The hydroxide anion OHˉ has been extensively studied by photodetachment, including rotationally resolved threshold photodetachment measurements by Lineberger and Wester as well as photodetachment microscopy experiments by Blondel. The rotationally-resolved infrared spectrum of OHˉ was measured by Saykally using velocity-modulated IR spectroscopy, while the open-shell X 2  ground state of the OH radical has been characterized extensively using microwave spectroscopy and other methods. This body of work leads to the energy level diagram in Figure , showing the rovibrational structure of the X 1  + anion ground state and the two spin-orbit manifolds of OH.