  The KLM+KLN Auger electron spectrum of rubidium (Z=37) emitted in the
electron capture decay of radioactive $^{83}$Sr in a polycrystalline platinum
matrix and also $^{85}$Sr in polycrystalline platinum and carbon matrices as
well as in an evaporated layer onto a carbon backing was experimentally studied
in detail for the first time using a combined electrostatic electron
spectrometer. Energies, relative intensities, and natural widths of fifteen
basic spectrum components were determined and compared with both theoretical
predictions and experimental data for krypton (Z=36). Relative spectrum line
energies obtained from the semi-empirical calculations in intermediate coupling
scheme were found to agree within 3$\sigma$ with the measured values while
disagreement with experiment exceeding 3$\sigma$ was often observed for values
obtained from our multiconfiguration Dirac-Hartree-Fock calculations. The
absolute energy of the dominant spectrum component given by the semi-empirical
approach agrees within 1$\sigma$ with the measured value. Shifts of +
(0.2$\pm$0.2) and - (1.9$\pm$0.2) eV were measured for the dominant KLM
spectrum components between the $^{85}$Sr sources prepared by vacuum
evaporation on and implanted into the carbon foil, respectively, relative to
$^{85}$Sr implanted into the platinum foil. A value of (713$\pm$2) eV was
determined for the energy difference of the dominant components of the KLM+KLN
Auger electron spectra of rubidium and krypton generated in the polycrystalline
platinum matrix. From the detailed analysis of the measured data and available
theoretical results, the general conclusion can be drawn that the proper
description of the KLM+KLN Auger electron spectrum for Z around 37 should still
be based on intermediate coupling of angular momenta taking into account
relativistic effects.
