The pure rotational spectrum of YbOH

The pure rotational spectrum of YbOH has been recorded and analyzed to produce fine and magnetic hyperfine parameters for the ${\tilde{X}}^2{\mathrm{\Sigma }}^{+}$(0,0,0) state. These parameters are compared with those determined from the optical study [Melville and Coxon, J. Chem. Phys. 115, 6974-6978 (2001)] and with the values for YbF [Dickinson et al. 115, 6979-6989 (2001)]. The results support the existence of an unobserved perturbing state near the ${\tilde{A}}^2{\mathrm{\Pi }}_{1\mathrm{/}2}$ state, similar to that previously found in YbF. The precisely determined parameters lay the foundation for laser cooling YbOH, which will aid in the search for new physics beyond the standard model.