THE FORMATION OF A MOLECULAR-STATE IN EXPANDED LIQUID RUBIDIUM

Inelastic neutron scattering experiments have been carried out on expa nded liquid rubidium in a density range between the melting point dens ity and about twice the critical density. The dynamics of the liquid s how metallic-like behavior even at relatively low densities. In expand ing the liquid further to about twice the critical density, where a br eakdown of the metallic properties is expected, S(Q, omega) changes it s shape considerably. It is now characterized by a localized excitatio n, indicating that molecules present in the dilute vapour may survive the condensation into the dense fluid. First principles total energy c alculations for Rb-lattices at 0 K support this view and predict that expansion favours spin pairing leading to the formation of Rb, molecul es with a continuous increase in vibron energy with decreasing density . Excellent agreement is found between the calculated vibron energy an d the experimental result. (C) 1998 Elsevier Science B.V. All rights r eserved.