The need for accurate laboratory data on spectral lines
For comparisons with quasar spectral data spectral accuracies on the order of 10-7 or better are required but for many lines these accuracies have not been reached yet in laboratory studies. Berengut et al. (Sydney) have produced a list of lines to be measured more accurately in order to facilitate searches for alpha variation. Our aim is to use our advanced laser systems and calibration techniques to provide transition frequencies of important lines at the desired level of accuracy, i.e. better than 10-7.
With the use of the XUV-laser system the very strong resonance line in neutral atomic carbon at 94.5 nm was calibrated with an accuracy of 4 x 10-8. A simultaneous recording of the 12C and 13C isotopes is displayed in the figure below. Also shown, on the right is the energy level scheme of atomic carbon. Redundant information was obtained by measuring all three spectral lines connecting the 3P fine structure ground state levels with the single 3S level that lies in the continuum above the ionization potential. This redundant information serves as a chack on the calibration procedures. For further information on the results see [Phys. Rev. A. 71, 040501(R) (2005)].
Recently a study was completed to calibrate 12 spectral lines in the spectrum of N I in the range 95.1-96.5 nm. The results are published in [J. Phys. B. 38, L383 (2005)].
Further work is in progress to investigate and calibrate lines in Mg I and O I.