THE JOURNAL OF CHEMICAL PHYSICS 139, 161101 (2013)

Communication: Ultrafast vibrational dynamics of hydrogen bond network terminated at the air/water interface: A two-dimensional heterodyne-detected vibrational sum frequency generation study Prashant Chandra Singh,1,a),b) Satoshi Nihonyanagi (ddddd),1,2,a) Shoichi Yamaguchi (dddd),1,a) and Tahei Tahara (dddd)1,2,c) 1

Molecular Spectroscopy Laboratory, RIKEN, Wako, Saitama 351-0198, Japan Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), Wako, Saitama 351-0198, Japan 2

(Received 23 August 2013; accepted 3 October 2013; published online 22 October 2013) Ultrafast vibrational dynamics of hydrogen bond network at the air/water interface is revealed by two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy. Three diagonal peaks are clearly observed in the 2D HD-VSFG spectrum, which correspond to the negative and positive hydrogen-bonded OH stretch bands and the positive “free” OH stretch band in the steady-state HD-VSFG spectrum. A diagonally elongated bleaching lobe of the hydrogen-bonded OH at 0 ps indicates that it is partly inhomogeneously broadened. This diagonal elongation vanishes in a few hundred femtoseconds, implying the ultrafast spectral diffusion in the hydrogen-bonded OH band. Off-diagonal cross peaks between each OH oscillator are clearly observed instantaneously within the time resolution of 0.2 ps, suggesting that they are vibrationally coupled through anharmonicity and/or energy transfer. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4826095] The uniqueness of water originates from its ability to form a three-dimensional hydrogen bond network. To understand the microscopic origin of the unique properties of water, it is crucial to clarify the structure and dynamics of the hydrogen bond network. Vibrational spectroscopy is a direct probe of the structure and dynamics of the hydrogen bond network because the OH stretch frequency is very sensitive to the hydrogen bond strength.1 Steady-state IR spectroscopy provides the time-averaged vibrational absorption that indicates the static properties such as the average strength of the hydrogen bond. Femtosecond time-resolved IR and twodimensional infrared (2D IR) spectroscopies reveal the time evolution of vibrational excitations in the hydrogen bond network, which elucidate the ultrafast dynamics of bulk water.1–5 At the water surface (i.e., the air/water interface), the hydrogen bond network is sharply terminated, which renders the surface more heterogeneous than bulk water. Because of its surface sensitivity, vibrational sum frequency generation (VSFG) has been intensively utilized for surface characterization.6, 7 An early VSFG study showed the existence of “free” OH (i.e., dangling OH) at the air/water interface, which is the clear evidence of the hydrogen bond network truncated at the water surface.8 Recently, heterodyne-detected vibrational sum frequency generation (HD-VSFG) revealed that the negative and positive bands appear in the hydrogen-bonded OH stretch region, indicating a) P. Chandra Singh, S. Nihonyanagi, and S. Yamaguchi contributed equally

to this work.

b) Present address: Indian Association for the Cultivation of Science,

Jadavpur, Kolkata 700032, India.

c) Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0021-9606/2013/139(16)/161101/4/$30.00

the existence of two distinct OH oscillators.9, 10 The negative band is assignable to the hydrogen-bonded OH group of “liquid-like” interfacial water that has hydrogen-down orientation, whereas our group assigned the low-frequency positive band to the strongly hydrogen-bonded pair at the surface by rigorous comparison between experimental data and molecular dynamics simulation.10 These HD-VSFG studies have revealed that the three distinct OH oscillators are present at the air/water interface. It is of particular interest to elucidate vibrational dynamics at the water surface, especially in terms of the interactions between the three OH oscillators at the air/water interface. Studies of the ultrafast dynamics of the hydrogen bond network at water interfaces have been started.11–15 In particular, Bonn and co-workers carried out two-dimensional (2D) VSFG spectroscopy at the surface of heavy water (D2 O).16 So far, however, the time-resolved and 2D VSFG experiments at the air/water interfaces have been performed with the conventional intensity-detection scheme that can only provide |χ (2) |2 (χ (2) : second-order nonlinear optical susceptibility). As already shown for steady-state measurements, spectral distortion in the |χ (2) |2 spectra very often causes wrong interpretation, and hence obtaining imaginary χ (2) (Imχ (2) ) spectra with heterodyne detection is crucial.10, 17–19 Thus, it is very important to carry out time-resolved VSFG measurements with heterodyne detection.14, 15 Very recently, Skinner and co-workers theoretically calculated 2D HD-VSFG spectra of OD oscillators of HOD in H2 O at the air/water interface,20 demonstrating the importance of heterodyne detection. Very recently, 2D HD-VSFG spectroscopy has been experimentally realized by our group15 and the Zanni group21 independently. In particular, we reported 2D HD-VSFG

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FIG. 1. (a) Steady-state Imχ (2) spectrum of the air/water (H2 O) interface. (b)–(g) 2D HD-VSFG spectra of the air/water (H2 O) surface at 0.0–2.0 ps after photoexcitation. The color scale of Imχ (2) is shown in the left bottom. Imχ (2) is normalized in each 2D spectrum. The 2D spectra are plotted with evenly spaced contours. Dashed straight lines are the diagonal line. Red solid straight lines (1, 2, 3) in (b) are the anti-diagonal lines. Black solid markers stand for peak ω2 wavenumbers of vertical cuts at each ωpump wavenumber. Black solid straight lines are fits for the black solid markers.

