Effects of Rare Gases on Sonoluminescence Spectrum of the K

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Effects of Rare Gases on Sonoluminescence Spectrum of the K Atom Yuichi Hayashi* and Pak-Kon Choi Department of Physics, Meiji University, 1-1-1, Higashimita, Tama-ku, Kawasaki 214-8571, Japan ABSTRACT: Multibubble-sonoluminescence spectra from Ar-saturated KCl aqueous solutions were measured in the temperature range of 15−40 °C at frequencies of 48 kHz, 148 kHz, and 1 MHz. The effects of dissolved rare gases and the ultrasonic frequency on the shape of the K atom emission spectrum were examined. The line width of the K doublet was independent of the solution temperature, whereas the K line intensity decreased with increasing temperature. The spectra from Xe- and Ar-saturated solutions at 148 kHz exhibited a red-shifted and asymmetrically broadened doublet of K lines. The spectrum from a He-saturated solution, on the other hand, exhibited symmetrically broadened K lines, which were slightly blue-shifted. The observed effects of rare gases are in good agreement with those obtained by gas-phase spectroscopy. These results strongly indicate that the excited K atoms are perturbed by rare gases inside bubbles. The spectra from Xe- and Arsaturated solutions also indicated that the K doublet is composed of two types of peaks, shifted broadened lines and unshifted narrow lines. These two types of peaks were clearly separated at a frequency of 48 kHz with a high acoustic power in the case of Ar-saturated solutions. The intensity of the broadened lines relative to that of the narrow lines markedly decreased at 1 MHz. Although the narrow lines as well as the broadened lines may have originated from inside the bubbles, the exact mechanism is still unclear.

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INTRODUCTION Multibubble sonoluminescence (MBSL) is the phenomenon of light emission from a large number of bubbles irradiated with intense ultrasound.1,2 MBSL results from the high energy focused by the rapid and violent collapse of bubbles, which generates extremely high temperatures and pressures inside the bubbles. It is known that the MBSL spectrum obtained from water saturated with a rare gas consists of a continuum extending from UV to IR wavelengths with OH-radical emission peaks around 310 nm.3−6 The origin of the continuum emission has been suggested to be bremsstrahlung, blackbody radiation, and/or excited-state molecular emission.2 Because light emission also results from several chemical reactions involved in the decomposition of gases and vapor within the bubbles at collapse, MBSL can be used as a spectroscopic probe of the species produced at bubble collapse. Studies on MBSL from alkali-metal salt solutions have revealed emission from the excited alkali-metal atoms.6−17 However, the emission mechanism from nonvolatile species of alkali metals is still unclear; a key issue is whether alkali-metal emission occurs in the gas phase7−10 or liquid phase.11−13 Two different models have been proposed to explain the mechanism of sonoluminescence from nonvolatile species: a shell model and an injected droplet model.10,17 The shell model suggests that the emission from alkali-metal atoms occurs in the liquid layer on the bubble surface. Alkali-metal ions in the layer are electronically reduced and excited by certain chemical species generated in bubbles at their collapse. According to the injected droplet model, alkali-metal ions in the liquid enter the bubbles as droplets, which might be formed by interfacial instabilities © 2012 American Chemical Society

(capillary surface waves and microjet formation) of the bubbles, with subsequent thermolysis and reduction of the alkali-metal ions and the excitation of alkali-metal atom emission inside the bubbles. The coalescence and fragmentation of bubbles are also likely to induce droplet formation in a complicated multibubble field.15 This model indicates that the emission occurs in the gas phase inside the hot core of the collapsing bubble. Sehgal et al.7 measured MBSL spectra from NaCl and KCl aqueous solutions and estimated the final temperature and pressure at bubble collapse from the broadening or shift of the alkali-metal spectral lines, assuming that the emission arises from the highly compressed gas phase within bubbles. The proposition that atomic emission originates from the gas phase is also supported by Lepoint-Mullie et al.,8 who demonstrated that the blue satellite that accompanies broadened Rb lines is due to the B−X transition of alkali-metal/rare-gas van der Waals molecules within the bubbles. Choi et al.9 investigated the effects of ethanol on the line width of Na atom emission. The Na line width broadened and the line intensity decreased with increasing ethanol concentration because the gases decomposed from ethanol affected the Na atom emission. This result indicated that Na atom emission occurs in the gas phase. On the other hand, Flint and Suslick11 studied the effects of solvent vapor pressure and an inert gas on the K lines, which are known to affect the final temperature inside the bubble when it collapses. From the result that neither the line width Received: April 7, 2012 Revised: June 9, 2012 Published: June 11, 2012 7891

