THE SPECTRUM OF Li+:

The spectrum of the Li+ ion has been reinvestigated under high resolution in the region 10,000 to 1000 Å with a view to determining the Lamb shifts in the ground state and first-excited state. The hyperfine structure of the line 5485 Å has been studied under improved resolution both with the natural isotopic mixture and with a 96% Li6 sample. For many of the higher levels the hyperfine structure is larger than the fine structure and for large n and l it is even larger than the singlet–triplet separation. The coupling conditions in these cases are discussed in some detail and theoretical intensity distributions in the combined line structure and hyperfine structure are presented for comparison with the partially resolved observed structures. The selection rule ΔG = 0 for the quantum number of the total spin G = I+S is found to be obeyed.On the basis of this detailed analysis a table of energy levels of Li+ is presented including, where possible, the hyperfine splittings. The absolute accuracy of the energy values is only ±3 cm−1 because of the error introduced by the resonance lines at 1 1S–n1P below 200 Å not remeasured in the present work. However, the relative accuracy of the levels with reference to the 2 3S level is considered to be within ±0.05 cm−1 in most cases.The limits of the observed series have been determined with an accuracy of ±0.10 cm−1 and lead to an ionization potential ofFormula: see textwith an error (±3 cm−1) caused entirely by the error in the resonance lines. The term values (ionization potentials) of the 2 3S and 2 1S statesFormula: see textare much more accurate. The difference of the observed ionization potential of Li+ and that calculated from relativistic quantum mechanics is −8.0±3 cm−1, which agrees within the rather large limit of error with the predicted Lamb shift (−7.8 cm−1).