The visible and near infrared spectra of L and T-type brown dwarfs exhibit prominent features corresponding to resonance lines of alkali-metal atoms [1]. Similar features are expected to appear in the spectra of extrasolar giant planets (EGPs) [2,3]. The spectra of cool brown dwarfs are dominated in the visible and near-infrared by the resonance lines of Na at 589 nm and of K at 770 nm. Because of the large concentrations of gaseous H₂ and He in the atmosphere of these objects at temperatures around 1000 K, the profiles of Na and K lines are strongly broadened by collisions up to 100 nm on each side of the line core. This broadening is pressure and temperature sensitive and consequently the ability to model precisely those alkali lines is a powerful tool to retrieve the atmospheric conditions of cool brown dwarfs and EGPs.
To improve the data available for modeling, we have designed a spectroscopic experiment to measure absolute absorption coefficients of alkali vapors colliding with H₂ and He at relevant temperatures. Our set-up allows us to use the “hook” method to determine the atom number density of the alkali element. If available, we will discuss our first measurements on the absorption spectrum of potassium vapors mixed with He buffer gas.

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