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nor the peak shift of the alkali-metal spectral line depend on these parameters, they concluded that alkali-metal emission originates from the liquid phase. In surfactant solutions such as sodium pentanesulfonate, the intensity of Na atom emission was enhanced. 18 This is caused by the higher local concentration of Na+ at the surfactant-coated bubble surface compared with that in the bulk solution. On the basis of this observation, Ashokkumar et al.12 suggested that the reduction of Na+ occurs at the bubble/liquid interface. Na atom emission has also been investigated in studies on single-bubble sonoluminescence (SBSL), which is the phenomenon of light emission from a single oscillating bubble trapped at the pressure antinode of a standing wave. Matula et al.19 compared the spectra of MBSL and SBSL obtained from NaCl aqueous solutions and found that Na atom emission occurred in MBSL but not in SBSL. They explained that the injection of liquid droplets into the heated core of bubbles is essential for Na atom emission in MBSL. In SBSL, stable bubble oscillation probably does not involve droplets because of the highly symmetric bubble collapse and the absence of surface deformation. This hypothesis is supported by Flannigan and Suslick,20 who investigated alkali-metal atom emission in sulfuric acid solutions. They observed Na or K atom emission from an unstable luminescing bubble, which randomly moved around a pressure antinode. Their results suggested that droplet injection due to the development of a capillary wave at the bubble surface is the main mechanism for the introduction of alkali-metal ions into the bubble interior. On the other hand, Na atom emission was observed from a “stable” luminescing bubble in aqueous solutions containing Na+, which were partially saturated with Ar.21−23 To bridge the gap between MBSL and SBSL, further studies on the relationship between the surface stability of bubbles and alkali-metal emission are required. In addition to the above studies, the reduction process of alkali-metal cations and the excitation process of alkali-metal atoms have also been discussed. Recent observations of MBSL from sulfuric acid solutions and aqueous solutions revealed the spatial separation of alkali-metal emission from the continuum emission.10,15,24 Sunartio et al.16 directly compared photographs of MBSL from aqueous solutions containing Na+ with those of sonochemical luminescence from luminol solutions. From the spatial similarities of both sets of photographs, they concluded that Na atom emission originates from sonochemically active bubbles, which differ from sonoluminescing bubbles. It has been pointed out that the mechanism of alkali-metal emission is closely related to the chemical reaction rather than the thermal excitation at bubble collapse, but the mechanism of these processes is not fully understood. It is still under debate where alkali-metal emission occurs, how alkali-metal ions are reduced and how alkali-metal atoms are excited. The main purpose of the present study is to investigate the effects of dissolved gases on the line width and shift of alkali-metal emission. K atom emission is favorable for evaluating the line broadening, peak shift and symmetry because the two lines of the K doublet are more widely separated than those of the Na doublet. We measured MBSL spectra from KCl aqueous solutions saturated with Xe, Ar, or He gas at temperatures in the range 15−40 °C and frequencies of 48 kHz, 148 kHz, and 1 MHz. The results suggested that the spectrum of K atom emission is composed of two types of peaks, unshifted narrow lines and shifted broadened lines. The broadened lines originate from the gas phase within bubbles.

