New sonochemiluminescence involving solvated electron in Ce(III)/Ce(IV) solutions.

The moving single-bubble sonoluminescence of Ce3+ in water and ethylene glycol solutions of CeCl3 and (NH4)2Ce(NO3)6 was studied. As found, a significant part of intensity of the luminescence (100% with cerium concentration less than 10-4 M) is due to the sonochemiluminescence. A key reaction of sonochemiluminescence is the Ce4+ reduction by a solvated (or hydrated in water) electron: Ce4+ + es (eaq) → *Ce3+. Solvated electrons are formed in a solution via electrons ejection from a low-temperature plasma periodically generated in deformable moving bubble at acoustic vibrations. Reactions of heterolytic dissociation of solvents make up the source of electrons in the plasma. In aqueous CeCl3 solutions, the Ce4+ ion is formed at the oxidation of Ce3+ by OH radical. The latter species originates from homolytic dissociation of water in the plasma of the bubble, also penetrating from the moving bubble into the solution. The sonochemiluminescence in cerium trichloride solutions are quenched by the Br- (acceptor of OH) and H+ ions (acceptor of eaq). In water and ethylene glycol solutions of (NH4)2Ce(NO3)6, the sonochemiluminescence also quenched by the H+ ion. The sonochemiluminescence in CeCl3 solutions is registered at [Ce3+] ≥ 10-5 M. Then the sonochemiluminescence intensity increases with the cerium ion concentration and reaches the saturation plateau at 10-2 M. It was shown that sonophotoluminescence (re-emission of light of bubble plasma emitters by cerium ions) also contributes to the luminescence of Ce3+ in solutions with [Ce3+] ≥ 10-4 M. If the cerium concentration is more than 10-2 M, a third source contributes to luminescence, viz., the collisional excitation of Ce3+ ions penetrating into the moving bubble.

Introduction

Sonochemiluminescence (SCL) is one of the types of light radiation during ultrasonic irradiation of solutions [1] in addition to sonoluminescence (SL) associated with emitting photons by cavitation bubbles [2]. SCL occurs in the reactions of products that arise and enter the solution during the decomposition of low-temperature plasma. The latter is periodically generated in the bubbles from the compounds of the solution during the acoustic vibrations [3]. Such SCL is known for many systems, e.g. aqueous solutions of luminol [3], acridinium NHS ester [4], chelated Tb3+, Ru(bpy)32+, Ru(bpy)33+ complexes [5], [6], [7] and non-aqueous solutions of various organic compounds [8], [9], [10]. Among them, a particular attention is paid to the systems whereby chemiluminescence (CL) arises in the reactions involving the sonolytically generated solvated electron (es). This reagent has an unusually high reducing ability. Therefore, exothermic redox reactions with its participation usually results in the electronically excited products. For example, in aqueous solutions of ruthenium(III)-bipyridyl complex, the excited MLCT state of the Ru(bpy)32+ (*Ru(bpy)32+) emerges directly in the elementary act of reduction upon transferring electron to the ligand-centered orbital unoccupied in the initial Ru(bpy)33+ complex. This luminescent reaction confirmed the electron ejection from the plasma of the bubble and previously unknown generation of the hydrated electrons (eaq) during moving single-bubble sonolysis of water (1):besides the processes involving other primary products of the sonolysis (2) [7]:

Obviously, similar reactions can also occur during sonolysis of other substances whereby their reduction with the hydrated (or solvated, in a general case) electron generates the electronically excited emitters. This should expand the list of SCL systems.

Based on this hypothesis, this work aims searching for SCL associated with the reduction of the Ce4+ ions during moving single-bubble sonolysis of the Ce4+ and Ce3+ solutions in water and ethylene glycol (Ce4+ is formed during the sonolysis from Ce3+). The choice of cerium compounds for this purpose is due to the high exothermicity of the Ce4+ reduction with eaq (and probably of Ce4+ with es in ethylene glycol) and possible formation of excited ion *Ce3+, which has a high quantum yield of photoluminescence (PL) in water. Note that the formation of *Ce3+ has not been previously detected despite the above considerations and numerous studies on reduction process Ce4+ → Ce3+ during sonolysis and radiolysis [11], [12].

