Observation of a collective excitation in the ejected-electron spectra of Yb and Ba

Ejected-electron spectra of Yb and Ba are presented. In both spectra, structure was seen which is attributable to autoionization to ground and excited states of $M^{+}$ ($M=\mathrm{Ba}\mathrm{or}\mathrm{Yb}$), and to Auger decay of highly excited $M^{*+}$ states to the ground state of $M^{2+}$. In Yb, Auger decay was also observed to populate excited states of $M^{2+}$. Both spectra are strongly affected by a collective resonance in the excitation of a $5p$ electron. For energies above the $5p$ ionization threshold, two-step autoionization to $M^{2+}$ was found to be dominant.