Article

EXPERIMENTAL SECTION

We measured MBSL spectra in the wavelength range 270−800 nm from KCl aqueous solutions with a concentration of 1 M. The experimental apparatus used for measuring the MBSL spectra was similar to that described in ref 25. We used two types of transducers, a sandwich-type transducer with a frequency of 28 kHz, and a piezoceramic transducer with a frequency of 1 MHz and focal length of 36 mm. The sandwichtype transducer operated at its harmonic frequencies of 48 and 148 kHz. The focusing transducer was used to obtain a high sound pressure at a focal point for facilitating cavitation. The cylindrical cell equipped with the transducer was made of stainless steel, and the sample container was 46 mm in diameter and 150 mm in length. The top and bottom faces of the cell were equipped with a quartz glass window and the transducer, respectively. The temperature of the sample was controlled between 15 and 40 °C by circulating water. The solution was carefully degassed and then saturated with Ar, Xe, or He gas for at least 2 h. Because the sample container was hermetically closed, no air was introduced during the experiments. The signal from a function generator (Agilent, 33250A) was amplified using a power amplifier (NF Circuit, HSA4014). The emitted light was analyzed using a system consisting of a monochromator (Acton Research, SpectraPro-300i) and a cooled CCD detector (Princeton, Pixis 100). Broad-band spectra were collected using a grating of 600 grooves/mm blazed at 300 nm. Narrow-band spectra around the K lines were collected using a grating of 1200 grooves/mm blazed at 500 nm. The instrumental bandwidth at half width at half-maximum (HWHM) for the latter case was estimated to be 0.16 nm from the measurement of the He−Ne laser line. The spectral response was calibrated against a standard halogen lamp and Xe lamp to ensure detection efficiency. The total power of the irradiated ultrasound was determined by calorimetry using a type-K thermocouple.

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RESULTS Broad-band spectra of MBSL from Ar-saturated KCl aqueous solutions were measured for the solution temperature range 15−40 °C at intervals of 5 °C. The results are presented in Figure 1 only for the cases of 15, 20, and 35 °C for clarity. The ultrasonic frequency and power used were 148 kHz and 2.3 W, respectively. Each spectrum consists of a continuum extending from UV to IR wavelengths and several peaks. The peak at about 310 nm has previously been identified as the emission from electronically excited OH radicals, which can also be observed from pure water saturated with a rare gas.3−5 The double peaks at approximately 770 nm are attributed to emission from electronically excited K atoms; these peaks have much higher intensities than the peak attributed to OH radicals. A blue satellite peak at about 735 nm accompanied the K lines. This satellite peak has been reported in previous works7,20 and has been suggested to originate from the K−Ar* exciplex. The total MBSL intensity, of which the continuum is the main contributor, decreased with increasing solution temperature. The inset in Figure 1 shows high-resolution spectra of K atom emission at 15 and 30 °C, which are normalized by their maximum intensities. A flame spectrum of KCl was also measured and is indicated as a dashed line in the inset. The double peaks from the flame spectrum are at 766.5 nm (2P3/2 → 2S1/2) and 769.9 nm (2P1/2 → 2S1/2). As can be seen in the inset, the K lines did not shift from the flame spectrum and 7892

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Figure 1. MBSL spectra from Ar-saturated KCl aqueous solutions at temperatures of 15 (a), 20 (b), and 35 °C (c). The ultrasonic frequency and power used were 148 kHz and 2.3 W, respectively. The inset shows the normalized high-resolution spectra of K atom emission at 15 (solid line) and 30 °C (dotted line). The dashed line in the inset represents the flame spectrum obtained from KCl, with the intensity divided by two for clarity. Figure 3. Spectra of K atom emission from KCl aqueous solutions saturated with Xe (a), Ar (b), and He (c) at a temperature of 15 °C. The ultrasonic frequency and power used were 148 kHz and 2.3 W, respectively. The collection times of the spectrometer were 1, 3, and 30 min for the solutions saturated with Xe, Ar, and He, respectively. All spectra are normalized at their maximum intensities. The dashed lines in each figure indicate the normalized flame spectrum of KCl.

