Influence of Ultrasonic Frequency on Swan Band Sonoluminescence and Sonochemical Activity in Aqueous tert-Butyl Alcohol Solutions

Abstract

The multibubble sonoluminescence (MBSL) spectra of t-BuOH aqueous solutions submitted to power ultrasound at 20, 204, 362, and 613 kHz show emissions for the Δυ = -1 to Δυ = +2 vibrational sequences of C2* Swan system (d(3)Πg → a(3)Πu). The Δυ=+2 emission overlaps with the CH(A-X) emission band. The maximal Swan band emission is observed when the MBSL of water itself is almost completely quenched. In general, MBSL is more intense at high-frequency compared to 20 kHz ultrasound. However, in the presence of Xe, the MBSL of C2* at 20 kHz is so bright that it can be seen by the unaided eye as a blue glow in the close vicinity of the ultrasonic tip. The intensity of the C2* band emission exhibits a maximum vs t-BuOH concentration: 0.1-0.2 M at 20 kHz and (1-8) × 10(-3) M at high-frequency ultrasound. Such a huge difference is attributed to a much smaller bubble size at high ultrasonic frequency or, in other words, to a much higher bubble surface/volume ratio providing more efficient saturation of the bubble interior with t-BuOH vapors and to the fact that high frequency bubbles remain active for many more cycles than 20 kHz ones, thus accumulating more hydrocarbon decomposition products. Simulation of the emission spectra using Specair software demonstrated the absence of thermal equilibrium for C2* radicals (Tv > Tr), where Tv and Tr are the vibrational and the rotational temperature, respectively. In Ar, Tv decreases with increasing t-BuOH concentration reaching a steady value in the concentration domain that corresponds to C2* emission maximum intensity. In the presence of Xe an extremely high Tv is obtained, which is explained by the relatively low ionization potential of Xe providing a higher electron temperature of nonequilibrium plasma generated during bubble collapse. Analysis of the gaseous products of t-BuOH sonolysis reveals a significant sonochemical activity even at high t-BuOH concentration when MBSL is totally quenched, indicating that drastic conditions could be produced also within nonsonoluminescing cavitation bubbles.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…Similarly, at 20 and 513 kHz (10 W/cm 2 and 1.5 W/cm 2 respectively) the zero order rate of production of H 2 O 2 increases in the order of He < Ar < Kr [220]. Pflieger et al simulated the spectral emission from inside the bubble during sonication for Ar and Xe in consideration of electron (T e ), vibrational (T v ), rotational (T r ) and translational (T g ) temperatures contributing to nonequilibrium plasma formation [259]. Xe showed a significant increase in vibronic temperatures (T v ,T e ) compared to Ar due to its lower ionising potential.…”

Toward a new paradigm for sonochemistry: Short review on nonequilibrium plasma observations by means of MBSL spectroscopy in aqueous solutions

“…Therefore, a much stronger SL quenching is obtained at high frequency for the same concentration of t-BuOH. Significant sonochemical activity is observed in the whole studied range of t-BuOH concentrations even when the sonoluminescence is completely quenched [56]. The high yield of acetylene at these concentrations unequivocally indicates that the drastic conditions inside the cavitation bubbles are not necessarily accompanied by sonoluminescence.…”

“…7) probably due to the quenching of C 2 ⁄ species with t-BuOH molecules or with the products of its sonochemical degradation. The optimal t-BuOH concentration for C 2 ⁄ emission at 20 kHz was found to be 100 times higher than at high-frequency ultrasound [56]. Such striking difference was explained by two arguments.…”

“…The increase of Dm = +1/Dm = 0 ratio indicates the increase of vibrational temperature of C 2 ⁄ radical. Reproduced with permission from [56]. radicals generated inside the bubbles; in argon, the vibrational temperature at 362 kHz is higher than at 20 kHz; and in the presence of Xe an extremely strong vibrational excitation (T v = 14,000 K, T r = 2500 K) is observed even at 20 kHz.…”

“…The most intense emission lines of C 2 ⁄ radical known as Swan band system (d 3 P g -a 3 P u ) are easily detectable in the visible spectral range for the Dm = À3 to Dm = +2 sequences and the relative intensities of these Dm transition emissions are sensitive to C 2 vibrational temperature [55]. A recent spectroscopic study of MBSL in aqueous tert-butanol solutions demonstrated that both the intensity and the shape of C 2 ⁄ radical emission spectra are strongly influenced by the experimental parameters, such as t-BuOH concentration, ultrasonic frequency, and saturating gas [56]. Some typical MBSL spectra of C 2 ⁄ are shown in Fig.…”