The I2 D'(2g)-A'(2u) transition has been excited by collisionally induced optical double resonance. This system has been rotationally analysed for more than 50 vibrational levels of the A'(2u) state. The long range behaviour of the A' state has been studied. The dissociation energy of this state has been determined to be Dc = 2 505.7 ± 1.9 cm-1. The D' state has been identifed with the α state of King et al. (Chem. Phys. 56 (1981) 145). The energies of the D' and A' states have been determined relative to X 1∑g+(ν = 0, J = 0) to be T0 = 40 331.6 ± 1.6 cm-1 and To = 9 988.7 ± 1.6 cm-1, respectively. The mechanism of this double resonance is discussed. Efficient collisional transfer to the D' state is observed and helps to shed some light on the operation of the optically pumped I2 D'-A' laser

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.

A rotational analysis of several bands of the D«-X system of 123 Sb -123 Sb is carried out. It is shown that the hitherto assumed vibrational classification of the D X system is certainly incorrect, as well as a previously given value for the rotational constant of the X state. B(X 1 2g + , £ = 0) is found equal to 0.044263 cm -1 . The perturbations appearing in the various vibrational levels are interpreted in terms of interactions with a new electronic state, labelled L.