prominent, particularly for the Xe-saturated solutions. This suggests that the spectrum of K atom emission is composed of two types of peaks: double unshifted narrow lines and double shifted broadened lines. As can be seen in Figure 3a, the widths of the unshifted narrow lines are nearly the same as those in the flame spectrum. In contrast to the cases of Xe and Ar, the K lines from the He-saturated solutions in Figure 3c have a particular shape. The K lines broadened symmetrically and shifted to the blue side by 0.21 nm relative to the flame spectrum. No narrow lines appeared to be contained in the spectrum from the He-saturated solutions. We investigated the experimental conditions under which unshifted narrow lines and shifted broadened lines could be clearly separated. Further experiments were performed on Arsaturated solutions at a frequency of 48 kHz. The results are shown in Figure 4 for ultrasonic powers of 3.1, 5.3, and 6.3 W. All the spectra in the figure are normalized by the intensity of the shorter-wavelength peak of the doublet. It is apparent that the K lines consist of unshifted narrow lines and shifted broadened lines. Only the broadened lines gradually shifted to the red side with increasing ultrasonic power. At 6.3 W, the broadened lines are clearly separated from the narrow lines and shifted to the red side by approximately 0.6 nm relative to the flame spectrum. The predicted profiles of the narrow lines and broadened lines at 6.3 W are shown as lines I and II in Figure 4, respectively, assuming that the line width of narrow lines is the same as that of flame spectrum from KCl and the intensity ratio between the doublet is two as has been theoretically predicted. The intensity reached a maximum at 5.3 W and then decreased to two-thirds of the maximum at 6.3 W owing to excessive sound pressure.26 Figure 5 shows a comparison of the K lines from Ar-saturated solutions sonicated at 1 MHz with those of solutions sonicated at 48 and 148 kHz. The ultrasonic powers

Figure 2. Relative intensities of continuum and K atom emission from Ar-saturated solutions as a function of solution temperature. Open circles and closed squares indicate the intensities of continuum and K atom emission, respectively.

broadened asymmetrically toward the red side (longer wavelength) compared with those obtained from the flame spectrum. Figure 2 shows the intensities of the continuum and K lines relative to their values at 15 °C as a function of solution temperature. The intensities of the continuum and K lines exhibit a similar decrease with increasing temperature. The salient feature of Figures 1 and 2 is that the K line shape is independent of the solution temperature, whereas the intensity rapidly decreases with increasing solution temperature. Parts a−c of Figure 3 show the normalized spectra of K atom emission from KCl aqueous solutions saturated with Xe, Ar, and He, respectively, at a temperature of 15 °C. The ultrasonic frequency and power used were 148 kHz and 2.3 W, respectively. The exposure times of the spectrum were 1, 3, and 30 min in the cases of Xe, Ar, and He, respectively. The relative intensities of the K atom emission are 810, 720, and 1 for Xe-, Ar-, and He-saturated solutions, respectively. It appears that the K lines from Xe-saturated solutions, as well as those from Ar-saturated solutions, did not shift and broadened asymmetrically toward the red side relative to the flame spectrum. The results show that the broadened line shapes in these three cases are different from each other. Sharp peaks are 7893

