Sonoluminescence
Long exposure image of multi-bubble sonoluminescence created by a high intensity ultrasonic horn immersed in a beaker of liquid. Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.
History The effect was first discovered at the University of Cologne in 1934 as a result of work on sonar. H. Frenzel and H. Schultes put an ultrasound transducer in a tank of photographic developer fluid. They hoped to speed up the development process. Instead, they noticed tiny dots on the film after developing, and realized that the bubbles in the fluid were emitting light with the ultrasound turned on. It was too difficult to analyze the effect in early experiments because of the complex environment of a large number of short-lived bubbles. (This experiment is also ascribed to N. Marinesco and J.J. Trillat in 1933 which also credits them with independent discovery). This phenomenon is now referred to as multi-bubble sonoluminescence (MBSL). More than 50 years later, in 1989, a major advancement in research was introduced by Felipe Gaitan and Lawrence Crum, who were able to produce single bubble sonoluminescence (SBSL). In SBSL, a single bubble, trapped in an acoustic standing wave, emits a pulse of light with each compression of the bubble within the standing wave. This technique allowed a more systematic study of the phenomenon, because it isolated the complex effects into one stable, predictable bubble. It was realized that the temperature inside the bubble was hot enough to melt steel. Interest in sonoluminescence was renewed when an inner temperature of such a bubble well above one million kelvins was postulated. This temperature is thus far not conclusively proven, though recent experiments conducted by the University of Illinois at Urbana-Champaign deduced the temperature at about 20,000 kelvins.
Properties Sonoluminescence may or may not occur whenever a sound wave of sufficient intensity induces a gaseous cavity within a liquid to quickly collapse. This cavity may take the form of a preexisting bubble, or may be generated through a process known as cavitation. Sonoluminescence in the laboratory can be made to be stable, so that a single bubble will expand and collapse over and over again in a periodic fashion, emitting a burst of light each time it collapses. For this to occur, a standing acoustic wave is set up within a liquid, and the bubble will sit at a pressure anti-node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained. Some facts about sonoluminescence: ? The light flashes from the bubbles are extremely short between 35 and a few hundred picoseconds long, with peak intensities of the order of 1-10 mW. ? The bubbles are very small when they emit the light about 1 micrometre in diameter depending on the ambient fluid (e.g. water) and the gas content of the bubble (e.g. atmospheric air). ? Single-bubble sonoluminescence pulses can have very stable periods and positions. In fact, the frequency of light flashes can be more stable than the rated frequency stability of the oscillator making the sound waves driving them. However, the stability analysis of the bubble show that the bubble itself undergoes significant geometric instabilities, due to, for
example, the Bjerknes forces and Rayleigh-Taylor instabilities. The addition of a small amount of noble gas (such as helium, argon, or xenon) to the gas in the bubble increases the intensity of the emitted light. The wavelength of emitted light is very short; the spectrum can reach into the ultraviolet. Light of shorter wavelengths has higher energy, and the measured spectrum of emitted light seems to indicate a temperature in the bubble of at least 20,000 kelvins, up to a possible temperature in excess of one megakelvin. The veracity of these estimates is hindered by the fact that water, for example, absorbs nearly all wavelengths below 200 nm. This has led to differing estimates on the temperatures in the bubble, since they are extrapolated from the emission spectra taken during collapse, or estimated using a modified Rayleigh-Plesset equation (see below). Some estimates put the inside of the bubble at one gigakelvin. These estimates are based on models which cannot be verified at present, and may include too many unsupported assumptions. Temperatures this high make the study of sonoluminescence especially interesting for the possibility that it might produce a method for achieving thermonuclear fusion. If the bubble is hot enough, and the pressure in it is high enough, fusion reactions like those that occur in the Sun and other stars could be produced within these tiny bubbles. This possibility is sometimes referred to as bubble fusion. On January 27, 2006, researchers at Rensselaer Polytechnic Institute claimed to have produced fusion reactions by sonoluminescence, without an external neutron source, according to a paper published in Physical Review. To date, these results have not been reproduced by other members of the scientific community. Recent experiments (2002, 2005) of R. P. Taleyarkhan, et.al., using deuterated acetone, show measurements of tritium and neutron output consistent with fusion, but these measurements have not been reproduced outside of the Taleyarkhan lab and remain controversial. Brian Naranjo of the University of California, Los Angeles, has recently completed an analysis of the Taleyarkhan results claiming that Taleyarkhan had most likely misinterpreted the radioactive decay of standard lab materials for the byproducts of nuclear fusion. Writing in Nature, chemists David J. Flannigan and Kenneth S. Suslick study argon bubbles in sulfuric acid and show that ionized oxygen , sulfur monoxide, and atomic argon populating high-energy excited states are present implying that the bubble has a hot plasma core. They point out that the ionization and excitation energy of dioxygenyl cation is 18 electronvolts, and thus cannot be formed thermally; they suggested it was produced by high-energy electron impact from the hot opaque plasma at the center of the bubble (Nature 434, 52 - 55 (03 March 2005); doi:10.1038/nature03361). Fluid Mechanics The dynamics of the motion of the bubble is characterized to a first approximation by the Rayleigh-Plesset equation This is an approximate equation that is derived from the compressible Navier-Stokes equations, and describes the motion of the radius of the bubble R as a function of time t. Here, ç is the viscosity, p the pressure, and ã the surface tension. This equation, though approximate, has been shown to give good estimates on the motion of the bubble under the acoustically driven pressure collapse of the bubble. Mechanism of phenomenon The mechanism of the phenomenon of sonoluminescence remains somewhat unsettled, though many theories have been shown to have greater or lesser degrees of robustness. These include: hotspot, bremsstrahlung radiation, collision induced radiation and corona discharges, nonclassical light, proton tunneling, electrodynamic jets, fractoluminescent jets (now largely discredited due to contrary experimental evidence), and so forth.
