The atomic nucleus and its electrons are often thought of as independent systems that are held together in the atom by their mutual attraction. Their interaction, however, leads to other important effects, such as providing an additional decay mode for excited nuclear states, whereby the nucleus releases energy by ejecting an atomic electron instead of by emitting a γ-ray. This 'internal conversion' has been known for about a hundred years and can be used to study nuclei and their interaction with their electrons. In the inverse process-nuclear excitation by electron capture (NEEC)-a free electron is captured into an atomic vacancy and can excite the nucleus to a higher-energy state, provided that the kinetic energy of the free electron plus the magnitude of its binding energy once captured matches the nuclear energy difference between the two states. NEEC was predicted in 1976 and has not hitherto been observed. Here we report evidence of NEEC in molybdenum-93 and determine the probability and cross-section for the process in a beam-based experimental scenario. Our results provide a standard for the assessment of theoretical models relevant to NEEC, which predict cross-sections that span many orders of magnitude. The greatest practical effect of the NEEC process may be on the survival of nuclei in stellar environments, in which it could excite isomers (that is, long-lived nuclear states) to shorter-lived states. Such excitations may reduce the abundance of the isotope after its production. This is an example of 'isomer depletion', which has been investigated previously through other reactions, but is used here to obtain evidence for NEEC.

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The Kβ x-ray spectra of the elements from Ca to Ge have been systematically investigated using a highresolution antiparallel double-crystal x-ray spectrometer. Each Kβ 1,3 natural linewidth has been corrected using the instrumental function of this type of x-ray spectrometer, and the spin doublet energies have been obtained from the peak position values in Kβ 1,3 x-ray spectra. For all studied elements the corrected Kβ 1 x-ray lines FWHM increase linearly as a function of Z. However, for Kβ 3 x-ray lines this dependence is generally not linear in the case of 3d elements but increases from Sc to Co elements. It has been found that the contributions of satellite lines are considered to be [KM] shake processes. Our theoretically predicted synthetic spectra of Ca, Mn, Cu, and Zn are in very good agreement with our high-resolution measurements, except in the case of Mn, due to the open-shell valence configuration effect (more than 7000 transitions for diagram lines and more than 100 000 transitions for satellite lines) and the influence of the complicated structure of the metallic Mn.

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Relativistic multiconfigurationDirac-Fock (MCDF) calculations in the average-level (MCDF-AL) version with the inclusion of the transverse (Breit) interaction, self-energy, and vacuum polarization corrections have been carried out for molybdenum, palladium, lanthanum, and holmium to elucidate the structure of the KnL" lines in their x-ray spectra. The examination of the effect of removing electrons from M shell on the principal Ka(KaL ) lines indicates that the most significant is the effect of removing 3p and then 3s subshe11s and that the effect of additional holes strongly increases with the atomic number. Detailed calculations have been performed on palladium to shed light on the structure of the KaL" satellite lines of its x-ray spectrum, which is the first systematic theoretical study on the structure of these lines of a heavy atom. It has been found that in all cases the "average" positions of the groups of lines corresponding to the transitions of the type 1s '2p "~2p '" " obtained in the present work are very close to the recently measured experimental positions of appropriate KaL" satellite lines. On the other hand, the structure of the satellite lines has been shown to be much more complex than can be observed experimentally. The results of this work can be used to construct different theoretical KuL" spectra for palladium, satisfactorily reproducing the shape of various experimental spectra generated by different inducing projectiles.

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Very extensive multiconfiguration Dirac-Fock calculations including the transverse ͑Breit͒ interaction and quantum electrodynamics corrections have been carried out for all 3d transition metals to explain reliably the dependence of K␤-to-K␣ x-ray intensity ratios on the changes in configurations of the valence electrons. For all considered atoms the greatest values of the K␤-to-K␣ intensity ratios are for 3d mϪ2 4s 2 configuration type, then for 3d mϪ1 4s 1 , and the smallest values are for 3d m type. It has been found that for each type of electronic configuration the K␤-to-K␣ intensity ratios increase evidently with the atomic number and for a particular atom they are quite sensitive to the changes of the valence electronic configuration. The greatest relative increase of the K␤-to-K␣ intensity ratios ͑4%͒ as a result of transition from electronic configuration of the 3d mϪ1 4s 1 type to the 3d mϪ2 4s 2 type takes place for Sc and the smallest relative increase ͑2%͒ takes place for Cu. The presented results make it possible to carry out reliable interpretation of various experimental K␤-to-K␣ x-ray intensity ratios for 3d transition metals in their compounds and alloys and can also provide quantitative information about the changes of the valence electronic configurations of these metals in considered systems.