Quantum-beat spectroscopy of the A(2)Sigma(+) state of the OH free radical.

We have demonstrated the first reported use of quantum-beat spectroscopy for the precision measurement of energy-level splittings. We have measured the magnetic-field-level crossings of different hfs states of the OH free radical as well as the g factors, including the rotational and anisotropic contributions, of several states. We have constructed a Hamiltonian whose eigenvalues are in excellent agreement with our measurements. The hfs constants are b = 719.0 (1.2) MHz and c = 160.5 (0.9) MHz.