Fine-Structure Wavelength Measurements in Two - Phosphorus and Calcium.

The wavelengths of two n = 2 state fine-structure transitions have been measured for the first time in helium -like phosphorus (Z = 15) and calcium (Z = 20). The two transitions, 1s2s ('3)S(,1) - 1s2p ('3)P(,2) and 1s2s ('3)S(,1) - 1s2p ('3)P(,2), were studied in P('13+) and Ca('18+) by means of fast-ion spectroscopy. All observations were carried out in the vacuum ultraviolet spectral region using a one meter normal incidence monochromator and windowless photon detector. The wavelengths of these ('3)S - ('3)P transitions exhibit large contributions due to relativistic as well as quantum electrodynamic effects in the atomic structure. Precise measurements of the two transition wavelengths in phosphorus and calcium allow detailed investigation of these contributions in highly-ionized two-electron ions. Comparison of the measured wavelengths with recent calculations shows good consistency. The measured wavelengths values for the transitions are, in phosphorus (('3)S(,1) - ('3)P(,0)): 813.16 (+OR-) 0.05 (ANGSTROM), (('3)S(,1) - ('3)P(,2)): 739.98 (+OR-) 0.02 (ANGSTROM); in calcium (('3)S(,1) - ('3)P(,2)): 466.78 (+OR-) 0.08 (ANGSTROM).