Abstract
Recently there have been several new experiments in two-photon bremsstrahlung in electron-atom collisions which point to the need for inclusion of polarizational or “atomic” bremsstrahlung (PB) in the theory of two-photon emission. Experiments with 9–15 keV electrons in the 90° geometry are found to disagree with the Born approximation but agree well with the nonrelativistic Coulomb theory for higher radiated photon energies. However, there is a significantly higher two-photon rate for lower radiated photon energy. An experiment with 70 keV electrons in the 90° geometry resolves the two order-of-magnitude discrepancy with theory reported in the 1985 experiment at 75 keV, and is in good agreement with the relativistic first Born approximation. On the other hand, while experiments with 70 keV electrons in the ±45° geometry find order-of-magnitude agreement with the first Born approximation for some photon energies, there is a discrepancy with the simple Z 2 dependence predicted by the Born approximation. A significant discrepancy with the Born theory is also seen for Fe and Cu when one of the two radiated photons has an energy near the K X-ray energy of the target atom. None of the available calculations include PB effects, which might be expected to lead to photon energy dependent enhancements.
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