MONATOMIC-MOLECULAR TRANSITION IN EXPANDED RUBIDIUM

S(Q,omega) for liquid rubidium measured along the liquid vapor coexist ence line exhibits monatomic behavior from normal density down to twic e the critical density. At this density we observe excitations charact eristic of a harmonic oscillator. We interpret this as evidence for th e passage of the fluid from a monatomic to a molecular state. First pr inciple total energy calculations for lattices of ribidium at 0 K pred ict that expansion favors spin pairing and leads to a lattice of dimer s with an increase in vibron energy with decreasing density. The excel lent agreement of the calculated vibron energy with the experimental r esult provides theoretical support for the appearance of molecules.