Barium Autoionization for Efficient Ion Trap Loading

Abstract

We report a theoretical and experimental investigation of autoionizing resonances from the $5d6p\,{}^3\mathrm{D}_1^o$ manifold in neutral barium for efficient loading of ion traps. Our calculations predict large resonant cross sections for many narrow autoionizing resonances, but we find experimentally that for most of these, Doppler broadening during trap loading depresses the effective cross sections that can be achieved in practice. We identify and demonstrate a strong, broad transition at $531\,\mathrm{nm}$, and show that it furnishes an order-of-magnitude increase in trap loading efficiency compared to other demonstrated resonances.

Figures (7)

• Figure 1: Level diagram of measured photoionization schemes in this work. The $413\,\mathrm{nm}$ + $531\,\mathrm{nm}$ scheme on the far right is found to be the most efficient for loading barium ions into the trap.
• Figure 2: Spectrum of the autoionizing resonances originating from the $5d6p\,{}^{3}\mathrm{D}_1^o$ state. Data points show the measured cross section, and red lines indicate the theoretical calculations. The calculated peak positions are in reasonable agreement with the measured frequencies, although the peak heights of the narrower resonances exhibit a mismatch.
• Figure 3: Autoionization resonances to the $5d\,{}_{3/2}15d\,{}_{3/2} \,\,J=2$ state (right) and the $5d\,{}_{3/2}15d\,{}_{5/2} \,\,J=1$ state (left). The disagreement in the height of the autoionizing peaks relative to the theoretical calculations (red) is explained by Doppler broadening, as evidenced by the much closer agreement of the cross section produced by convolving the theoretical curve with a Gaussian profile(blue).
• Figure 4: Spectrum of the autoionizing resonances from the $5d6p\,{}^3\mathrm{D}_1^o$ state. The measured spectrum (black points) agrees well with the theoretical prediction (red lines), without the application of additional broadening.
• Figure 5: Average number of ions loaded per ablation shot as a function of 530.8 nm laser power. The red dashed line shows a linear fit to the data ($\chi^2/\mathrm{dof} = 1.01$), indicating that the loading remains linear over the explored power range with no evidence of saturation.