Molecular Emission and Temperature Measurements from Single-Bubble Sonoluminescence

Single-bubble sonoluminescence (SBSL) spectra in ${\mathrm{H}}_2\mathrm{O}$ show featureless continuum emission. From an acoustically driven, moving bubble in phosphoric acid (${\mathrm{H}}_3{\mathrm{PO}}_4$), we observe very strong molecular emission from excited OH radicals ($\sim 310\mathrm{nm}$), which can be used as a spectroscopic thermometer by fitting the experimental SBSL spectra to the OH $A{}^2\Sigma ^{+}–X{}^2\Pi$ rovibronic transitions. The observed emission temperature ($T_{\mathrm{em}}$) ranges from 6200 to 9500 K as the acoustic pressure ($P_a$) varies from 1.9 to 3.1 bar and from 6000 to $>10000\mathrm{K}$ as the dissolved monatomic gas varies over the series from He to Xe.

Figure 1

Photographs (with different exposure times) of a moving sonoluminescing bubble in 65% ${\mathrm{H}}_3{\mathrm{PO}}_4$ regassed with 50 torr Ar at an acoustic pressure of 3.1 bar as determined from hydrophone measurements.

Figure 2

(a) SBSL spectra from 65% ${\mathrm{H}}_3{\mathrm{PO}}_4$ regassed with 50 torr Ar at different acoustic pressures. All spectra were collected from the same rapidly moving bubble driven at different $P_a$, as labeled above the corresponding spectra. The peak around 310 nm arises from OH ($A{}^2\Sigma ^{+}–X{}^2\Pi$) emission. The spectra were acquired with a $300\mathrm{\text{grooves}}/\mathrm{mm}$ grating blazed at 250 nm. (b) Higher resolution spectra of OH ($A{}^2\Sigma ^{+}–X{}^2\Pi$) emission from the SBSL of 65% ${\mathrm{H}}_3{\mathrm{PO}}_4$ at different acoustic powers regassed with 50 torr Ar compared to the best-fit synthetic spectra (underlying black lines). The underlying continuum has been subtracted; spectra are normalized to the highest intensity at $\sim 309\mathrm{nm}$. The calculated temperatures were obtained by fitting experimental SBSL spectra with synthetic spectra using the LIFBASE software package [41] and assumed thermal equilibrium and a Lorentzian profile. $1200\mathrm{\text{grooves}}/\mathrm{mm}$ grating blazed at 330 nm.

Figure 3

The effects of dissolved noble gases on SBSL spectra. (a) SBSL spectra from 65% ${\mathrm{H}}_3{\mathrm{PO}}_4$ with 50 torr of various noble gases at an applied acoustic pressure of 2.4 bar. (b) High resolution spectra and calculated effective emission temperatures from SBSL with different noble gases at the acoustic pressure of 2.4 bar compared to the best-fit synthetic spectra (underlying black lines). The calculated temperatures were obtained by fitting experimental SBSL spectra with synthetic spectra using the LIFBASE software package [41]. The thermal conductivity of He is $156.7\mathrm{mW}/\mathrm{m}\mathrm{K}$, Ne $49.8\mathrm{mW}/\mathrm{m}\mathrm{K}$, Ar $17.9\mathrm{mW}/\mathrm{m}\mathrm{K}$, and Kr $9.5\mathrm{mW}/\mathrm{m}\mathrm{K}$ [53].