron nitride or aluminium oxide. The wavelengths of the 1s 2 1 S-1snp 1 P (n=2-4) resonance lines in N VI and O VII (17-30 AA) have been determined with uncertainties ranging from 0.2 to 0.7 mAA, by measurements in up to the 8th spectral order. Previous experimental results are confirmed, but the substantially increased accuracy of the present values reveals small but significant and systematic discrepancies with theoretical predictions for the 1s 2 1 S-1s2p 1 P transition in N VI and O VII.