From left to right : apparition of bubble, slow expansion, quick and sudden contraction, emission of light
In 2002 M. Brenner, S. Hilgenfeldt, and D. Lohse, published a long (60 page) review "Single bubble sonoluminescence" (Reviews of Modern Physics 74, 425) which contains a detailed explanation of the mechanism. An important factor is that the bubble contains mainly inert noble gas such as argon or xenon (air contains about 1% argon, and the amount dissolved in water is too great. To get sonoluminescence, the concentration must be reduced to 20-40% of its equilibrium value) and varying amounts of water vapor. Chemical reactions cause nitrogen and oxygen to be removed from the bubble after about one hundred expansion-collapse cycles, which are initially needed before the bubble emits light. Once the bubble is "cleaned", it emits light with no delay - "Evidence for Gas Exchange in Single-Bubble Sonoluminescence", Matula and Crum, Phys. Rev. Lett. 80 (1998), 865-868). During bubble collapse inertia of surrounding water causes high speed and high pressure in its final stage - gas in the bubble's interior reaches temperatures around 10000 K, which causes ionisation of a small fraction of the noble gas - small enough for the bubble to remain transparent, allowing volume emission (if there were surface emission, it would produce more intense light of longer duration, dependent on wavelength, contradictory to experimental results). Electrons from ionised atoms interact mainly with neutral atoms causing radiation (thermal bremsstrahlung). When collapse stops the temperature drops, electrons recombine with atoms, and light emission ceases due to lack of free electrons. This allows about a 160ps light pulse for argon (even a small drop in temperature causes a large drop in ionization, due to the large ionization energy relative to photon energy). This is a very simplified description - the bubble oscillation cycle described in the literature consists of many steps of different duration some take 15 microseconds (expansion), some other 100 picoseconds (emission). Computations based on the theory they present, albeit with some simplifications (e.g. assuming a uniform temperature in the entire bubble), produce radiation parameters (intensity and duration time versus wavelength) that match experimental results with errors no larger than expected due to these simplifications, so it seems the phenomenon of sonoluminescence is at least roughly explained, although some details of the process remain obscure. Exotic proposals An unusually exotic theory of sonoluminescence, which has received much popular attention, yet is considered to have a marginal effect on the mechanism of SBSL by the scientific community at large, is the Casimir energy theory proposed by Claudia Eberlein, a physicist at the University of Sussex. In 1996, it was suggested that the light in sonoluminescence is generated by the vacuum around the bubble in a process similar to Hawking radiation, the radiation generated by the edges of black holes. Quantum theory holds that a vacuum is filled with virtual particles, and the rapidly moving interface between water and air converts virtual photons into real photons. This is related to the Unruh effect or the Casimir effect. If true, sonoluminescence may be the first observable example of quantum vacuum radiation. It is, however, argued that the mechanism leading to the above effects do not occur on the proper time scales to describe the observed spectrum of SBSL, which is thought to likely obey a classical cavitation collapse; and thus the Casimir model has been largely relegated to the position of an ancillary remnant of the field at large. Shrimpoluminescence Pistol shrimp (also called snapping shrimp) produce a type of sonoluminescence from a collapsing bubble caused by quickly snapping a specialized claw. The light produced is of lower intensity than the light produced by typical sonoluminescence, and is not visible to the naked eye. It most likely has no biological significance, and is merely a byproduct of the shock wave, which these shrimp use to stun or kill prey. However, it is the first known instance of an animal producing light by this effect, and was whimsically dubbed "shrimpoluminescence" upon its discovery in October of 2001.
