Laser Spectroscopy on Os⁻: A Prerequisite for the Laser Cooling of Atomic Anions

Laser cooling of neutral atoms or positive ions is today routinely employed in numerous experiments. Negative ions, in contrast, have distinct characteristics which hamper the application of lasers for cooling. But in 1999, the discovery of the unique bound–bound electric dipole transition in the negative osmium ion provided the motivation for a first cooling attempt. This thesis presents the first milestones toward the ultimate goal of laser cooling negative osmium, including high-resolution laser spectroscopy of the relevant bound–bound E1 transition. Its frequency – between the ground 4F9/2 and the 6DJe1 (bound) excited states – was determined to be 257.831190(35) THz in 192Os-, in agreement with a previous measurement, but two orders of magnitude more precise. The determination of the resonant cross-section implicitly provided the corresponding Einstein A coefficient, which was found to be A ~ 330 s-1. Furthermore, the isotope shift of the E1 transition and the hyperfine structure constants of the ground and excited state were obtained, for the first time, from the analysis of the spectra of all naturally abundant isotopes. The hyperfine structure revealed the heretofore unknown total angular momentum of the excited state to be Je1 = 9/2. Finally, laser spectroscopy in an external magnetic field confirmed the expected line splitting for Os- due to the Zeeman effect. Based on all these experimental results the prospect of laser cooling negative osmium is reviewed