Evidence for a plasma core during multibubble sonoluminescence in sulfuric acid.

Multibubble sonoluminescence (MBSL) is the light generated during the implosive collapse of many bubbles in liquids irradiated with ultrasound.1 Recently, we have investigated the single-bubble sonoluminescence (SBSL) of sulfuric acid, which is >1000 times brighter than prior SBSL, and discovered new emission lines (i.e., noble gas neutrals and ions, O2, SO); from these lines, temperatures greater than 15 000 K and pressures in excess of 1500 bar have been experimentally determined.2 In addition, the observation of molecular and atomic ions is the first experimental demonstration of an inner ionized plasma core during single-bubble cavitation. We have now examined MBSL in sulfuric acid. The spectrum under Ar, at relatively low acoustic power, consists of a broad continuum extending into the UV with SO and Ar emission lines on top of this continuum. The observation of the Ar lines strongly suggests that an optically opaque plasma is generated inside the bubble during cavitation. Interestingly, we find three distinct light emitting morphologies for multibubble sonoluminescence from an ultrasonic horn; as the acoustic intensity is increased, the bubble cloud emission changes from filamentous, to bulbous, to cone shaped, and it is only from the filamentous emission that strong atomic and molecular emissions are observed. Historically, MBSL had been studied mainly in water, which gives a spectrum consisting of a broad continuum extending into the UV and an often broadened peak around 310 nm from the excited-state of OH•.3 Low vapor pressure liquids such as long chain alkanes and silicone oils with volatile organometallic solutes have provided more information about the conditions generated during MBSL.4 MBSL of these liquids produced new well-resolved emission lines from excited states of C2, CH, CN, and metals such as Fe, Cr, and Mo. Using these emission lines as probes of the conditions within the bubbles, temperatures around 5000 K and pressures of hundreds of bar have been measured.4 The production of low molecular weight sonolysis products (e.g., H2, CH4, and C2H2) can prove to be problematic. These volatile products can have limited solubility in the liquid and therefore accumulate in the bubbles; subsequent cavitation events will thereafter dissipate much of their energy in the increased heat capacity (i.e., polyatomic vibrations and rotations) and especially bond dissociations.6 Liquids that have both a low vapor pressure and highly soluble sonolysis products are therefore preferred for the generation of higher temperatures during cavitation. Sulfuric acid is one such liquid because it has a very low vapor pressure (e.g., <1.5 mTorr for 95 wt % H2SO4 at 25 °C), and its sonolysis products (e.g., SOx, trace amounts of H2S, and elemental sulfur)7 are either highly soluble or solids. Prior MBSL studies in sulfuric acid have shown that the light intensity is much greater than in water, and recently, low-resolution spectra have revealed two broad peaks, assigned to SO and SO2. The sonoluminescence spectrum from concentrated (95 wt %) sulfuric acid saturated with Ar is shown in Figure 1.9 The spectrum includes emission from a broad continuum extending into the UV (due to blackbody or bremsstrahlung) together with SO (B3Σ-X3Σ-), Ar (4p-4s manifold), and oxygen atom (at 777 nm) emission. Effective emission temperatures can be determined by comparing the observed Ar lines to synthetic thermalized spectra.4,10 By such analysis, we find an effective temperature of ∼8000 K (Figure 2). Interestingly, at this temperature, there would not be any appreciable number of Ar atoms thermally excited into the 4p state (which is ∼13 eV above the ground state). This apparent paradox indicates that, as with SBSL,2 an optically opaque plasma11 is probably formed at the core of the collapsing bubble in sulfuric acid, and that the Ar emission results from collision excitation with higherenergy ions (e.g., electrons12). The presence of a plasma during MBSL has been postulated numerous times,13 but until now, there has been no experimental evidence. The SO and O lines observed in Figure 1 originate from the sonolysis of trace amounts of sulfuric acid that enter the cavitating bubble as either a vapor or injected as liquid microdroplet. The production of SO2 is confirmed by the absorption profile of the H2SO4 after sonication with strong absorption below ∼350 nm. SO2 absorbs strongly in the UV. This absorption increases in Figure 1. MBSL spectrum from concentrated sulfuric acid under Ar. Sonication at 20 kHz (14 W/cm2) with a Ti horn directly immersed in 95 wt % sulfuric acid, ∼298 K.