Monatomic-Molecular Transition in Expanded Rubidium

Abstract

S͑Q, v͒ for liquid rubidium measured along the liquid vapor coexistence line exhibits monatomic behavior from normal density down to twice the critical density. At this density we observe excitations characteristic of a harmonic oscillator. We interpret this as evidence for the passage of the fluid from a monatomic to a molecular state. First principle total energy calculations for lattices of ribidium at 0 K predict that expansion favors spin pairing and leads to a lattice of dimers with an increase in vibron energy with decreasing density. The excellent agreement of the calculated vibron energy with the experimental result provides theoretical support for the appearance of molecules.[S0031-9007(97)03149-9] PACS numbers: 61.25.Mv, 61.12.Ex, 64.70.Ja, 71.30. + h Liquid alkali metals at high densities close to their triple points can be regarded as monatomic fluids, where each atom contributes one single electron to a half-filled conduction band [1]. This view is consistent with several experimental observations: The dynamic structure factor S͑Q, v͒ for liquid rubidium measured at conditions close to the melting point exhibits distinct collective excitations at a high wave vector [2] typical for dense metallic fluids. The dispersion relation of these excitations is in accord with the result from computer simulations employing an interatomic pseudopotential typical for monatomic metals [3].Measurements of the static structure factor S͑Q͒ for fluid rubidium and cesium [4,5], carried out along the liquid vapor coexistence line for several densities between the melting point and the critical point, are consistent with this view. The main structural effect of the density decrease with increasing temperature is a reduction in the average number of nearest neighbors within the first coordination shell around an atom, while the mean interparticle distance remains nearly unchanged. This behavior changes dramatically if the metals are evaporated to the dilute gas phase close to their triple points. Under these conditions, alkali metal vapors behave like typical insulators with electrical conductivities less than 10 25 V 21 cm 21 [6]. Measurements of optical [7,8] and magnetic [9] properties and of equation of state data [8,10], as a function of density indicate that molecular aggregates such as dimers or trimers do exist in the gas phase. As the vapor is compressed along the coexistence line the experimental results indicate an increase in concentration of the molecular species to about 25% at conditions close to the critical point [9]. Resent theoretical considerations of these data reach a similar conclusion [11]. The question to emerge from these findings is, up to what density can the pairing mechanism present in the vapor survive the passage from the dilute vapor to the dense liquid [1,12].Recently, we were able to extend our measurements of the dynamic structure factor S͑Q, v͒ for liquid ru-bidium to densities as low as twice the critical density, where deviations from the metallic behavior are already significant ...

Structural studies of expanded fluid rubidium up to the supercritical regions

“…The shortening of r 1 is seemingly consistent with the appearance of diatomic molecules as suggested by the inelastic neutron scattering measurements [20]. However, the observed structural features within the density range from 1.1 to 0.5 g cm À3 cannot be simply interpreted as such a physical picture that the amount of diatomic molecules in the fluid increases with decreasing fluid density [21].…”

Trends in the properties and structures of the simple metals from a universal local pseudopotential

“…The simplicity of our approach makes it well suited for the study of liquid metals, 4,60,61 expanded metals, 5,62 and solid or liquid alloys. For the solids at zero or high pressure, we have mapped the equilibrium crystal structures in the r s Ϫz plane.…”

First-principles molecular dynamics of liquid rubidium at low density

“…In the case of rubidium, neutron scattering experiments carried out along the vapor-liquid coexistence curve by Pilgrim and coworkers show interesting changes in the shape of the dynamic structure factor S(q, x): while at moderate liquid densities this function has broad shoulders indicative of highly damped collective excitations, at the lowest density investigated (about twice the critical density) a well-defined side peak appears [7][8][9][10][11]. Pilgrim and coworkers have interpreted these results as evidence of a transition from a monoatomic to a molecular state, attributing the observed excitation peak to vibrations of a rubidium dimer [11].…”