Precision study of theA Σ+2state of the OH radical

We report precision measurements and calculations of several parameters of the $A {}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ state of the OH free radical. Data from quantum beats in the fluorescence of laser-excited molecules and direct optical spectroscopy have been combined to yield the following values for the parameters of our model Hamiltonian: $\gamma =6862\mathrm{}\left(43\right)\mathrm{}$ MHz, $b+\frac{c}{3}=774.1\mathrm{}\left(0.4\right)\mathrm{}$ MHz, $c=168.9\mathrm{}\left(0.8\right)\mathrm{}$ MHz, $g_r=0.00248\mathrm{}\left(0.6\right)\mathrm{}{\mu }_B$, and $g_A=0.0242\mathrm{}\left(11\right)\mathrm{}{\mu }_B$. Calculations that include coupling to the ground $X {}_{}{}^2{}_{}{}^{}\Pi$ states through rotational or spin-orbit interactions produce estimates of these parameters that agree very well for all except $g_A$.