Measurement of electron-impact–excitation cross sections for very highly charged ions

We report the first measurements of electron-impact–excitation cross sections for very highly charged ions (${\mathrm{Ba}}^{46+}$), and introduce a powerful new technique for studying these ions. Approximately 2×${10}^4$ ${\mathrm{Ba}}^{46+}$ ions/cm were trapped inside the space charge of an ≃120-mA electron beam and their x-ray emission spectra observed with Si(Li) and Bragg-crystal–diffraction spectrometers. Cross sections for several N=3 levels excited from the neonlike ${\mathrm{Ba}}^{46+}$ ground state were measured relative to radiative recombination on the same ions.