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therefore difficult to make the k = 0 extrapolation in Eq. (2) when u) -0.This molecular-dynamics calculation shows that the vaf of a dense gas at a near-critical temperature behaves asymptotically like at~3 /2 ; the value of the coefficient a can be deduced from the transport coefficients v and D. These results confirm those obtained by Alder and Wainwright 1 and by Wood 9 for systems of 500 and 4000 hard spheres.The authors are particularly grateful to L. Verlet and J. J. Weis for their help in realizing this work.*Laboratoire associg au Centre National de la Recherche Scientifique. 1 B. J. Alder and T. E. Wainwright, Phys. Rev. A 1,The unique properties of He II have been associated with the existence of a macroscopic wave function which determines the behavior of the superfluid component. From this idea Onsager 1 and Feynman 2 predicted that He II should exhibit vorticity with circulation quantized in units of h/m 9 where h is Planck's constant and m is the mass of the helium atom. For almost two decades physicists have been exploring theoretically and experimentally the phenomena associated with these vortices. 3 A wealth of convincing information is available which supports the existence of vortices, the most direct experiments being those which proved that circulation in He II is quantized 4 ' 5 and that He II comes into rotation in a series of quantum steps. 6 There has still been one experiment which has enticed workers for some time: actually to make directly visible the discrete vortex lines in the rotating He II. (An analogous experiment has been done to visualize fluxoids in a superconductor. 7 ) This Letter describes the first successful experiment which records the positions of the vortex lines in helium. We point out that according to current ideas the vortex 18 (1970). 2 J. R. Dorfman and E. G. D. Cohen, Phys. Rev. A 15, 776 (1972). 3 Y. Pomeau, Phys. Rev. A 5, 2569 (1972); M. H.

The spectrum of the luminescence emitted at the collapse of single laser-induced bubbles in water is measured for different maximum bubble radii. Bubbles as large as 2 mm show a molecular OH(*) band at 310 nm in the spectrum, which otherwise can be fitted approximately with a blackbody curve at a temperature of 7800 K. This finding provides a connection between the light emission of single bubbles and multibubble sonoluminescence, since in the latter case the same molecular band is observed. Surface instabilities are observed in the larger bubbles, and may be connected with the OH(*) emission.

An initial model of the superfluid X transition is constructed with use of vortex-ring excitations, as originally suggested by Onsager and Feynman. A real-space renormalization technique generates a screened vortex energy and core size, and gives rise to a transition where rings of infinite diameter are excited as the superfluid density approaches zero at T c . Although the model satisfies the Josephson hyperscaling relation, it is not yet a complete theory: The superfluid density exponent is v=0.53, and does not match the known value v = 0.67. PACS numbers: 67.40.Kh, 67.40.Db, 67.40.Vs The superfluid transition of liquid He has been studied intensively for nearly 50 years. In spite of this, there is still little known about the underlying physical mechanism. Onsager and Feynman l proposed in their original papers that vortex excitations might be responsible for the transition. Since then many authors have reiterated this proposal, and at least a partial listing of these papers is given in the references of Kotsubo and Williams. 2 The key role of vortex-ring excitations, in particular, was emphasized by Popov, 3 Wiegel, 4 Banks, Myerson, and Kogut, 5 and Nelson and Toner. 6 These papers gave a qualitative and physical picture of the transition, but quantitative predictions remained elusive. On the other hand, perturbation-series techniques such as the high-temperature expansion and the 4 -e expansion proved to be successful in the calculation of the critical exponents of the transition. 7 However, these are formal and mathematical procedures that are hard to apply in many situations, and the momentum-space representation that is used makes it difficult to identify the excitations causing the fluctuations. 8 The role of vortices in the superfluid transition has recently been made much more concrete by the numerical results of Kohring, Shrock, and Wills, 9 who used a Monte Carlo simulation of the three-dimensional (3D) XY model to give strong evidence that vortices play a crucial part in the transition. Vortices had previously only been known to be important in two dimensions (2D), as shown theoretically by Kosterlitz and Thouless 10 and as confirmed by experiments in thin helium films.' x The question of how the transition evolves as one goes from 2D to 3D geometries has recently arisen in experiments on helium films adsorbed in packed powders. 2 The results there led to speculation that the nature of the transition did not seem to change much as one varied the film thickness to the point where the powder pores fill completely with helium. The finite-size 2D transition appears to merge smoothly with the finitesize 3D transition, with little sign that the vortices cease to play a role in the 3D case. This conclusion has now been corroborated by the Monte Carlo results.In this Letter the superfluid density is calculated near the transition by consideration of the screening effects of circular vortex rings on an applied superflow. The starting point is to realize that vortex rings are eigenfunctions of the Landa...

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