Structure, energetics, and reactions of alkali tetramers

Electronic structure calculations have been carried out for all possible alkali tetramers that can be formed from X₂ X₂ → X₂X_2, X₂ Y₂ → X ₂Y₂, and XY XY → X₂Y₂ alkali dimer association reactions. Vibrationally stable rhombic (D_2h) and planar (C_s) structures are found for all possible tetramers formed from the alkali metals, Li to Cs. All tetramer formation reactions (from ground state singlet homonuclear or heteronuclear dimers) are found to be exothermic with binding energies ranging from 6282 cm^-1 for Li ₂Li₂ to 1985 cm^-1 for Cs₂Cs ₂. Extensive calculations, carried out at long-range for several reactant pairs, indicate that there are barrier-less pathways for the formation of tetramers from dimer association reactions. At low temperatures, direct formation of tetramers is unlikely, owing to the large exothermicity associated with these association reactions, but atom exchange reactions (X₂ Y₂ XY XY) are possible for some species.