1. Introduction

Water is one of the most important substances in nature and has been extensively investigated over the past decades [1–5]. Despite its apparent molecular simplicity, it has been long considered with complex properties. More importantly, as a common solvent, water can dissolve various solutes. It is always a fascinating research field because of the role in chemistry, biology, electrochemistry, geography and so on [6–9]. Many researches have been conducted to analyze the structure and properties of water over the past decades. Experiment probes include nuclear magnetic resonance, X-ray diffraction, neutron scattering, IR absorption, and Raman spectroscopy [10–13]. Among them, SRS has been proved a useful tool to investigate water structure under different conditions [14–17]. SRS study of liquid water can give important information about H-bonds network structure and dynamics. Particularly, OH vibration modes of water molecules can be easily and accurately identified by the sharp peaks in SRS spectra [18–22]. With the discovery of the SRS phenomenon, the study of water structure is presented with a novel breakthrough [16–18].

However, Raman scattering cross section of water is inherently small, which is about 10⁻³⁰ cm² for a single molecule. High-quality Raman spectra require not only a long integration time and high-power excitation, but also the low excitation wavelength. Due to the basic and practical application of SRS method in detecting water, how to provide a cheap but convenient method to enhance the SRS signals of water has been of great interest. Previously, several different methods have been attempted to solve this problem, including cooling [16], focusing [19,23], and pumping with stimulated Brillouin scattering [24]. These methods are very important and useful, but have relatively high requirements on external environment and experimental equipment. Some studies have achieved SRS enhancement through the resonance method, but need to limit the wavelength of the incident laser [20].

In this paper, we realized an obvious enhanced SRS signal by dissolving KOH in liquid water. The spectra of water and KOH-H₂O solution exhibit different characteristics. Only one peak appears in the FSRS (forward stimulated Raman scattering) spectrum of water, comparing with two peaks in KOH-H₂O solution. Especially, a relatively low-wavenumber peak appears at 3200 cm⁻¹ in BSRS (backward stimulated Raman scattering) spectrum of KOH-H₂O solution, representing the formation of ice Ih-like structure. Meanwhile, the normalized SRS peak intensity of KOH-H₂O solution is also higher than that of water at same conditions. The reason for enhanced SRS signals can be divided into two aspects: i) the electric field generated by KOH softens and elongates the O-H bond and stiffens and shortens the O:H nonbond, leading to the enhancement of H-bonds network in water molecules; and ii) dissolving KOH in water increases the third-order Raman susceptibility, thereby increases the Raman gain coefficient of water molecules. Compared to previous research, the solute in our research undergoes dissociation in water forming mono-atomic ions (K⁺ and OH^-) that are not Raman-active. This makes our stimulated Raman spectra simpler, which also excludes some influencing effects (for example, the assignment of some overlapping peaks). SRS enhancement phenomenon is explained not only from hydrogen bonds structure but also from the mechanism of stimulated Raman scattering. We quite optimistically believe that our results can open the way to understand the interaction between water molecules and other alkali hydroxides. More importantly, from the application point of view, we provide a simple but convenient method for enhanced SRS signals of water molecules.

2. Experimental step

The pure water and the liquid water of KOH-H₂O solution (1 M) have been deionized from triple distilled water. The resistivity of the water is 17.3 M.Ω·cm. Samples are kept in a quartz cell whose width, height, and length are 1, 5, and 10 cm, respectively. The focal length of the lens is 15 cm. The length, width and thickness of dichroic (transmission: reflectivity=50%: 50%) are 80, 80 and 1.1 mm, respectively. Second harmonic of the pulsed Nd-YAG laser with a wavelength of 532 nm is used as excitation laser and focused in the quartz cell. The pulse duration is 12 ns, and the repetition rate is 10 Hz. Optical power density and diameter are 1.4×10¹² Wcm⁻² and 4 mm, respectively. Spectra are obtained by using an objective lens with times 50 long working distance. Output light is accepted by a spectrometer and signals are analyzed by a computer. The experimental setup is shown as Fig. 1.

The scanning speed of spontaneous Raman spectra is 10 cm⁻¹/s and the spectral resolution is 1 cm⁻¹. Samples are kept in a quartz disk of 10 mm radius and excited by an Argon laser at 514.5 nm with an output power of 10 mW. A 50× long working distance objective lens is located in different samples. Spectra are measured by Renishaw InVia Raman micro spectrometer and detected with a CCD detector. THMSG Linkam G600 heating-freezing with 0.1 K accuracy is used to control the temperature of ice Ih in spontaneous Raman scattering process. All experiments are carried out at ambient pressure.

 

Fig. 1. Schematic diagram of the experimental setup to measure SRS spectra. Insert: SRS spectra of liquid water.