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MBSL intensity with increasing temperature was attributed to two factors. One was the decrease in the number of cavitation bubbles following a decrease in the gas concentration. The second was the increase in the water vapor/gas ratio in bubbles. Water vapor has a lower specific heat ratio than Ar, which decreases the maximum temperature at bubble collapse. Studies on SBSL have also clarified that the amount of water vapor trapped in a collapsing bubble is a key parameter determining the intensity and final temperature of the bubble at its collapse.29−31 The water at a bubble surface vaporizes into the bubble during its expansion to maintain an equilibrium vapor pressure. The heat absorption of water vapor trapped in the interior prevents the final temperature at bubble collapse from increasing. Thus, a higher solution temperature leads to the quenching of SBSL. As shown in Figure 2, the relative intensities of the continuum and K lines exhibited similar rates of decrease with increasing temperature. The mechanisms of continuum and alkali-metal atom emissions are different, and the bubbles that cause each emission are known to have different spatial distributions in a multibubble field.10,15,16,24 The continuum emission arises from higher-temperature bubbles and the alkalimetal atom emission originates from lower-temperature bubbles.24 If the quenching effect of water vapor, as discussed above for the case of SBSL, is the main reason for the temperature dependence in Figure 2, the intensities of the continuum and K lines should show different behavior. Then the measured temperature dependence could be explained by the decrease in the population of sonoluminescing bubbles due to the difficulty of bubble formation. The increase in the solution temperature causes a decrease in gas solubility and an increase in the water vapor content inside bubbles. The solubility of Ar gas at 40 °C is two-thirds of that at 15 °C. The energy needed for a bubble to expand increases with increasing water vapor content, as pointed out by Sehgal et al.27 These factors lead to a decrease in MBSL intensity. If we assume that the origin of K atom emission is a gas phase inside bubbles, two effects of water vapor on K atom emission should be taken into account: a quenching effect and a broadening effect. Water vapor trapped inside a bubble is partially dissociated by heat at bubble collapse, and H2 gas is mainly produced. In a spectroscopic study, Earl and Herm32 studied the effects of collisional quenching by foreign gases on excited alkali-metal atoms. They reported that water vapor and H2 gas cause deactivation of the excited 4 2P state of K atoms, although Ar does not quench the excited K atoms. Regarding the broadening effect, Jongerius et al.33 measured collisional broadening and the shift rate of the Na-D lines using several types of perturbers in flames and vapor cells. They showed that the broadening rates induced by water vapor and H2 perturbers were nearly the same as that induced by Ar perturbers. Storey et al.,29 who performed a numerical simulation on the effect of water vapor for an Ar bubble at 25 °C and 26.5 kHz, showed that the amount of water vapor trapped inside a bubble at collapse is 14% of the total bubble content. At the ultrasonic frequency of 148 kHz used in the present experiment, the amount of water vapor trapped in the bubbles will be much less than the quantity of saturated Ar gas owing to the frequency effect.34,35 Even if the amount of water molecules inside the bubbles is enhanced by the increase in the solution temperature or droplet injection, the change in such an amount of water molecules may be insensitive to induce broadening of the K lines, because no change of spectral profile in the temperature

Figure 4. Spectra of K atom emission from Ar-saturated KCl aqueous solutions sonicated at 48 kHz with ultrasonic powers of 3.1 (a), 5.3 (b), and 6.3 W (c). The temperature of the solution was 15 °C. All spectra of K atom emission are normalized by the intensity of the shorter-wavelength peak of the doublet. The two components with lower intensities are used to explain the peak splitting at 6.3 W: I, unshifted narrow lines; II, shifted broadened lines.

Figure 5. Effect of ultrasonic frequency on the K lines at a temperature of 15 °C. From top to bottom, the ultrasonic frequency and power are 48 kHz and 3.1 W, 148 kHz and 2.3 W, and 1 MHz and 4.7 W, respectively. All spectra are normalized by their maximum intensities.

used at 48 kHz, 148 kHz, and 1 MHz were 3.1, 2.3, and 4.7 W, respectively. The line widths of the spectrum at 1 MHz appear to be much smaller than those at the lower frequencies. The peak splitting, as observed at 48 kHz at the power of 6.3 W in Figure 4, was not observed at 1 MHz for all powers and temperatures. This behavior at 1 MHz is due to the intensity of the shifted broadened lines being much less than that of the unshifted narrow lines. Thus, the intensity ratio of the narrow lines to the broadened lines depends not only on the dissolved gas but also on the ultrasonic frequency. The origin of the two types of peaks is discussed in the next section.

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DISCUSSION The dependences of MBSL and K atom emission on solution temperature are discussed in terms of the effect of water vapor in bubbles. Sehgal et al.27 reported that the temperature dependence of MBSL from rare-gas saturated water exhibits exponential behavior, similar to the present result in Figure 2. They interpreted their result as being due to the difference in the free energy of cavity formation, which is mainly determined by the vapor pressure. Didenko et al.28 measured the MBSL spectra of Ar-saturated water at several frequencies in the temperature range from 11 to 70 °C. The observed decrease in 7894

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Table 1. Experimental Values of the Half Width and Shift of K Lines and the Relative Densities Estimated for the He-Saturated Solutions HWHM transition

broadening ratea (×10−2 nm/amg)

relative density

experimental (×10−1 nm)

shift ratea (×10−3 nm/amg)

relative density

P1/2 → 2S1/2 P3/2 → 2S1/2

1.54 1.66

3.7 4.5

41.4 37.1

1.37 1.35

2.9 3.2

47.0 42.6

2 2

a

shift

experimental (nm)

