Letter pubs.acs.org/JPCL
Bend Vibration of Surface Water Investigated by HeterodyneDetected Sum Frequency Generation and Theoretical Study: Dominant Role of Quadrupole Achintya Kundu,†,⊥ Shogo Tanaka,§,⊥ Tatsuya Ishiyama,§,# Mohammed Ahmed,† Ken-ichi Inoue,† Satoshi Nihonyanagi,†,‡ Hiromi Sawai,§ Shoichi Yamaguchi,†,∇ Akihiro Morita,*,§,∥ and Tahei Tahara*,†,‡ †
Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan ∥ Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan ‡
S Supporting Information *
ABSTRACT: Heterodyne-detected vibrational sum frequency generation spectroscopy was applied to the water surface for measuring the imaginary part of second-order nonlinear susceptibility (Im χ(2)) spectrum in the bend frequency region for the first time. The observed Im χ(2) spectrum shows an overall positive band around 1650 cm−1, contradicting former theoretical predictions. We further found that the Im χ(2) spectrum of NaI aqueous solution exhibits an even larger positive band, which is apparently contrary to the flip-flop orientation of surface water. These unexpected observations are elucidated by calculating quadrupole contributions beyond the conventional dipole approximation. It is indicated that the Im χ(2) spectrum in the bend region has a large quadrupole contribution from the bulk water.
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applications of heterodyne-detected VSFG (HD-VSFG) techniques greatly improved our understanding of structure10,14−21 and dynamics22−27 of aqueous interfaces. Although most of prior VSFG studies of water have been focused on the OH stretch vibration due to its large transition dipole moment, observing the bend mode of water could provide complementary information to that obtained from the stretch mode. Because each water molecule has one HOH bend mode, in contrast to two coupled OH stretching modes, the transition dipole of the bend mode is uniquely related to the molecular orientation. The bend mode has smaller transition moment, and thus it is much less influenced by intramolecular coupling than the stretching vibrations.28,29 Therefore, the bend mode is expected to provide a clear clue to the molecular orientation of water at interfaces. However, the bend mode of water has been much less explored than the stretch mode in previous VSFG studies due to its weak intensity, and the understanding on the nature of the bend mode remains to be established. Vinaykin and Benderskii first reported the VSFG spectra of water in the bend frequency region at the air/water interface by
he air/water interface has been drawing great interest in many fields of science and engineering,1−3 including atmospheric and geosciences and mechanical engineering, etc. To understand the fundamental processes occurring at the water interface, it is important to know the structure and dynamics of water at the air/water interface. The water structure at the air/water interface has been extensively studied by measuring the OH stretching vibration of water using vibrational sum frequency generation (VSFG) spectroscopy.4−6 The frequency of the OH stretching vibration is sensitive to the hydrogen bond, and the surface-selective VSFG measurement provides valuable information on the hydrogen bonding network of surface water. VSFG is based on the second-order nonlinear optical processes which are forbidden in a bulk of centrosymmetric medium under the electric dipole approximation. This makes VSFG a powerful interface-selective spectroscopic tool. The conventional VSFG measurements provide the absolute square of second-order nonlinear susceptibility (|χ(2)|2), while heterodyne-detected and phase-sensitive VSFG spectroscopies7−12 directly provide the imaginary (Im χ(2)) and real (Re χ(2)) parts of a complex χ(2) spectrum. The sign of Im χ(2) gives decisive information about up/down orientation of interfacial molecules, and the line shape in Im χ(2) directly reflects the vibrational resonances.13 The recent development and © XXXX American Chemical Society
Received: March 23, 2016 Accepted: June 20, 2016
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DOI: 10.1021/acs.jpclett.6b00657 J. Phys. Chem. Lett. 2016, 7, 2597−2601
Letter
The Journal of Physical Chemistry Letters using conventional VSFG spectroscopy.28 Isaienko et al. approached the water bend mode at a buried silica/water interface via a combination band in the near-infrared region.30 Recently computational simulations of the bend mode of water were also performed. Nagata et al.31 reported a calculated and experimental |χ(2)|2 spectrum of water in the bend frequency region at the air/water interface, which is essentially consistent with the previous report by Vinaykin and Benderskii. Nagata et al. also predicted the Im χ(2) spectrum in the bend frequency region consisting of a negative peak at ∼1650 cm−1 and a positive peak at ∼1730 cm−1.31 Ni and Skinner29 also reported the theoretical calculations of |χ(2)|2 and Im χ(2) in the bend mode, which are similar to those of Nagata et al. However, Im χ(2) spectrum of