Measurement of the two-photon spectral distribution from decay of the 1s2s;{sup 1}S{sub 0} level in heliumlike nickel

A measurement of the shape of the spectral distribution of two-photon decay of the 1s2s{sup 1}S{sub 0} level in heliumlike nickel is described. Uncertainties in detector efficiencies which had limited the precision of earlier measurements were eliminated by comparing the continuum emission from two-photon decays of H-like and He-like nickel. Our results are in agreement with the nonrelativistic calculation of Drake and the fully relativistic calculation of Derevianko and Johnson and suggest a method for testing relativistic atomic many-body theory in strong fields. {copyright} {ital 1999} {ital The American Physical Society}