Experimental part

The bidistilled water was used for the preparations of the solutions; ethylene glycol and sulfuric acid of a high degree of purity were additionally distilled. Salts CeCl3·7H2O, (NH4)2Ce(NO3)6, and KBr of a high-purity grade were from Sigma-Aldrich and used without further purification. The single-bubble sonolysis of the solutions was performed in a spherical glass flask resonator with the piezoelectric transducers oppositely glued (volume ~ 100 mL, resonant frequency ~ 26 kHz). To avoid the reaction of es with dissolved oxygen all the solutions were prepared with evacuation at 0.01 Torr for 30–40 min to remove the dissolved gases and further saturation with argon by bubbling the gas through the solution (flow rate 5 mL·s−1, 5–6 min). Then the solutions were additionally evacuated for 30 min. This procedure is also necessary to obtain reproducible and bright single-bubble sonoluminescence (SBSL).

The sonoluminescence spectra and concentration dependences of SL intensity were registered with spectrofluorimeter Aminco Bowmen (Hamamatsu R3896 photomultiplier, Δλ = 2 nm). The photon emission from the cavitation bubble in the center of the flask was delivered to the entrance slit of the spectrofluorimeter using the quartz fiber (the diameter is 0.4 mm). Its starting point was located in 5 mm from the bubble. The relative spectral sensitivity of the detection system was determined using a lamps with a known spectral distribution of its radiation intensity: DDS-30 deuterium lamp in the range of 250–400 nm and KGM-650 tungsten halogen lamp at 350–600 nm. When recording SBSL, the temperature was maintained in the range of 2–5 °C. The acoustic pressure (pa) in the middle of the resonator flask was measured with hydrophone 8103 Brüel & Kjӕr. Ultrasonic treatment was carried out only when spectra are recording (about 1.5 min). To obtain the averaged spectra the treatment of one solution was carried out 3 times, but no more, after which fresh solutions were prepared. The final spectra were obtained by averaging 10 experimental spectra. PL and absorption spectra were recorded respectively with spectrofluorimeter Fluorolog-3 Horiba Jobin Ivon and spectrophotometer Shimadzu UV-1800. The photographs of SBSL were taken with camera Nikon 3000D.

Results and discussion

Luminescence of *Ce3+ during sonolysis of CeCl3 solutions in water

Fig. 1 shows the spectra of the single-bubble sonoluminescence of water and CeCl3 aqueous solutions with different concentrations at the acoustic pressure of 1.16–1.32 bar. When the latter equals 1.2 bar, there is a stable, spherically symmetric and pulsating with a standing wave frequency but motionless bubble (Fig. 2, photo b). The SBSL spectrum of water at this pressure represents the broad band continuum due to the superposition of the emission spectra of several emitters (Fig. 1, spectrum 1): OH*, H2O*, possibly bremsstrahlung and thermal radiation of electrons and other “hot” particles of the non-equilibrium low-temperature plasma, periodically emerging in the bubble during acoustic vibrations at the end of the compression [3]. A similar spectral continuum of SL of the bubble (hereinafter, solvent or water continuum) is typical for many liquids. Against this background, in the SBSL spectra of the CeCl3 aqueous solutions with concentrations starting from 10–4 M, we observe the exited *Ce3+ ion emission band with the maximum at ~349 nm (hereinafter, the Ce3+ band).

Fig. 1

Luminescence spectra during the sonolysis of the CeCl3 aqueous solutions at various acoustic pressures and absorption spectrum: 1 – SBSL (1.2 bar) of H2O or 5·10–5 M Ce3+ in H2O, and moving SBSL (1.32 bar) of H2O or 5·10–5 M Ce3+ in 0.1 M H2SO4; 2, 3, 4 – SBSL (1.2 bar) of Ce3+ in H2O, and moving SBSL (1.32 bar) of Ce3+ in 0.1 M H2SO4 for different cerium ion concentrations: 10–3, 0.01, and 0.1 M (SBSL only), respectively; 5 – absorption (cuvette l = 1 cm) of 10–4 M Ce3+ in H2O; 6 – moving SBSL (1.16 bar) of H2O or 5·10–5 M Ce3+ in 0.1 M H2SO4, no glow; 7 – moving SBSL (1.16 bar) of 5·10–5 M Ce3+ in H2O (the ordinate of the spectrum is multiplied by 3); 8 – moving SBSL (1.32 bar) of 5·10–5 M Ce3+ in H2O.

Fig. 2

The photographs of the moving bubble (a, c) and the motionless bubble (b). Exposure 0.1 s. Photographs (b, c) were taken in the light of intrinsic luminescence of the bubble, photograph (a) was taken in the light reflected from an external source. Acoustic pressure, bar: 1.16 – a, 1.2 – b, 1.32 – c.