The broadening and shift rates at a temperature of 3000 K are taken from the figures in ref 40.

amg and shift rates of 3.2 × 10−3 and 2.9 × 10−3 nm/amg for the 2P3/2 → 2S1/2 and 2P1/2 → 2S1/2 transitions, respectively, at a temperature of 3000 K. Here the unit of amg (amagat) is defined as the number density at standard temperature and pressure. The temperature of 3000 K is the assumed final temperature at collapse, which was estimated from the Cr emission for He bubbles.41 The comparison gives a relative density of 42 ± 3.5 (standard deviation). The relative density of 42 is equivalent to 7.5 kg/m3 for He gas. Table 1 summarizes these relative densities calculated from the line width and shift for He-saturated solutions. The average relative density is roughly consistent with the values for Ar and Xe data, which were estimated after the removal of unshifted narrow lines. Sehgal et al.7 obtained a relative density in the range from 31 to 50 for Na and K atom emissions. Lepoint-Mullie et al.8 obtained a relative density of 18 from the line shift of Rb atom emission in Ar-saturated RbCl and rubidium 1-octanolate solutions. Choi et al.9 reported a relative density of 60 from the line width of Na atom emission in NaCl aqueous solutions. Our relative densities are derived from both the line width and shift for the 2P3/2 → 2S1/2 and 2P1/2 → 2S1/2 transitions of K atom emission, and they are consistent with each other as shown in Table 1. Compared with the above values, the present value of relative density appears to be reasonable. Note that the reference rates of broadening and shift at 3000 K, which we adopted here, are a crucial factor in determining the relative density. These rates at 3000 K are considerably different from those at hundreds of degrees Kelvin,40 which have frequently been used as reference values in calculating the relative density of alkali-metal emission in MBSL. The temperature and pressure at the occurrence of K atom emission can be calculated assuming that the bubble collapse is adiabatic and obeys a perfect gas law, as reported by Sehgal et al.7 and Hatanaka et al.15 Our calculation gave a final temperature of 3480 ± 280 K and a pressure of 585 ± 120 atm. Here we assumed that the initial gas temperature and pressure within the bubbles were 288 K and 1.14 atm, respectively. We next discuss how K+ cations are introduced into the bubble interior. On the basis of the effects of rare gases presented here, the injected droplet model appears to be a reasonable mechanism of line broadening because the effects strongly indicated that the broadened lines originate from the gas phase inside the bubbles. The solution containing alkalimetal ions enters the bubbles as droplets through the development of surface instabilities. The K atoms excited in the bubbles through a certain reduction process of K+ cations are perturbed by foreign rare-gas atoms during their emission, leading to line broadening under the highly compressed gasphase condition within the bubbles. For the injected droplet model, an important point is the conditions that are required to induce the surface instabilities of bubbles. These conditions appear to be strongly related to the bubble size. In the multibubble field, the mean bubble size is larger at a lower