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Preparation of ice Ih samples: Oriented single crystals of H₂O are grown inside sample cell using the following technique. Cold air from a laminar flow cooler is directed across the sample cell. The temperature is lowered a few degrees below the freezing point of water to initiate freezing. Slowly cool the sample to avoid the generation of bubbles. If little bubbles still generated during this process, warm solution can be injected to milt the surrounding water molecules, and then completely eliminate the bubbles. The temperature of the cold air can be adjusted to melt back the polycrystalline mass to one seed crystal which may be regrown to fill the entire cooled volume of the sample cell. Reorientation of a crystal whose axis is not quite horizontal is accomplished by the following steps: the melting zone of the crystal is rarely completely parallel to the walls of sample cell in a warm air flow. A slanted interface forms between solid and liquid. As the crystal melts it tends to rotate about a horizontal axis as it floats upward. The rotation can be stopped by abruptly lowering the temperature of the air flow a few degrees below the freezing point. Then reoriented crystal can be regrown to fill the cooled sample cell volume.

3. Results and discussions

Figure 2(a) is FSRS spectra of liquid water and 1 M KOH-H₂O solution. As shown in Fig. 2(a), the Raman shift of the main peak in liquid water is 3390 cm⁻¹, which corresponds to the ordinary OH stretching vibration of water molecules [2,25]. However, two peaks appear approximately at 3350 and 3409 cm⁻¹ in the spectrum of KOH-H₂O solution. The assignment of peak at 3409 cm⁻¹ (of KOH-H₂O solution) is the same as the main peak of pure water. The peak at 3350 cm⁻¹ attributes to OH stretching vibration of water in the liquid phase. According to previous researches, the Raman shift is inversely proportional to the bond energy [26–28]. When the hydrogen bond energy is lower than 15 kJ/ mol, it belongs to weak hydrogen bond [27,28]; Otherwise, it belongs to strong hydrogen bond. Hydrogen bond strength of the peak at 3350 cm⁻¹ (15.5 kJ/mol, strong hydrogen bond) is greater than that of 3390 cm⁻¹ (13 kJ/mol, weak hydrogen bond), as shown in Fig. 3.

 

Fig. 2. SRS spectra of liquid water and 1 M KOH-H₂O solution in (a) forward direction and (b) backward direction.

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Fig. 3. The relationship between OH stretching vibration and hydrogen bonds energy. Purple: pure water. Light blue: KOH-H₂O solution. The horizontal axis is attained from electronic structure calculations. The model used in calculation is Lewis structure mode [29].

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Simultaneously, the BSRS spectra of liquid water and KOH-H₂O solution are also measured under same conditions as shown in Fig. 2(b). The BSRS spectrum of liquid water shows two characteristic peaks which cannot be observed in FSRS. The stronger peak appears at 3359 cm⁻¹. The shoulder peak appears at 3385 cm⁻¹. Meanwhile, there are also two peaks appear in the spectrum of KOH-H₂O solution in backward direction. The Raman shifts of them are 3200 and 3359 cm⁻¹. Hydrogen bonds energies of the peaks at 3200 (22.3 kJ/mol, strong hydrogen bonds) and 3359 cm⁻¹ are respectively higher than that of 3359 and 3385 cm⁻¹ as shown in Fig. 3. The SRS peak intensity of KOH-H₂O solution is also much greater than that of pure water in both forward and backward direction.

These phenomena indicate an obvious improvement in the SRS signal of liquid water with the presence of KOH. In addition, hydrogen bonds network in KOH-H₂O solution is also stronger than that of pure water. We obtain the spontaneous Raman spectrum (at 253 K) and FSRS spectrum of ice Ih at the same time (Fig. 4). These spectra show that the characteristic peak of ice Ih is at around 3200 cm⁻¹. Therefore, the peak at 3200 cm⁻¹ of 1 M KOH-H₂O solution represents the formation of ice Ih-like structure. The appearance of ice-like structure further verifies the idea that hydrogen bonding structure of water molecules is strengthened in KOH-H₂O solution.

 

Fig. 4. (a) Spontaneous Raman spectrum of ice Ih at 253 K; (b) FSRS spectrum of ice Ih.

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Figure 5 is spontaneous Raman spectra of pure water and KOH-H₂O solution in the range from 2800 to 4000 cm⁻¹. Raman shifts and FWHMs of peaks are listed in Table 1. The bands of pure water at 3190 and 3418 cm⁻¹ are assigned to the symmetric and asymmetric O-H stretching vibrations, respectively. Three Raman peaks appear at 3200, 3432 and 3612 cm⁻¹ in the spectra of 1 M KOH-H₂O solution. The assignments of the peaks at 3200 and 3432 cm⁻¹ are the same as the peak at 3190 and 3418 cm⁻¹. The shoulder peak at 3612 cm⁻¹ attributes to free OH vibrational mode. Raman spectra show that the FWHMs (full width at half maximum) of peaks in 1 M KOH-H₂O solution are narrower than that of pure water. As it known to all, the Raman gain coefficient (g_ss) is proportional to the probability of Raman transition $\left( {\frac{{dw{f_i}}}{{d({\hbar w} )}}} \right)$, but it is inversely proportional to the FWHM ($\hbar {\Gamma }$) as shown in Eq. (1) [30].

