We have observed an unusual transition which is predicted to result in visible and near-uv emission from very highly charged titaniumlike ions spanning the entire upper half of the periodic table. Measurements of the wavelengths of the 3d 'D2-D3 transitions in Ba+ and Xe+ are in surprisingly poor agreement with ab initio calculations. This work was carried out in an electron beam ion trap and demonstrates that such a device can be an important tool for visible spectroscopy of highly charged ions. PACS numbers: 32.70.n, 31.25.v, 52.25.b, 52.70.KzAlthough ions having identical numbers of electrons display similar spectral structure, the scale of many atomic properties changes dramatically as the nuclear charge (Z) is increased along an isoelectronic sequence. One example is the rapid decrease in transition wavelengths leading to the characteristic emission of light in the x-ray region of the spectrum for very highly charged ions. It was therefore unusual when Feldman, Indelicato, and Sugar [1] reported calculations predicting a small set of observable visible and near-uv magnetic dipole (Ml) lines for highly charged ions in the titaniumlike isoelectronic sequence. The upper levels of these transitions lie within the ground term, and, therefore, in low density plasmas (~10'3 cm '), where competition from electron

The transition probability for the 3d 4 5 D 2 ← 5 D 3 magnetic-dipole transition in Ti-like Xe ͑Xe 32ϩ ) has been measured using an electron-beam ion trap. The unusually weak dependence of the transition energy on nuclear charge Z, and the fact that the transition wavelength remains in the 320-to 400-nm range for 54ϽZϽ92, makes this transition promising as a plasma diagnostic tool. Our measurement of the transition probability yields 465͑30͒ s Ϫ1 , corresponding to a lifetime of 2.15͑14͒ ms, in good agreement with the theoretical value of 2.4 ms. ͓S1050-2947͑97͒07506-9͔

This paper surveys the ongoing physics experiments at the Electron Beam Ion Trap (EBIT) facility at NIST, with particular attention paid to the underlying physical principles involved. In addition, some new data on the performance of our EBIT are presented, including results related to the determination of the trap width, ion temperature, and number of highly charged ions in the trap.

In isoelectronic sequences, transition wavelengths ordinarily move rapidly to shorter wavelengths as Z increases. However, it has been predicted that a particular sequence of magnetic dipole (M 1) transition wavelengths for the Ti-like ions, Xe ϩ32 through U ϩ70 , remain relatively constant in the 320-400-nm range. In the present paper we extend the experimental identifications of the Ti-like M 1 transitions from Ba and Xe to Nd ͑Nd ϩ38 ͒ and Gd ͑Gd ϩ42 ͒ to verify this behavior. Using the newly acquired wavelengths to adjust atomic parameters, we have also calculated improved wavelengths for all such M 1 transitions between Xe ͑Zϭ54͒ and Os ͑Zϭ76͒.