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A spectroscopy experiment has been performed on lithium atoms in the presence of a strong magnetic field (4.9 T) using multistep laser excitation of high lying levels. In this experiment, the peculiar excitation scheme is designed in order to selectively tests the dynamical symmetry of the excited states. This allows the direct observation of the transition to chaos from the breaking of these symmetries. The comparison between the experimental spectrum and a calculated one for hydrogen shows a remarkably good agreement.

The sub-Doppler analysis of the (A1Σ+–X1Σ+) system is performed for 6LiH and 7LiH by crossing an effusive beam of LiH by a cw tunable laser beam in the near UV; 384 lines are measured between 357 and 385 nm and assigned to 6LiH and 7LiH with an accuracy of 5 × 10−3cm−1. Accurate Dunham coefficients and their mass-dependent corrections are deduced for the A1Σ+electronic state of both isotopomers from the combination of previously published data and these new ones.

A spectroscopy experiment has been performed on lithium atoms in the presence of a strong magnetic field (4.9?T) using multistep laser excitation of high lying levels. In this experiment, the peculiar excitation scheme is designed in order to selectively tests the dynamical symmetry of the excited states. This allows the direct observation of the transition to chaos from the breaking of these symmetries. The comparison between the experimental spectrum and a calculated one for hydrogen shows a remarkably good agreement. Une exp?rience de spectroscopie a ?t? r?alis?e sur des atomes de lithium plong?s dans un fort champ magn?tique (4.9?T) en utilisant une excitation laser ? plusieurs ?tages des ?tats proches de la limite d'ionisation. Un sch?ma d'excitation particuli?rement bien choisi permet de tester s?lectivement la sym?trie dynamique des ?tats excit?s. Cette exp?rience nous a permis d'obsever, pour la premi?re fois, la transition vers le chaos, comme r?sultant d'une brisure de ces sym?tries. la comparaison entre le spectre exp?rimental enregistr? et le spectre calcul? pour l'hydrog?ne montre un remarquablement bon accord.