Absorption spectroscopy and pressure broadening diagnostics for a three species alkali noble gas comagnetometer

The Global Network of Optical Magnetometers for Exotic physics searches (GNOME) looks for correlated transient signals that could arise from ultralight dark matter or other exotic fields coupled to atomic spin. We are developing a three species comagnetometer using potassium, rubidium, and helium 3 to increase sensitivity to spin dependent energy shifts while rejecting common mode magnetic noise. In this design, optically pumped rubidium polarizes helium 3 by spin exchange, and the resulting spin dynamics are read out with optical rotation of potassium probe light tuned near 770 nanometers. Accurate vapor and buffer gas densities are required to model the response and optimize operating conditions. We record alkali absorption spectra versus cell temperature and fit them with calculated spectra to extract potassium and rubidium number densities and their ratio. We also quantify pressure broadening of the alkali lines from collisions with helium 3 at about three atmospheres, providing an independent estimate of the helium 3 density. These spectroscopy based calibrations inform cell selection, temperature set points, and expected sensitivity for the GNOME comagnetometer upgrade and future science runs.