The Ce3+ band intensity increases with the cerium concentration (Fig. 1, spectra 2–4). At the same time, the intensity of the shortwave region of the continuum (less than 300 nm) is significantly reduced due to the absorption of the emission by Ce3+ (Fig. 1, spectrum 5). The PL quantum yield of Ce3+ in water is close to 100% [13]. This leads to the efficient re-emission of absorbed SL of the bubble, i.e. sonophotoluminescence (SPL) of the Ce3+ emitter.

It should be noted that mass transfer through the liquid–bubble interface is limited in the case of the symmetric stationary bubble. Penetrating the non-volatile cerium ion into the bubble as a result of evaporation is inefficient and decreasing the probability of its excitation by the collision with “hot” particles of the plasma. A weak mass transfer also lowers the flow of the water sonolysis products from the bubble into the solution and the possibility of SCL of cerium ion in solution. Therefore, SPL was considered the main mechanism for the Ce3+ SBSL generation as, besides, for the multibubble sonoluminescence (MBSL) of this emitter [14], [15], [16]. However, the stationary state of the bubble takes place only in the limited range of acoustic pressure pa affecting the bubble. Increasing and decreasing pa relative to the optimal value ~1.2 bar moves the bubble around the stabilization center (Fig. 2, photos a, c). Herewith, the shape of the bubble is deformed. Indeed, there are surface waves and micro-jets, which increase the mass transfer through the interface of the bubble (in both ways, inwards and outwards) [17], [18]. When p = 1.32 bar, moving SBSL of water is observed. The intensity of moving SBSL continuum in the highly degassed solution has insignificant changes as compared with the SBSL intensity of the stable bubble (Fig. 1, spectrum 1). Although in our experiments, there is a general tendency for luminescence intensity to increase with increasing of pa, the expected enhancement in intensity with increasing pressure to 1.3 after 1.2 bar is compensated by the appearance of moving bubble deformations, which reduce the compression efficiency and the achievable degree of collapse, on which the continuum intensity depends. When pa = 1.16 bar, there is no water continuum of moving SBSL (Fig. 1, background curve 6). Obviously, at this value of pa, the energy of the bubble compression is insufficient for the appearance of a continuum due to collisional excitation of emitters. Nevertheless, even for dilute solutions of CeCl3 in water (5·10–5 M), the Ce3+ emission is registered (Fig. 1, spectrum 7). At the same time, SPL of Ce3+ is not detected at this concentration due to the absence of SL of the bubble. Obviously, SPL is not detected at this concentration under pa = 1.32 bar but the Ce3+ band intensity in the case of moving SBSL (Fig. 1, spectrum 8) is close to the SPL intensity of the 0.1 M Ce3+ solution with the stationary bubble (pa = 1.2 bar). The Ce3+ band intensity for moving SBSL also increases with the cerium ion concentration.

As found, the Ce3+ band at pa = 1.32 bar is quenched in acidic solutions. Thus, when H2SO4 added (the final acid concentration is 0.1 M) to the CeCl3 solutions (5·10–5, 10–3, 0.01 M), the Ce3+ band intensity is reduced to the level corresponding to the SPL mechanism at pa = 1.2 bar (Fig. 1, spectra 1, 2, 3 respectively). The quenching by acid (0.1 M) also takes place at 0.1 M Ce3+ concentration. However, there is a slight excess over the SPL intensity (the corresponding spectrum is not shown in Fig. 1). The reason for incomplete quenching at the high Ce3+ concentration will be considered below. Herewith, there is no significant effect of the acid addition on the solvent continuum (see also Fig. 3).

Fig. 3

The moving SBSL spectra of CeCl3 aqueous solutions at pa = 1.32 bar: 1 – H2O; 2 – 3∙10–5 M Ce3+; 3 – 5·10–5 M Ce3+, 0.1 M H2SO4; 4 – 5·10–5 M Ce3+, 10–3 M KBr. 5 – PL spectra (λexc = 253 nm) of 5∙10–5 M Ce3+ in: H2O, aqueous solutions of 0.1 M H2SO4 or 10–3 M KBr.