Èerenkov radiation Èerenkov radiation (also spelled Cerenkov or Cherenkov) is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Èerenkov radiation. It is named after Russian scientist Pavel Alekseyevich Èerenkov, the 1958 Nobel Prize winner who was the first to rigorously characterize it. Physical origin While relativity holds that the speed of light in a vacuum is a universal constant (c), the speed at which light propagates in a material may be significantly less than c. For example, the speed of the propagation of light in water is only 0.75c. Matter can be accelerated beyond this speed during nuclear reactions and in particle accelerators. Èerenkov radiation results when a charged particle, most commonly an electron, exceeds the speed at which light is propagating in a dielectric (electrically insulating) medium through which it passes. Moreover, the velocity that must be exceeded is the phase velocity rather than the group velocity. The phase velocity can be altered dramatically by employing a periodic medium, and in that case one can even achieve Èerenkov radiation with no minimum particle velocity a phenomenon known as the Smith-Purcell effect. In a more complex periodic medium, such as a photonic crystal, one can also obtain a variety of other anomalous Èerenkov effects, such as radiation in a backwards direction (whereas ordinary Èerenkov radiation forms an acute angle with the particle velocity). As a charged particle travels, it disrupts the local electromagnetic field (EM) in its medium. Electrons in the atoms of the medium will be displaced and polarized by the passing EM field of a charged particle. Photons are emitted as an insulator's electrons restore themselves to equilibrium after the disruption has passed. (In a conductor, the EM disruption can be restored without emitting a photon.) In normal circumstances, these photons destructively interfere with each other and no radiation is detected. However, when the disruption travels faster than light is propagating through the medium, the photons constructively interfere and intensify the observed radiation. It is important to note, however, that the speed at which the photons travel is always the same. That is, the speed of light, commonly designated as c, does not change. The light appears to travel more slowly while traversing a medium due to the frequent interactions of the photons with matter. This is similar to a train that, while moving, travels at a constant velocity. If such a train were to travel on a set of tracks with many stops it would appear to be moving more slowly overall, i.e. have a lower average velocity, despite having a constant higher velocity while moving.
The geometry of the Èerenkov radiation. A common analogy is the sonic boom of a supersonic aircraft or bullet. The sound waves generated by the supersonic body do not move fast enough to get out of the way of the body itself. Hence, the waves "stack up" and form a shock front. Similarly, a speed boat generates a large bow shock because it travels faster than waves can move on the surface of the water. In a similar way, a charged particle can generate a photonic shockwave as it travels through an insulator. In the figure, the particle (red arrow) travels with speed vp and we define â = vp / c where c is speed of light. n is the refractive index of the medium and so the photons (blue arrows) travel at speed vem = c / n.
The left corner of the triangle represents the location of the superluminal particle at some initial moment (t=0). The right corner of the triangle is the location of the particle as some later time t. In the given time t, the particle travels where as the electromagnetic waves are constricted to travel Note that since this ratio is independent of time, one can take arbitrary time periods and achieve similar triangles. The angle remains unchanged, meaning that subsequent waves generated between the initial time t=0 and final time t will form similar triangles with coinciding right endpoints to the one shown. Characteristics Intuitively, the overall intensity of Èerenkov radiation is proportional to the velocity of the inciting charged particle and to the number of such particles. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Èerenkov radiation is continuous. The relative intensity of one frequency is proportional to the frequency. That is, higher frequencies (shorter wavelengths) are more intense in Èerenkov radiation. This is why visible Èerenkov radiation is observed to be brilliant blue. In fact, most Èerenkov radiation is in the ultraviolet spectrum - it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum. There is a cut-off frequency for which the equation above cannot be satisfied. Since the refractive index is a function of frequency (and hence wavelength), the intensity doesn't continue increasing at ever shorter wavelengths even for ultra-relativistic particles (where v/c approaches 1). At X-Ray frequencies, the refractive index becomes less than unity (note that in media the phase velocity may exceed c without violating relativity) and hence no X-Ray emission (or shorter wavelength emissions such as gamma rays) would be observed. However, X-rays can be generated at special energies corresponding to core electronic transitions in a material, as the index of refraction is often greater than 1 at these energies. As in sonic booms and bow shocks, the angle of the shock cone is inversely related to the velocity of the disruption. Hence, observed angles of incidence can be used to compute the direction and speed of a Èerenkov radiation-producing charge. Nuclear reactors Èerenkov radiation is used to detect high-energy charged particles. In pool-type nuclear reactors, the intensity of Èerenkov radiation is related to the frequency of the fission events that produce high-energy electrons, and hence is a measure of the intensity of the reaction. Èerenkov radiation is also used to characterize the remaining radioactivity of spent fuel rods. [edit] Astrophysics experiments When a high-energy cosmic ray interacts with the Earth's atmosphere, it may produce an electron-positron pair with enormous velocities. The Èerenkov radiation from these charged particles is used to determine the source and intensity of the cosmic ray, which is used for example in the Imaging Atmospheric Èerenkov Technique (IACT), by experiments such as VERITAS, H.E.S.S., and MAGIC. Similar methods are used in very large neutrino detectors, such as the Super-Kamiokande, the Sudbury Neutrino Observatory (SNO) and IceCube. Èerenkov radiation can also be used to determine properties of high-energy astronomical objects that emit gamma rays, such as supernova remnants and blazars. This is done by projects such as STACEE, a gamma ray detector in New Mexico. Particle physics experiments Èerenkov radiation is commonly used in experimental particle physics for particle identification.
In common use, phosphorescence also refers to the emission of light by bioluminescent plankton, and some other forms of chemoluminescence. Phosphorescent powder under visible light, ultraviolet light, and total darkness.
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur less often in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours.