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.
Rapidly quenched Kosterlitz-Thouless (KT) superfluid transitions are studied by solving the Fokker-Planck equation for the vortex-pair dynamics in conjunction with the KT recursion relations. Power-law decays of the vortex density at long times are found, and the results are in agreement with a scaling proposal made by Minnhagen and co-workers for the dynamical critical exponent. The superfluid density is strongly depressed after a quench, with the subsequent recovery being logarithmically slow for starting temperatures near T(KT). No evidence is found of vortices being "created" in a rapid quench; there is only decay of the existing thermal vortex pairs.
An exact analytic solution for the dynamics of vortex pairs is obtained for rapid temperature quenches of a superfluid film starting from the line of critical points below the critical temperature TKT . An approximate solution for quenches at and above above TKT provides insights into the origin of logarithmic transients in the vortex decay, and is in general agreement with recent simulations of the quenched XY model. These results confirm that there is no "creation" of vortices whose density increases with the quench rate as predicted by the Kibble-Zurek theory, but only monotonic decay of the thermal vortices already present at the initial temperature. PACS numbers: 64.60.Ht, 67.25.dj, 67.25.dk, 67.25.dpAlthough the phase-ordering kinetics of temperaturequenched thermodynamic systems have been studied for many decades [1], there are only a few exact results for the dynamics of the recovery to equilibrium [2,3]. For many systems, progress in the field has been made by asserting that dynamic scaling should apply to a quenched system, that for a system with non-conserved order parameter the dynamics will be characterized by a growing length scale ξ(t) = ξ 0 t 1/z , where z ≈ 2 is the dynamical exponent of model A in the classification of Halperin and Hohenberg [4] and t is the time from the quench to low temperature. Scaling holds if solutions involving a length scale r only depend on the ratio r/ξ. The growing length scale characterizes the domain growth of the topological defects of the order parameter as the system becomes completely ordered at long times. A phenomenological argument [1,5] is commonly used to predict the time dependence of the decaying defect density:where n is the number of components of the order parameter. Superfluids are in the n = 2 universality class, where the defects are quantized vortices, so if z = 2 then dynamic scaling predicts a 1/t decay of the vortex density. Computer simulations of spin systems with varying n gave general agreement with Eq. (1), though only at long time and length scales [6]. However, a problematic case for dynamic scaling has been two-dimensional superfluids, where the defects are the vortex pairs of the Kosterlitz-Thouless theory [7] characterizing the equilibrium phase transition occurring at the critical temperature T KT . The above arguments would give a vortex density decaying as t −1 for quenches from well above T KT to very low temperatures. Simulations of the XY model [8][9][10], however, showed the initial vortex decay to be considerably slower than this, and then only at very long times finally approached the predicted exponent of −1. The behavior could be modeled as a ln t/t variation, but this requires altering the dynamic length scale to vary as (t/ ln t) 1/z for initial temperatures above T KT . This change has been cited [11] as a breakdown of dynamic scaling, though others [12] find such a sudden change in the dynamic scale still fully consistent with scaling. To further complicate the issue, numerical solutions of the Fokker-Planck equation f...
A precursor luminescence pulse consisting of atomic line emission is observed as much as 150 nanoseconds prior to the collapse point of laser-created bubbles in alkali-metal solutions. The timing of the emission from neutral Na, Li, and K atoms is strongly dependent on the salt concentration, which appears to result from resonant radiation trapping by the alkali atoms in the bubble. The alkali emission ends at the onset of the blackbody luminescence pulse at the bubble collapse point, and the duration of the blackbody pulse is found to be reduced by up to 30% as the alkali-salt concentration is increased.
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