This indicates the insignificant penetration of the low-volatile acid into the bubble at this concentration (0.1 M) even under conditions of moving SBSL and weak effect on the processes of the generation of the sonolysis products. Separate experiments showed that this concentration of sulfuric acid has no significant effect on the shape and intensity of the Ce3+ band in the PL spectrum of CeCl3 in water (Fig. 3, spectrum 5). Thus, the PL quantum yield of the Ce3+ ions in the presence of acid does not change. Therefore, quenching of moving SBSL is associated with the influence of acid on the excitation stage of cerium ion in the bulk solution. In addition, the quenching of moving SBSL of Ce3+ is also observed in the presence of 1 mM KBr (Fig. 3, spectrum 4). As in the case of H2SO4, KBr in this concentration has no significant influence on the continuums of SBSL, moving SBSL, and PL of Ce3+.

H2SO4 and KBr, more strictly the H+ and Br− ions, used for quenching of Ce3+ ion moving SBSL, are known acceptors of hydrated electrons and OH radicals, respectively [19].

Based on the above facts, we consider that, except of the SPL contribution, moving SBSL of the Ce3+ ions represents the sonochemiluminescence. The latter consists of the sequential redox reactions occurring after the transfer of products heterolytic (eaq) and homolytic (OH) water dissociation from the bubble to the CeCl3 solution (3) and (4):

As noted in the Introduction, the formation of Ce4+ from Ce3+ via reaction (3) is a well-known process at the sonolysis and radiolysis of solutions of the trivalent cerium compounds [11], [12]. As follows from the standard potentials eaq (–2.9 V) and Ce4+/Ce3+pair (1.4 V) [12], the energy of the reduction process (no less than –2.9 –1.4 = –4.3 eV) is sufficient for population of the high-energy level of the electronically excited state of Ce3+ (about 4.1 eV [13]) according to reaction (4). This chemiluminescent reaction is illustrated using the energy diagram of the valence sublevels of cerium ions in Supplementary material. There also is a detailed calculation of exothermicity (free energy change) of reaction (4).

The quenching of SCL is due to the reactions with acceptors (5), (6), competing with reactions (3), (4), respectively:

Notably, 100 times lower concentration of Br− as compared with H+ is enough to quench SCL. Apparently, reaction (3) is substantially slower than reaction (5). Their rate constants are k3 = 2.9·108 and k5 = 1·1010 M−1·s−1, respectively [12], [19]. At the same time, the reaction rates of (4), (6) are significantly higher and close to each other: k4 = 6.6·1010 and k6 = 2.3·1010 M−1·s−1 [12], [19].

Luminescence of *Ce3+ during sonolysis of (NH4)2Ce(NO3)6 solutions in water

The emergence of reaction (3) was proved with special experimental studies of SBSL and moving SBSL in ceric ammonium nitrate solutions. This salt of tetravalent cerium dissolved in water manifests significant absorption in the region of Ce3+ band and, especially, in the short-wave region of the SBSL water continuum (its bound shifts to 340 nm, Fig. 4, spectrum 2) but the Ce4+ ion is not a luminophore.

Fig. 4

The spectra of SBSL at 1.2 bar (1, 2) and moving SBSL at 1.32 bar (3): 1 – H2O; 2, 3 – 5·10–5 M (NH4)2Ce(NO3)6. 4 – Absorption spectrum (cuvette l = 0.5 cm) of 10–5 M (NH4)2Ce(NO3)6 in H2O. The dashed line shows the “restored” Ce3+ band in spectrum 3 in the absence of light absorption by the Ce4+ ion.

As expected, in the absence of SPL, there is no luminescence except the unabsorbed part of the continuum in the SBSL spectra at pa = 1.2 bar of ceric ammonium nitrate solutions. However, in the case of the low-concentration solutions (5·10–5 M), the unabsorbed part of Ce3+ band is observed in the spectrum of moving SBSL at pa = 1.32 bar (Fig. 4, spectrum 3). This fact confirms the *Ce3+ formation because of Ce4+ reduction during the sonolysis. The Ce3+ band is quenched by acidification of the solution but not in the presence of KBr. These experiments confirm the fact of the *Ce3+ generation in reaction (3).

Luminescence of *Ce3+ during sonolysis of (NH4)2Ce(NO3)6 solutions in ethylene glycol and estimation the possibility of chemiluminescence at Ce4+ reduction by reagents, other than es

We were able to register the well-resolved Ce3+ band in the moving SBSL mode in the ethylene glycol solution of (NH4)2Ce(NO3)6 (Fig. 5).1 In this solvent, the Ce4+ ion absorption in the region of the Ce3+ band is lower than in water, so it does not prevent for its registration. The Ce3+ band in ethylene glycol is quenched by H2SO4 as in water.

