2 Experimental

LaO vapor was obtained by heating a mixture of LaI₃ powder with metallic lanthanum, in presence of traces of oxygen, in the tantalum heater of a special furnace filled with argon as a buffer gas. At the operating temperature, between 1600 and 1800 K, the pressure was typically between 15 and 20 Torr. A description of the furnace can be found elsewhere [11, (Shenyavskaya et al. 1973)]. LaO molecules were then excited by the light of a fixed-frequency argon-ion or krypton-ion laser which was beamed into the heater. Only Ar⁺ 501.7-nm and Kr⁺ 520.8-nm lines were found to give rise to resonance transitions in the spectral region of the $B{\rightarrow}X$ system. The induced fluorescence spectra were recorded from $16\:500$ to $18\:600$ cm^-1, with a commercial Fourier transform spectrometer (Bomem DA 3). With this apparatus, a resolution of about 0.06 cm^-1 is currently achieved, corresponding to an uncertainty of about 0.007 cm^-1 in the determination of the positions of the sharpest lines. More details on the experimental procedure have been given by [12, Shenyavskaya et al. (1993)].