Measurement of the Zeeman effect in an atomic anion: Prospects for laser cooling of ${\text{Os}}^{-}$

The negative osmium ion ${\text{Os}}^{-}$ is one of very few atomic anions potentially suitable for laser cooling. We have made a measurement of the Zeeman splitting of a bound-bound transition in ${}^{192}$${\text{Os}}^{-}$ by studying the laser excitation from the 5$d^7$6$s^2$ ${}^4$$F$${}_{9/2}^e$ ground to the 5$d^6$6$s^2$6$p$ ${}^6$$D$${}_{9/2}^o$ excited state in a homogeneous external magnetic field. The experimental Landé factors $g_J=1.31\left(7\right)$ and $g_J=1.50\left(8\right)$, respectively, agree well with calculated values. Both levels are found to split into 10 Zeeman sublevels, resulting in 28 allowed transitions of different relative intensities, in agreement with calculations based on pure and composite $LS$ states. In view of the experimental results, the prospects for laser cooling of ${\text{Os}}^{-}$ are discussed.

Figure 1

Experimental setup for transverse spectroscopy in the homogeneous-magnetic-field region of a superconducting magnet. The ion beam (gray) and laser beam (red) are superimposed perpendicularly at the center of the magnet.

Figure 2

Measured absorption spectra in an external magnetic field of 35 mT. Top: $2\times$ 9 peaks of the $\sigma$ transitions; bottom: 10 peaks of the $\pi$ transitions. Experimental data are represented by black dots with vertical error bars, and the corresponding fits (see text) by solid lines.

Figure 3

Landé $g$ factors extracted from the fits of the experimental Zeeman spectra (see text). The weighted means are indicated as the bands (1$\sigma$ uncertainties) between the dashed horizontal lines.

Figure 4

Partial energy-level diagram of ${}^{192}$${\text{Os}}^{-}$ in an external magnetic field. Zeeman sublevels are not to scale. The potential laser cooling ($\pi$) transition is represented as a green arrow, and the undesired (${\sigma }^{+}$) decays to dark states are indicated by red arrows.