Two-photon decay in gold atoms

We have measured the energy differential transition probabilities for the two-photon decay of K vacancies in gold atoms (nuclear charge Z=79). This is the heaviest atom for which this information has been obtained, and so is most sensitive to relativistic effects. The experiment determined the shape of the continuum radiation for the transitions 2s -> 1s, 3s -> 1s, 3d -> 1s, and (4s+4d)-> 1s at an emission pair opening angle theta=pi/2. Our results for 3d -> 1s and (4s+4d)-> 1s extend to energies above and below the region of the intermediate state resonances. No relativistic calculations exist for Au, so we compare with calculations by Mu and Crasemann and Tong for Ag (Z=47) and Xe (Z=54). For equal-energy, back-to-back two-photon decay, the calculations show an increase in transition probability with Z for the 2s -> 1s and 3d -> 1s transitions. In contrast, our data, at Z=79, corrected for the angular distribution, give a smaller transition probability than the lower-Z experimental results of Ilakovac and Mokler for Ag and Xe. The shapes of the two-photon continua in our data are in general agreement with theory except that we find anomalously high values for the differential two-photon transition probability for the 3s -> 1s transition near y=0.35, where y is the fraction of the transition energy carried by the lower-energy photon.