Abstract A consistent set of improved rotational constants is derived for the X2Σ+, A′2⊿r, A2Πr, B2Σ+,and C2Πr electronic states of lanthanum oxide from the reanalysis of all available data. These constants are obtained from a global fit of the respective bands by a "Direct Approach" technique using complete Hamiltonian matrices for the description of Π and ⊿ states. In order to fit the lines accurately it is shown necessary to include centrifugal distortion effects in the spin-orbit interactions (AJ, AJ and AJJ respectively in A′, and A and C states) as well as in Λ-doubling (PJ pJJ,qJ in C state) and spin-rotation interactions (γJ in B state). More reliable values for the internal partition function and dissociation equilibrium constant of La O are computed for temperatures between 1000 and 8000 K, by taking account of all the known electronic states and using present or recent values for the molecular parameters and dissociation energy.
