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SONOLUMINESCENCE FLASHES CAN HAVE A DIPOLE SHAPE. Sonoluminescence (SL) is a process in which sound waves strike a tiny gas bubble, trapped in a liquid, causing the bubble to oscillate and emit picosecond pulses of light. The mechanism by which the bubble converts and concentrates sound energy into light energy is largely unknown. An experiment at UCLA by Seth Putterman and his colleagues shows now that the SL light emission pattern can have a dipole shape. This implies that the collapse of the bubble is not spherically symmetric. Furthermore, the dipole pattern can persist for a period equivalent to 100 bubble cycles (Keith Weninger et al., Physical Review E, Sept. 1996.) Meanwhile, scientists at MIT and the University of Marburg (Germany) have put forward a theory which addresses the new data. Michael Brenner (brenner@math.mit.edu) and his collaborators assert that the large energy focusing of the SL process can be explained as the storage of acoustic energy over many oscillations (and not just one bubble cycle as in the standard shock theory of SL); essentially, the bubble is a storage tank, patiently soaking up acoustic energy before re-emitting the energy in the form of sharp light pulses. This model, the MIT researchers say, accounts for the persistence of the dipole pattern in the UCLA observations, and points to the possibility that successive light flashes may not be independent but actually correlated in some way. (Michael Brenner et al., upcoming article in Physical Review Letters; journalists can obtain the articles by contacting physnews@aip.org.)

SONOLUMINESCENCE RESEARCH VIBRATES WITH ACTIVITY. At last week's joint meeting of the Acoustical Societies of America and Japan in Honolulu, researchers presented the latest results on sonoluminescence (SL), the mysterious phenomenon in which acoustic waves aimed at a water tank create oscillating bubbles which collapse and release ultrashort light flashes representing trillion-fold concentrations of the original sound energy. Presenting new experimental results, groups at Yale, the University of Washington (UW), and UCLA bolstered the front-running explanation for SL, namely, that a collapsing bubble creates an imploding shock wave which heats up gas inside the bubble and generates light. The three groups all recorded sharp acoustical pops during the SL process, suggesting the creation of shock waves. UW's Tom Matula presented preliminary results of SL experiments on a NASA astronaut-training plane showing that the same maximum light output was produced in high gravity, microgravity, and normal gravity. These results weakened recent speculations that SL occurs when an imploding bubble forms a needle-like spike or "jet" on one side which punctures the other side of the bubble to produce a flash of light, since different gravity conditions would surely vary the shape of the bubble and the formation of any resulting jets.

THE DURATION OF SONOLUMINESCENCE (SL) PULSES has been resolved for the first time. Sonoluminescence is the conversion of sound into light, in which acoustic waves aimed at a water tank create bubbles which collapse to release light flashes lasting less than a billionth of a second. Previously, researchers could only establish an upper limit for the length of SL flashes, but not their actual duration. Applying a single-photon-detection technique originally adapted for SL at the University of Stuttgart, UCLA researchers (Seth Putterman, 310-825-2269) were able to determine that the entire spectrum of colors in an SL flash shines for the same amount of time--from 35-380 picoseconds for mixtures of various gases in the water. This rules out the "adiabatic heating" hypothesis, in which an imploding bubble would first emit red, then add higher-energy colors as it collapsed further and got hotter. It supports a picture in which the collapsing bubble launches a shock wave which heats up the bubble to form a dense, relatively cold plasma. In this scenario, the light would be produced by a process called "thermal Bremsstrahlung," in which the plasma electrons collide with each other, and as they speed up and slow down at different rates they would create light of all different colors. (Hiller et al., upcoming article in Physical Review Letters.)

ISOTOPE EFFECTS IN SONOLUMINESCENCE have been observed by Seth Putterman and Robert Hiller at UCLA. sonoluminescence (SL) is a mysterious phenomenon in which acoustic energy is transduced into light energy; high frequency sound waves are absorbed by tiny bubbles in water. The bubbles, oscillating wildly, re-emit the energy in the form of tiny, focused light bursts. Many things about SL are still unknown, such as the nature of the light-emitting process or why the light pulses are so short. The UCLA work has established one new fact: substituting heavy water (D2O) for ordinary water (H2O) as the liquid medium causes the SL spectrum to dramatically shift from ultraviolet toward red wavelengths. This result seems to represent yet a new mystery. According to the researchers, "The shift is remarkably large, especially in view of the small difference in chemical and elastic properties between light and heavy water." (Robert A. Hiller and Seth Putterman, upcoming article in Physical Review Letters; journalists can obtain copies from AIP Public Information, physnews@aip.org)

PHOTONS FROM SONOLUMINESCENCE MAY PEAK IN THE ULTRAVIOLET. Sonoluminescence (SL) is a phenomenon in which sound waves create bubbles in a sample of water and cause them to oscillate. The excited bubbles in turn emit light. Last year, Seth Putterman (213- 825-2269) and his group at UCLA discovered that the light pulses were very short, only 50 psec, and that the conversion of sound energy into light energy represented in effective energy concentration of 12 orders of magnitude. Now the UCLA scientists have studied the spectra of SL photons and found that if a peak exists it must be at an energy above 6 eV; detection of photons with energies above this value could not be calibrated. The UCLA researchers believe SL can be used for producing a broadband light source in the wavelength range of 190-750 nm. (Robert Hiller et al., 24 August, Physical Review Letters.)

CAN FUSION BE INITIATED BY SONOLUMINESCENCE? In sonoluminescence carefully tuned sound waves cause bubbles in a fluid to oscillate; in the collapsing part of this motion the bubbles emit short (50 psec) bursts of light, most likely by some implosion effect. At a recent meeting of the Acoustical Society of America, Livermore physicist William Moss presented computer simulations which show that peak temperatures (as high as 1 million K) and pressures inside the bubbles could, with further experimental refinements, be sufficient to support nuclear fusion reactions. Experiments at several labs have not been able to measure such high temperatures; nor have they observed neutrons, an important product of nuclear fusion. One current experiment, at Livermore, is using bubbles filled with deuterium rather than air. (Science, 16 December 1994.)

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