Direct determination of the energy of the first excited fine-structure level in ${\mathrm{Ba}}^{6+}$

We present an experimental observation of the visible magnetic dipole transition between the fine structure levels in the ground term of ${\mathrm{Ba}}^{6+}$ ($5s{}^{2}5p^2$: ${}^{3}P_0-{}^{3}P_1$) with a compact electron beam ion trap. The wave number in vacuum of the transition is determined to be 15505.506(37) ${\mathrm{cm}}^{-1}$ by simultaneously observing the reference lines of neutral Ne. The present measurement reduces the uncertainty in the previously reported value obtained from observation of the electric dipole transitions to the ground terms in the extreme ultraviolet range [Phys. Scr. 46, 403 (1992)].

Figure 1

(a) Introduced Ne gas pressure dependence of visible emission spectra of ${\mathrm{Ba}}^{6+}$ and Ne. The introduced Ne gas pressure measured in the room temperature region of the CoBIT chamber are shown under each spectrum. The emission peaks of the neutral Ne atoms and the ${\mathrm{Ba}}^{6+}$ ions are indicated by the red arrows and the blue arrows, respectively. (b) External Ne lamp spectrum observed before the measurement of (a).

Figure 2

Comparison of the peak position between the external Ne lamp (red) and the internal emission from the CoBIT trap region for the neutral Ne line at 610 nm. The corresponding wave number calibrated by the Ne lamp spectrum is indicated on the upper side of the graph. The left vertical axis is for the external lamp (red) and the right axis for the internal emission (blue).

Figure 3

Typical visible emission spectrum of ${\mathrm{Ba}}^{6+}$ and neutral Ne without background subtraction as a function of column number. The corresponding wave number was scaled by the Ne lamp spectrum.

Figure 4

(a) Relation between the literature values [30] of the observed Ne peak wave numbers and the relative column position in the CCD sensor with respect the ${\mathrm{Ba}}^{6+}$ peak. The blue line is the calibration curve obtained by fitting with a cubic polynomial function. (b) Residual plots of each calibration Ne line from the calibration curve. The right vertical axis shows the corresponding wave number. The interval between the blue lines shows the range-of-confidence interval ($1\sigma$) of the calibration curve.

Figure 5

Experimental wave number (in vacuum) for (a) ${}^{3}P_0-{}^{3}P_1$ in ${\mathrm{Ba}}^{6+}$ determined with the seven calibration lines in Fig. 3, and for (b) the Ne No. 2 line in Fig. 3 determined with the six calibration lines without No. 2 (see text for details). The black solid line is the weighted average of this data set. The blue dashed lines show the range of the standard error. The purple solid line is the literature value [30].