Much work has been devoted in the past to the spectroscopy of transition metal monohalides, essentially because they are of considerable interest in the fields of catalytic chemistry and high-temperature chemistry. In particular, diatomic compounds of group IIIb atoms, Sc, Y, and La, were the best studied experimentally, and also theoretically, on account of their simpler electronic structure, as test cases for modeling the role of the d electron in the chemical bond.
In spite of these efforts, the knowledge of the electronic structures of such species remains still incomplete, even at low energies. Thus, as a continuation of our current research program on scandium and lanthanum halides [see Refs.
(1-4) and references therein], it appeared of strong interest to reinvestigate the electronic spectrum of lanthanum monofluoride, more specially in the infrared spectral region. The very first spectroscopic data concerning this molecule were obtained by Barrow et al. (5). The results were based on the rotational analyses of visible absorption band systems, e 3 ⌽, d 3 ⌽ Ϫ a 3 ⌬, and several singlet-singlet systems. The X 1 ⌺ ϩ symmetry and 6s 2 configuration of the ground state and the separation of singlet and triplet manifolds were later established by Schall et al. (6), who observed a state at 5478 cm Ϫ1 and ascribed it as ⍀ ϭ 2 ( 1 ⌬). More recently, several studies have mainly dealt with electronic states between 15 000 and 25 000 cm Ϫ1 in energy, involved in transitions toward either a 3 ⌬ or X 1 ⌺ ϩ states (7-9).
The production of LaF molecules was ensured in a special furnace at a temperature between 1500 and 1700 K, with a technique similar to that used in the cases of ScCl, ScI, or LaI (2-4). The thermal emission spectrum was analyzed at high resolution (between 0.020 and 0.060 cm Ϫ1 ) with the Fourier transform spectrometer of the Laboratoire Aime ´Cotton.
The dense LaF emission spectrum (more than 30 000 lines) covers the whole spectral range between 4500 and 12 000 cm Ϫ1 . It consists of numerous bands, with strongly developed structures, all degraded toward longer wavelengths, which may be grouped into six systems. Severe overlaps between band systems complicate the analyses. In the Table 1, the wavenumbers of the characteristic heads are listed together with the assignment of the corresponding bands. Some of the states involved in these transitions are characterized for the first time, i.e., the (1) 3 ⌸ and (1) 1 ⌸ from the 5d6s configuration, and, at higher energies, a 3 ⌽ and the components ⍀ ϭ 0 Ϯ of a 3 ⌸ state. Spectroscopic constants are derived from preliminary rovibrational analyses. The electronic states will be hereafter designated following the conventional notation used in theoretical papers.