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We present PHOENIX atmosphere models for metal-rich cool white dwarfs usingimproved line shapes for the Na I and K I resonance doublets. Profiles forcollisional broadening due to H2 and He based on the adiabatic representationshow strong deviations from Van der Waals interaction at short distances.Comparison with observed spectra that show extremely broadened Na I linesindicates that a He-rich atmospheric composition is required to explain theline strengths and spectral energy distributions. Our current syntheticspectra, using an expansion in powers of density to the third order optimisedfor brown dwarf atmosphere conditions, significantly underestimate the observedabsorption in the far wings, even predicting smaller total line strength than aLorentzian profile. This is due to the handling of multiple perturberinteractions becoming inadequate for the extreme densities of the coolest whitedwarfs. The density expansion would have to be extended at least to the 7thorder for an accurate treatment of such conditions and might break downaltogether in the densest objects. The results of a direct calculation of theunified profile should therefore be used for model atmospheres of coolmetal-rich white dwarfs. Qualitative comparison of the full adiabatic profileto the spectrum of WD2356-209 indicates good agreement with the observed lineshape. Observations of the coolest white dwarfs may therefore serve as alaboratory for testing the physics of the deeper atmospheres and interiors ofbrown dwarfs and giant planets.

Alkali Line Profiles in Ultracool White Dwarfs