range 15−40 °C was observed in Figure 1. It is possible that the water vapor inside each bubble efficiently quenches the K lines although its contribution to the broadening may be small. Figures 3 and 4 suggested that the spectrum of K atom emission is composed of two types of doublets: double shifted broadened lines and double unshifted narrow lines. We first discuss the origin of the broadened lines. In the He-saturated solutions, we obtained only broadened lines of the doublet, which exhibited symmetrical broadening and were slightly shifted to the blue side. In the Ar-saturated solutions, the broadened lines exhibited asymmetrical broadening and shifted to the red side. The spectra from the Xe-saturated solutions showed the same tendency as those from the Ar-saturated solutions. These effects of rare gases on the K lines are consistent with the results of spectroscopic studies in the gas phase.36,37 These spectroscopic studies revealed the effect of rare gas on the collisional broadening of alkali-metal emission. This collisional broadening is caused by the finite difference between the interaction energies in the initial and final states when an excited atom interacts with a rare gas atom. According to the results of a series of studies,36−38 the effect of He perturbers on the collisional broadening of alkali-metal emission differs markedly from those of Xe and Ar perturbers. He perturbers cause slightly asymmetric line broadening toward the blue side, whereas Xe and Ar perturbers induce asymmetric line broadening toward the red side. Similar results have been reported for Na lines.38 A comparison with these spectroscopic results supports the hypothesis that the collision of radiating K atoms with foreign rare gas atoms in bubbles causes line broadening that depends on the type of rare gas. Therefore, we conclude that the broadened lines of the K atom emission originate from a gas phase inside the bubble. From the above discussion, the broadening and shift of the K lines shown in Figure 3 can provide information on the thermodynamic conditions inside the bubbles when K atom emission occurs. We can estimate the relative density at bubble collapse by comparing the data for He shown in Figure 3c with the spectroscopic data. The spectra for He-saturated solutions will give a more accurate value than those for Ar and Xe because of the absence of unshifted narrow lines. The K doublet was numerically resolved into two lines on the assumption that the ratio between the line intensities is 2, as has been theoretically predicted, and that the line shapes are analogous to each other. Each resolved K line was fitted with a Voigt function. To analyze the line width, we used an approximate equation with an accuracy of 0.02%39 to convert the fitted Voigt profile to a Lorentz function. These calculations gave HWHMs of 1.66 and 1.54 nm and line shifts of 1.35 × 10−1 and 1.37 × 10−1 nm for the 2P3/2 → 2S1/2 and 2P1/2 → 2 S1/2 transitions, respectively. In a spectroscopic study, Mullamphy et al.40 calculated the temperature dependence of the broadening and shift rates of K lines perturbed by He. We used their broadening rates of 4.5 × 10−2 and 3.7 × 10−2 nm/ 7895

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ultrasonic frequency.34,42 The surfaces of large bubbles at low ultrasonic frequencies are strongly distorted by their own surface waves. Augsdörfer et al.43 numerically investigated the stability of the surface of a single bubble driven at 20.6 kHz. They reported that strong deformation was induced on the surfaces of larger bubbles owing to the Faraday instability, which may lead to the disruption of bubbles within one cycle of bubble oscillation, whereas the deformation on the surfaces of smaller bubbles due to the Rayleigh−Taylor instability is small. Additionally, in the multibubble field, one can easily expect the surfaces of larger bubbles to be distorted by adjacent collapsing bubbles. Bubbles driven at a low ultrasonic frequency undergo a violent collapse due to the high compression ratio at bubble collapse, which produces a strong outgoing shock wave and sound emission to adjacent bubbles.44,45 The bubble cloud and cluster often observed at low frequencies46 will also cause surface instabilities on bubbles. In fact, Weninger et al.45 directly observed large nonspherical bubbles at 27 kHz, whereas all the bubbles they observed at 1 MHz were spherical. Thus, large bubbles at low ultrasonic frequencies are favorable for the formation of droplets in the multibubble field. Figure 4 clearly shows that the K lines are composed of unshifted narrow lines and shifted broadened lines. Figure 5 indicates that the ultrasonic frequency strongly affected the apparent line width. On the basis of this observation, the frequency dependence of the line width in Figure 5 is interpreted as follows. At 48 kHz, the contribution of broadened lines to the K line intensity is large, as shown in Figure 4. An increase in ultrasonic frequency causes a decrease in the intensity of the broadened lines relative to that of the narrow lines, leading to a decrease in the apparent line width. As a result, the narrow lines remained at a frequency of 1 MHz with a small contribution of the broadened lines. The effect of the ultrasonic frequency can be explained by the injected droplet model as discussed above. At a lower ultrasonic frequency, the intensity of the broadened lines is enhanced by the higher efficiency of injection of droplets into larger bubbles. In fact, the absolute intensity of the K lines at 148 kHz, for which the broadened lines are the main contributor, was 40 times larger than that at 1 MHz, although the intensities of the continuum at the two frequencies differ less than 2 times. The spectra of the continuum and K lines at different frequencies are shown in ref 17. This finding shows that the population of bubbles that generates droplets was much greater at the lower ultrasonic frequency, whereas the total intensity of the continuum emission from the spherical bubbles hardly changed at the two ultrasonic frequencies. Thus, the increase in the intensity of the broadened lines due to the high efficiency of droplet injection is responsible for the increase in the apparent line width at the low ultrasonic frequency. At a low ultrasonic frequency, increasing the ultrasonic power causes the bubble collapse to become more violent, which causes an increase in the relative gas density in the bubble at collapse and eventually leads to the further shift to the red side and peak splitting shown in Figure 4. It has also been recognized that the ultrasonic frequency affects the amount of water vapor trapped inside bubbles.34,35 However, the amount of water vapor trapped inside bubbles appears to have little effect on the K line width, as shown in Figures 1 and 2. It is also interesting to consider the origin and mechanism of the unshifted narrow lines. The results in Figure 3 showed that the intensity of the narrow lines depends on the dissolved gas. The narrow lines from Xe-saturated solutions became much