Fig. 5

The spectra in ethylene glycol: 1 – SBSL (1.24 bar) without additives; 2 – moving SBSL (1.36 bar) of 5·10–5 M (NH4)2Ce(NO3)6; 3 – moving SBSL (1.36 bar) 5·10–5 M (NH4)2Ce(NO3)6, 0.1 M H2SO4. 4 – PL (λexc = 253 nm) of 10–5 M CeCl3. 5 – Absorption (cuvette l = 1 cm) of 5·10–5 M (NH4)2Ce(NO3)6.

The obtained data indicate that the solvated electron es is formed during the moving single-bubble sonolysis of ethylene glycol, which similarly to reaction (4) leads to the *Ce3+ generation (reaction (4')).

As is known, the solvated electron may be formed during the radiolysis of alcohols, amines and amides [19]. Based on the analogy in the radiolysis and sonolysis of liquids, the heterolytic dissociation of the solvent molecules with generating es at the moving single-bubble sonolysis is possible in polar liquids other than water. It should be noted that it was not possible to register moving SBSL of Ce3+ in CeCl3 ethylene glycol solutions more than provided by SPL values. Possibly, in contrast to aqueous solutions, the ethylene glycol analog of reaction (3) providing oxidation Ce3+ → Ce4+ is ineffective. This fact does not allow observing SCL.

Hydrogen atom and carbon-centered free radicals •CH2OH, •CH2CH2OH and •C(CH3)3 are also strong reducing agents formed by ultrasonic treatment of ethylene glycol solutions. The redox potentials of atom H and radicals •CH2OH, •C(CH3)3 are –2.3 V [12], –0.24 V and –2.1 V [20] respectively, and generally for hydrocarbon radicals of various nature not exceed –2.0 V [21]. Although the reduction of tetravalent cerium ion (reaction (7)) by atom H (which has the highest reducing ability among the known radical intermediates of ethylene glycol sonolysis) is exothermic, the SCL generation via reaction (7) is unlikely, as in aqueous acidic solutions, where H is the main reducing agent instead of eaq.

First, since the potential of the H+/H pair (–2.3 V) is 0.6 V less (in absolute value) of the standard potential of eaq, the exothermicity of reaction (7) is also less than the previously found exothermicity of reaction (4) (3.7 eV vs 4.3 eV). This energy is less than required for electronic excitation of Ce3+ ion, 4.1 eV. Second, even if the *Ce3+ formation by reaction (7) is possible, its rate is 103 times lower than the rate of reaction (4): k7 = 6.5·107 versus k4 = 6.6·1010 M−1·s−1 [22]. Therefore, chemiluminescence by reaction (7) in water, as well as similar reactions of Ce4+ reduction by radical products in ethylene glycol, is not registered. Instead of this, in acidic solutions, where H is formed according to reaction (6), quenching of Ce3+ SCL is observed, as a part of moving SBSL of this ion exceeding the luminescence by the SPL mechanism.

Obviously, due to the study of the Ce4+ reduction processes only in acidic solutions, the radio- and sonochemiluminescence due to reaction (4) have not been previously found [11], [12]. However, it should be noted that the Ce4+ reduction was repeatedly used in chemiluminescent methods of the analysis of various substances, e.g. phenolic compounds [23]. In these analytical reactions, rhodamine 6G (R6G) and phenols were used as Ce4+ reducing reagents, and R6G was a sensitizer of luminescence. It was assumed that *Ce3+ is the primary emitter of these chemiluminescent reactions and it is resulted from reduction Ce4+ → *Ce3+. R6G*, the secondary emitter with the detected luminescence, is generated by the energy transfer from the primary emitter. Meanwhile, the excitation of trivalent cerium ion in the described reaction is questionable. The reduction potential of R6G (about –0.8 V [24]) coupled with the oxidizing potential of Ce4+ is insufficient to create the energy reserve necessary for populating the excited state of Ce3+. However, there is no direct spectral identification of *Ce3+ in this and other similar studies [25] postulating reaction Ce4+ → *Ce3+.