The g$({\hbar \delta w})$ is joint state density of Raman transition. So, we draw a conclusion that the Raman gain coefficient of water molecules is increasing with the presence of KOH.

 

Fig. 5. Spontaneous Raman spectra of (a) KOH-H₂O solution and (b) pure water. Peak fitting of the spectra was achieved by Origin software.

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Table 1. Comparison of the Raman bands observed in pure water and KOH-H₂O solution.

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In summary, above all experimental results show that SRS signal is enhanced by the dissolution of KOH. Meanwhile, we also performed above experiments on NaOH-H₂O solution and obtained the same experimental results: SRS signal is also enhanced with the presence of NaOH.

We propose that the enhancement mechanism of SRS in water induced by KOH is as follows. It is known that the entire hydrogen bond is defined to be O:H−O [31,32]. The H−O bond represents the intramolecular polar-covalent bond and the O:H bond means the intermolecular van der Waals bond. The H−O bond (∼4.0 eV) is much shorter, stronger, and stiffer than the O:H bond (∼0.1 eV). H atom serves as the point of reference in the O:H−O system. Dissociation of KOH in water yields the monovalent K⁺ and OH^- ions. Each K⁺ and OH^- ion acts as a charge center, generating a radial electrostatic field (E), as shown in Fig. 6(a). The electric field modulates the two segments (H−O and O:H) cooperatively through the repulsion between electron pairs on the adjacent oxygen anions. It softens and elongates the O-H bond and stiffens and shortens the O:H nonbond, which has the same effect as high pressure to pure water [Fig. 6(b)] [30]. However, the softer O:H bond always relaxes more in length than the stiffer H−O bond does in the same direction. So, the hydrogen bonds between water molecules in KOH-H₂O solution are strengthened under the compression of the electric field. At the same time, due to the effect of the electric field, water molecules move towards the OH^- ions and aggregate together [30], resulting in the formation of more hydrogen bonds between the water molecules. So, an ice-like structure appears in this condition. Therefore, we consider that this electrostatic field causes the enhanced SRS of liquid water.

 

Fig. 6. (a) Schematic diagram of KOH dissolved in liquid water; (b) Cooperative relaxation of the segmented O:H-O bond in KOH-H₂O solution. White ball: oxygen atom; red ball: hydrogen atom; blue ball: the nonbonding lone pair; and dotted line: the original position of the water. H atom is the reference origin. This is only a schematic diagram and does not represent the actual molecular size and number of binding molecules.

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It is well known that the polarization of nonlinear light scattering (P_NL) is an expansion starting with a second-order P⁽²⁾ and third-order P⁽³⁾ polarization, followed by higher order terms [33].

SRS is a coherent and strong excitation nonlinear process: initially, excited photons and Stokes photons (${w_s}$) are generated by the collision of pump light incident photons (${w_p}$) and thermal vibrational phonons (${w_v} = {w_p} - {w_s}$). Then the incident photons collide with the excited phonons to produce Stokes photons and more excited phonons. Finally, an avalanche process is formed, as shown in Fig. 7 [34]. We only need to consider third-order P⁽³⁾ polarization at SRS process.

 

Fig. 7. Schematic diagram of stimulated Raman scattering.

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Where ${\varepsilon _0}$ and ${\chi ^{(3 )}}$ are the dielectric constant and third-order Raman susceptibility, respectively. Adding an electrostatic field (${E_{dc}}$) to water (${E_{dc}}$ of pure water is 0) does not influence the Stokes (${w_s}$) and thermal vibrational (${w_v}$) frequencies. Previous research shows that third-order susceptibility is proportional to the potential difference generated by the electrostatic field. So, the electrostatic field (generated by the dissolution of KOH in water) can enhance the third-order nonlinear Raman susceptibility ${\chi ^{(3 )}}$ of water molecules.

Raman gain coefficient (${G_{ss}}$) is proportional to ${\chi ^{(3 )}}$ [Eq. (4)]. Therefore, SRS signals are greatly enhanced by dissolving a small amount of KOH.

4. Conclusion

We investigate water and KOH-H₂O solution by SRS in both forward and backward directions. It is found that the hydrogen bonds in KOH-H₂O solution is stronger than that in pure water at same conditions. Importantly, we observe an ice Ih-like structure in KOH-H₂O solution. Moreover, the normalized SRS intensity of KOH-H₂O solution is also higher than that of liquid water. These phenomena indicate that KOH can enhance the stimulated Raman scattering of water molecules. The enhancement mechanism is attributed to the effect of electric field induced by KOH. This electric field not only strengthens the network structure in the water, but also increases the Raman gain coefficient, leading to the enhancement stimulated Raman scattering of water molecules. Our experimental results not only shed light on the interaction between alkali and water molecules, but also provide a simple and convenient method for enhancing stimulated Raman scattering of water molecules.