Plasma Quenching by Air during Single-Bubble Sonoluminescence

Abstract

We report the observation of sudden and dramatic changes in single-bubble sonoluminescence (SBSL) intensity (i.e., radiant power, Φ SL ) and spectral profiles at a critical acoustic pressure (P c ) for solutions of sulfuric acid (H 2 SO 4 ) containing mixtures of air and noble gas. Nitric oxide (NO), nitrogen (N 2 ), and atomic oxygen emission lines are visible just below P c . At P c , very bright (factor of 7000 increase in Φ SL ) and featureless SBSL is observed when Ar is present. In addition, Ar lines are observed from a dimmed bubble that has been driven above P c . These observations suggest that bright SBSL from H 2 SO 4 is due to a plasma, and that molecular components of air suppress the onset of bright light emission through quenching mechanisms and endothermic processes. Determination of temperatures from simulations of the emission lines shows that air limits the heating during single-bubble cavitation. When He is present, Φ SL increases by only a factor of 4 at P c , and the SBSL spectrum is not featureless as for Ar, but instead arises from sulfur oxide (SO) and sulfur dioxide (SO 2 ) bands. These differences are attributed to the high thermal conductivity and ionization potential of He compared to Ar.

Inside a Collapsing Bubble: Sonoluminescence and the Conditions During Cavitation

“…In addition, even if a small fraction of noble gas is mixed with a diatomic gas and dissolved into solution (or even air itself, which is 0.9% Ar), the SBSL from water behaves similarly to that observed from water containing the pure noble gas only. This is because the reactive diatomic gases are dissociated in the hot bubble interior and form soluble species (e.g., NO x ); the sonolysis products dissolve into the surrounding liquid, leaving only noble gas inside the bubble (57,(90)(91)(92)(93)(94)(95). The low thermal conductivities of the heavier Single-bubble sonoluminescence (SBSL) from 85 wt% H 2 SO 4 partially regassed with Ar.…”

Emission from Electronically Excited Metal Atoms during Single-Bubble Sonoluminescence

“…At low P a , a bubble can be formed and trapped, but does not emit. As the P a is increased to 3:4 bar, dim SL is observed, and the emission spectra consist of emission from Ar excited states (i.e., Ar ) with an underlying continuum attributed to radiative plasma processes [11,38]. As the P a is increased further, SL increases more than 100-fold, while the spectral profile remains essentially unchanged save for broadening of the Ar lines.…”

“…Pyrolysis of the droplets then occurs during bubble implosion, which can lead to a variety of redox reactions capable of producing the excited state alkalimetal atoms. Droplet entrainment also explains why Ar emission and SL decrease with increasing P a ; the liquid droplets will lead to a lower peak temperature due to endothermic processes including liquid vaporization, excitation of bond rotations, and vibrations (i.e., reduction in the polytropic ratio), bond dissociation, and ionization [38,41]. We find that changes in M-SBSL spectra coincide with changes in the macroscopic bubble motion; at low P a , we observe Ar emission lines and bubble motion that is smooth and elliptical, while at high P a we observe emission from electronically excited alkali-metal atoms and bubble motion that becomes increasingly erratic with frequent and abrupt changes in direction.…”

Temperature Nonequilibration during Single-Bubble Sonoluminescence

“…It is interesting to note that we did not observe any emission lines from atomic oxygen, though the 777 nm line has been seen in SBSL spectra from aqueous H 2 SO 4 containing air. 15 Figures 2 and 3 show expanded regions of the spectrum from Figure 1 compared to least-squares fit simulations. The simulations were done using PGOPHER 16 and published molecular constants 17, 18 and Franck−Condon factors 19 for the B and X states of SO.…”