more prominent than those from Ar-saturated solutions whereas the narrow lines disappeared in He-saturated solutions. Thus, the intensity of narrow lines increased with increasing molecular weight of the rare gas inside the bubbles. These results suggest that the origin of the narrow lines is also in the gas phase. Xu et al.10 and Hatanaka et al.15 succeeded spatial separation of two different types of sonoluminescing bubbles in Na2SO4 sulfuric acid solutions, one of which is the bubbles radiating only continuum emission and the other is the bubbles radiating Na atom emission. The spectrum of Na atom emission in Figure 10 obtained by Hatanaka et al.15 appears to contain unshifted narrow lines. Their observations strongly support our assumption that both the broadened lines and narrow lines come from the gas phase because if one or both lines arise from the liquid phase at bubble/liquid interfaces, then all the sonoluminescing bubbles should radiate Na atom emission. At the present stage, however, it is unclear why unshifted narrow K lines were observed from collapsing bubbles. As can be seen in Figures 4 and 5, the line width of narrow lines did not broaden and its peak position did not shift from that of flame spectrum of KCl even though the acoustic power and ultrasonic frequency was changed. From a standpoint of collisional broadening, this implies that the narrow lines originated from a phase with low relative density. The narrow lines may originate from a site that is unaffected by a high-density gas. Moreover, the timing of the narrow-line emission may be different from that of the broadened-line emission. This issue should be clarified and is currently being investigated.

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CONCLUSIONS The effects of a saturated rare gas on the sonoluminescence spectra of K atom emission strongly indicated that a gas phase inside the bubbles was the origin of the emission site. The spectra from Xe- and Ar-saturated KCl aqueous solutions at 148 kHz showed that the K doublet consists of two components: shifted asymmetrically broadened lines and unshifted narrow lines. The spectra from He-saturated solutions were shifted to the blue side and symmetrically broadened without accompanying unshifted narrow lines. This dependence of the broadened lines on the rare gas is in good agreement with the results of spectroscopic studies in the gas phase, which indicates that collisional interactions with the rare gas are responsible for the shift and broadening of K lines. The relative density inside bubbles at the timing of K atom emission was estimated to be 42 from a comparison of the line width and shift with the spectroscopic data. The K line width was independent of the solution temperature, whereas the intensity decreased with increasing temperature. Water vapor trapped inside bubbles caused a decrease in the population of sonoluminescing bubbles, resulting in the quenching of K lines without changing the K line shape. The intensity of the broadened lines decreased with increasing ultrasonic frequency, and narrow lines were prominent at 1 MHz. This ultrasonic frequency dependence may be related to the bubble size and efficiency of droplet injection. The narrow lines also originated from the gas phase inside the bubble. The exact mechanism generating the narrow lines is unclear and should be investigated in future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 7896

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. S. Hatanaka for useful advice in sample preparation. This research was supported by Grants-in-Aid for Scientific Research (21560056) from the Japan Society for the Promotion of Science.

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dx.doi.org/10.1021/jp3033287 | J. Phys. Chem. B 2012, 116, 7891−7897