We found only one previous work [26] with the CL emission spectrum confirming the formation of *Ce3+ during the reduction of tetravalent cerium. The Ce3+ ion is formed in heterogeneous system (NH4)2Ce(NO3)6–C6H6–H2O as a result of the interaction of the catalytically active surface of the (NH4)2Ce(NO3)6 crystals with water. The redox potentials of pairs Ce4+/Ce3+ (+1.4 V) and H2O/O2 (–1.23 V [27]) are not sufficient to populate the excited level of Ce3+, therefore, the mechanism for the *Ce3+ generation in this chemiluminescent system is also unclear. Thus, we have obtained the first confirmation of the chemiluminescent emitter generation providing the population of the high-energy level of the electronically excited state of the Ce3+ ion in the reduction of Ce4+ and its luminescence in the UV-region.

The concentration dependence of Ce3+ luminescence intensity and estimation the range of action of various mechanisms of its excitation during sonolysis of CeCl3 aqueous solutions

Fig. 6 shows the dependence of the intensity of moving SBSL Ce3+ on its concentration in the CeCl3 solutions with intensity reduction by SPL mechanism evaluated according to the SBSL data of the stable bubble. In the range of low concentrations (10–5–10–2 M) with a transit to saturation, the intensity is increased. The increase become less pronounced in the range starting from 10–2 M. Here, a linear dependence of the intensity on the concentration is observed if the intensity reached at low concentrations is not accounted. This section of linear relationship is the only one after the suppression of SCL in acidic solutions (Fig. 6).

Fig. 6

Dependence of the total moving SBSL intensity (1.32 bar) minus the SPL intensity (SBSL at 1.2 bar) on the concentration of Ce3+ in the CeCl3 aqueous solutions – (1); same curve in a solution of 0.1 M H2SO4 – (2); the concentration dependence of SCL intensity – (3). The inset b shows the dependence (1) in the concentration range 5∙10–5–10–3 M.

A similar general dependence was previously observed in the studies of radioluminescence in aqueous solutions of some dyes and aminobenzoic acids [28], [29]. These studies noted that at low luminophore concentrations, the luminescence intensity is defined by radiochemiluminescence2 whereas linear dependence at high concentrations is due to the luminophore excitation under the direct action of ionizing irradiation, i.e., collisions with charged particles (electrons, α-particles, etc.). We consider that in our case the linear part of the concentration dependence of moving SBSL is analogously determined by the collisional excitation (CE) of the Ce3+ ion in the bubble with “hot” particles due to the significant penetration of non-volatile components of the solution into the bubble because of injection of nanodroplets [17]. Previously, the existence of Ce3+ CE was not observed at studying MBSL of Ce3+ [14], [16]. Indeed, the contribution of this mechanism is small due to the low yield of Ce3+ CE [30] (despite the movement of the bubbles and the deformation injection of the solution under conditions of multibubble sonolysis). Therefore, the contribution of CE is difficult to separate from the contribution of SPL mechanism.

Thus, moving SBSL of Ce3+ in the aqueous solutions of inorganic salt CeCl3 has three mechanisms in contrast to SBSL having only the SPL mechanism. These are SCL at Ce3+ concentrations less than 10–4 M, SCL + SPL in the concentration range of 10–4–10–2 M, and SCL + SPL + CE at [Ce3+] > 10–2 M. The multibubble sonoluminescence of cerium ions, despite its apparent similarity to moving SBSL, represents only SPL at [Ce3+] = 10–4–10–2 M and, apparently, SPL + CE at large concentrations due to absence of eaq among the primary products of water decomposition in this mode of sonolysis [7], [31], [32] and accordingly SCL.

Conclusion

In this work, we have studied the moving single-bubble sonoluminescence of the Ce3+ ion in the solutions of CeCl3 and (NH4)2Ce(NO3)6 in water and ethylene glycol. It has been shown that the significant amount of intensity of this luminescence (100% under concentrations less than 10–4 M) is due to the sonochemiluminescence in the reactions of the products of water sonolysis (decomposition) in the bubble. The reduction of the tetravalent cerium ion by the solvated (or hydrated) electron Ce4+ + es → *Ce3+ is a key stage of this sonochemiluminescence.

CRediT authorship contribution statement

Glyus L. Sharipov: Conceptualization, Project administration, Writing - original draft, Writing - review & editing, Investigation, Validation, Funding acquisition. Bulat M. Gareev: Investigation, Writing - original draft, Validation. Kristina S. Vasilyuk: Investigation, Writing - original draft. Dim I. Galimov: Investigation, Project administration, Writing - original draft, Writing - review & editing. Airat M. Abdrakhmanov: Investigation, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.