water in the bend frequency region at any interface has not been experimentally measured so far, and thus the validity of the calculations has not been verified. Therefore, application of HD-VSFG to the bend frequency region has been desired to clarify the nature of the bend mode for interfacial water. In the present study, we measured the complex χ(2) spectrum in the bend frequency region using HD-VSFG spectroscopy for the first time. The observed Im χ(2) spectrum shows only one positive peak around 1650 cm−1, contradicting the former theoretical predictions. Furthermore, the comparison of the Im χ(2) spectra of neat water and that of NaI aqueous solution reveals that the obtained spectra cannot be rationalized under the simple dipole approximation. The theoretical calculation indicates that the quadrupole contribution is large in the water bend Im χ(2) signal. The optical configuration and Fourier transform analysis of HD-VSFG spectroscopy that we have developed has been described elsewhere.8,32 Briefly, a narrow-band ω1 pulse (795 nm) and a broad-band ω2 pulse (1500 cm−1 ∼ 1900 cm−1) were focused into thin y-cut quartz to generate local oscillator (LO). The transmitted ω1, ω2, and LO pulses are refocused by a concave mirror onto the surface of ultrapure water (Millipore, 18.2 MΩ cm resistivity), which is filled in a clean glass container. The sum frequency (SF, ω3 = ω1 + ω2) generated at the water surface and LO pulses are collinearly introduced into a polychromator and then detected by a charge coupled device. The observed interference spectrum is filtered in the time domain and normalized by the signal obtained from the D2O (Wako, 99.9%, NMR grade) reference surface.12,33 Note that the χ(2) of the D2O surface is a negative real constant (i.e., Im χ(2) is zero) because D2O has no absorption in the ω1, ω2, and ω3 region.33 The absolute values of the amplitude χ(2) spectra were calibrated using χ(2) of quartz (8.0 × 10−13 m V−1).34 The ω3, ω1, and ω2 pulses are S-, S-, and P-polarized, respectively (conventionally known as SSP polarization combination). The height of the water surface was monitored by a displacement sensor and kept constant during the measurements. The HDVSFG measurement was performed at 296 K under nitrogen purging to suppress the ambient water vapor which strongly absorbs the infrared ω2 pulse. The relative humidity under the N2 purge was typically less than 6%. The bandwidth of ω1 is about 25 cm−1 which limits the frequency resolution of the present spectrum. Imaginary and real parts of the χ(2) spectrum of water at the air/water interface in the bend frequency region are shown in Figure 1. The observed Im χ(2) spectrum of water shows a vibrationally resonant peak around 1650 cm−1, and the Re χ(2) spectrum shows a dispersive band shape with a constant negative nonresonant background whose amplitude well
Figure 1. Imaginary (red) and real (black) parts of the experimentally obtained χ(2) spectrum of water at the air/water interface in the bend frequency region. The corresponding magnitude square |χ (2) |2 spectrum is also shown in blue.
exceeds that of the resonant feature. A key finding in our present experiment is that the Im χ(2) spectrum of water in the bend frequency region shows only one positive band with the peak frequency at ∼1650 cm−1. Previously, Nagata et al. reported the theoretically calculated Im χ(2) spectrum of water in the bend frequency region at the air/water interface which exhibits a negative peak at ∼1650 cm−1 and a positive peak at ∼1730 cm−1.31 The direct measurement of the Im χ(2) spectrum reveals that the prediction by this prior MD simulation is incorrect, although it apparently reproduced the |χ(2)|2 spectrum. This further shows that it is very difficult to uniquely predict Im χ(2) spectrum from the corresponding squared spectrum by recovering the missing phase information.16,35,36 We also note that frequency dependence of Fresnel factors is not significant in the present experiment (see Figure S2). The overall spectral shape of our |χ(2)|2 spectrum (Figure 1, bottom) is essentially consistent with the |χ(2)|2 spectra previously reported by Vinaykin and Benderskii28 and Nagata et al.31 However, Vinaykin and Benderskii28 claimed that |χ(2)|2 shows two resonances at 1656 and 1752 cm−1,28 whereas Nagata et al. did not recognize the resonance at 1752 cm−1 in their |χ(2)|2 spectrum of water. In our measurement, it is difficult to discern the presence of a resonance at the 1752 cm−1 because of the limited S/N in our |χ(2)|2 spectrum. As a possible reason for the discrepancy, we note that the spectrum in the bend frequency region can be readily affected by the IR absorption due to ambient water vapor. The sign of Im χ(2) spectra of water is usually considered to reflect up/down orientation of water at the interface.8,13 This rule was exemplified in the OH stretch band of pure water and NaI solution.37 In the OH stretching region between 3200 and 3600 cm−1, the sign of Im χ(2) is entirely negative for pure water, while it is partially (
