The two-photon decay (2E1) as a test case of our understanding of the structure of heavy atoms or ions

Abstract

In this work we are dealing with the phenomenon of two-photon decay as a second order process in strong electromagnetic fields. In this process a transition between two quantum levels occurs via simultaneous emission of two photons. The energies of the individual photons form a continuous symmetric distribution with a maximum at half the transition energy.

The two-photon decay is of special importance, because to calculate the transition matrix element the whole structure of the atom is required. To calculate the transition probability for such a transition summing over all bound and continuum virtual intermediate states is required. The wave functions and the energies of the intermediate states can be calculated by theory. With a precise measurement of the spectral shape one can test the theoretical predictions. Because these predictions are found in a model wwith certain prerequsites, we therefore can test our understanding of the physical problem.

In the framework of atomic physics we studied the simultaneous emission of two E1 photons for three different cases. The special interest of this work lies on heavy one- and two-electron systems or the analogous multi-electron system of an atom with an inner-shell vacancy. The spectral distributions were measured by a photon-photon coincidence technique. Heliumlike ions are the most simplest systems where the competing influences of electron-electron correlation and relativistic effects on the wave functions and therfore on the spectral distribution of the two-photon decay can be measured. In this work the spectral distribution of the two-photon decay in heliumlike nickel (atomic number Z = 28) and heliumlike gold (Z = 79) has been measured. With the response function of the experimental set-up the matrix element of the two-photon transition could be derived for the first time. For many-electron systems the spectral distribution of the two-photon decay in silver (Z = 47) with an inner-shell vacancy was performed.

This investigation represents the first experiment showing the angular distribution of the two photons emitted in this process.

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