Spectroscopic experiments on molecules embedded in free clusters of liquid helium reveal a number of unusual features deriving from the unique quantum behavior of this nanoscale matrix environment. The apparent free rotation of small molecules in bosonic He4 clusters is one of the experimentally most well documented of these features. In this Focus article, we set this phenomenon in the context of experimental and theoretical advances in this field over the last ten years, and describe the microscopic insight which it has provided into the nature and dynamic consequences of quantum solvation in a superfluid. We provide a comprehensive theoretical analysis which is based on a unification of conclusions drawn from diffusion and path integral Monte Carlo calculations. These microscopic quantum calculations elucidate the origin of the empirical free rotor spectrum, and its relation to the boson character and superfluid nature of the quantum nanosolvent. The free rotor behavior of the molecular rotation is preserved because of inefficient angular momentum coupling between the dopant and its quantum liquid surroundings. This is consistent with the superfluid character of the droplet, and has significant implications for the hydrodynamic response of the local quantum fluid environment of the embedded molecule. The molecule–helium interaction appears to induce a local nonsuperfluid density component in the first quantum solvation shell. This can adiabatically follow the molecular rotation, resulting in a reduction of the rotational constant. The dynamic nature of this adiabatically following density, its relation to the magnitude of the gas-phase molecular rotational constant and to the anisotropy of the interaction potential, are characterized with several examples. The role of the local superfluid density is analyzed within a continuum hydrodynamic model which is subject to microscopic quantum constraints. The result is a consistent theoretical framework which unites a zero temperature description based on analysis of cluster rotational energy levels, with a quantum two-fluid description based on finite temperature analysis of local quantum solvation structure in the superfluid.

We present the first measurement of pseudorapidity densities of primary charged particles near midrapidity in Au+Au collisions at sqrt[s(NN)] = 56 and 130 GeV. For the most central collisions, we find the charged-particle pseudorapidity density to be dN/deta|(|eta|<1) = 408+/-12(stat)+/-30(syst) at 56 GeV and 555+/-12(stat)+/-35(syst) at 130 GeV, values that are higher than any previously observed in nuclear collisions. Compared to proton-antiproton collisions, our data show an increase in the pseudorapidity density per participant by more than 40% at the higher energy.

We report the preparation and characterization of self-assembled monolayers (SAMs) derived from the adsorption of partially fluorinated hexadecanethiols (CF 3 (CF 2 ) n-1 (CH 2 ) m SH; n ) 2, 4, 6, and 8; n + m ) 16) onto the surface of gold. The quality of the SAMs, as measured by electrochemical impedance spectroscopy, was found to be sensitive to the solvent used as SAMs prepared in dichloromethane exhibited higher resistances and lower capacitances than those prepared in liquid CO 2 . The extent of fluorination was observed to influence the wettability, structure, and capacitance of the films without significantly affecting their charge-transfer resistance or their stability against exchange by a competing adsorbate. Reflectance-absorption infrared spectra showed that the fluorocarbon chains were oriented more normal to the surface for n ) 8 but more parallel to the surface for n ) 2. Advancing contact angles of hexadecane increased sharply with fluorination from n ) 0 to 4 but slightly with additional fluorination, suggesting that hexadecane is insensitive to CH 2 groups that are buried by >4 fluorinated carbon atoms. We also were able to model the dielectric properties of these partially fluorinated SAMs as two capacitors in series: one due to the outer fluorocarbon region and one due to the inner hydrocarbon region. The dielectric constants of the fluorocarbon and hydrocarbon regions of these SAMs were quantified as ∼1.7 and ∼2.3, respectively, and were comparable to the values for the pure polymer counterparts.