measurements of interfacial water for the first time, which requires high sensitivity and high phase stability. Here we report the first 2D HD-VSFG study at the air/water interface. We present 2D HD-VSFG spectra in the OH stretch region and discuss spectral diffusion and the vibrational coupling between the distinct OH oscillators at the air/water surface. In the present 2D HD-VSFG experiment, visible ω1 (center wavelength: 795 nm, bandwidth: 25 cm−1 , pulse width: 0.5 ps) and infrared ω2 (center wavenumber: 3400 cm−1 , bandwidth: 250 cm−1 , pulse width: 0.1 ps) pulses were focused into a y-cut quartz crystal to generate sum frequency (ω1 +ω2 ) as a local oscillator (LO). The LO pulse passed through a glass plate (2 mm) to be delayed with respect to the ω1 and ω2 pulses by 3.5 ps. The LO, ω1 , and ω2 pulses were focused onto the surface of purified water (H2 O, Millipore, resistivity: 18.2 M cm) where an infrared ωpump pulse (bandwidth: 160 cm−1 , pulse width: 0.2 ps) was also focused for vibrational excitation. The ωpump pulse was set at 3200, 3300, 3400, 3500, 3600, and 3700 cm−1 . The ω1 + ω2 pulse generated at the water surface and the LO pulse were collinearly introduced into a polychromator and detected by a multichannel detector. The steady-state χ (2) spectrum of the water surface was obtained without using the ωpump pulse.17 In the present experimental condition, Imχ (2) spectra provide informative absorptive lineshape. Femtosecond timeresolved Imχ (2) (pump-induced change in Imχ (2) ) spectra were recorded and processed according to a procedure described in our previous paper.15 The ω1 + ω2 , ω1 , ω2 , and ωpump pulses were S-, S-, P-, and P-polarized, respectively. All the measurements were performed at 298 K. Figure 1(a) shows the steady-state Imχ (2) spectrum of the air/water interface. (We use the vertical axis for the ω2 wavenumber and the horizontal axis for Imχ (2) to read-

ily compare it with 2D spectra.) A sharp positive peak at 3700 cm−1 is due to the free OH at the air/water interface.8, 10 A broad negative band around 3450 cm−1 is attributed to the hydrogen-bonded OH pointing downward to bulk water,8, 10 whereas a small positive band around 3200 cm−1 has been assigned to the strongly hydrogen-bonded pair at the topmost surface.10, 22 Figures 1(b)–1(g) show the 2D HD-VSFG spectra of the air/water interface at 0.0–2.0 ps after vibrational excitation by the ωpump pulse. The horizontal and vertical axes represent the ωpump and ω2 wavenumbers, respectively. Red and blue colors in the 2D spectra stand for positive and negative Imχ (2) , respectively. The large red lobe around ωpump = ω2 = 3350–3550 cm−1 is a diagonal peak due to the bleaching of the negative hydrogen-bonded OH stretch band, while the small blue lobe around ωpump = ω2 = 3200–3300 cm−1 is attributable to the bleaching of the positive band due to strongly hydrogen-bonded OH. The negative Imχ (2) at ωpump = ω2 = 3700 cm−1 is due to the bleaching of the 0–1 transition of the free OH stretch. Figure 2(a) shows the diagonal cut of the 2D HD-VSFG spectrum at 0.0 ps along the diagonal line in Figure 1(b) (black dashed line). This cut clearly shows three diagonal peaks around 3200–3300 (negative), 3400–3500 (positive), and 3700 cm−1 (negative), which assures that each of the three OH oscillators at the air/water interface is photoexcited distinctly. In 2D spectroscopy, an inhomogeneously broadened band in a steady-state spectrum gives a bleaching lobe elongated along the diagonal line in a 2D spectrum just after photoexcitation, reflecting the memory of photoexcitation wavenumber. For a homogeneously broadened band, a 2D spectrum shows no diagonal elongation. Incomplete diagonal elongation implies a broadening mechanism in the

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FIG. 2. (a) Diagonal cut of the 2D HD-VSFG spectrum at 0.0 ps along the diagonal line. (b)–(d) Anti-diagonal cuts of the 2D HD-VSFG spectrum at 0.0 ps along the anti-diagonal lines (1, 2, 3) in Figure 1(b). Vertical thin lines indicate where ωpump is equal to ω2 .

intermediate modulation regime. As seen in Figure 1(b), the positive lobe in the 2D HD-VSFG spectrum at 0.0 ps shows very small but noticeable diagonal elongation for the 3300– 3600 cm−1 ω2 region. This diagonal elongation disappears in a few hundred femtoseconds owing to spectral diffusion. At 2.0 ps, the 2D HD-VSFG spectrum is dominated by a feature due to transient temperature increase that causes a high-wavenumber shift of the OH stretch band.14 The diagonal elongation at 0.0 ps can be more readily recognized by comparing the diagonal cut with the anti-diagonal cut passing through the diagonal line at 3450 cm−1 (line 2 in Figure 1(b)). The diagonal peak around ωpump = ω2 = 3400– 3500 cm−1 is substantially narrower in the anti-diagonal cut (Figure 2(c)) than that in the diagonal cut (Figure 2(a)). This bandwidth difference is a manifestation of the diagonal elongation of the bleaching of the negative hydrogen-bonded OH band. The bandwidth ratio of the diagonal peak in the diagonal cut to that in the anti-diagonal cut is plotted against time delay in Figure 3. This ratio decays from 1.29 at 0.0 ps to 1.07 at 0.8 ps. The diagonal elongation can also be evaluated from the slope of black straight lines drawn in the 2D HD-VSFG spectra (Figures 1(b)–1(g)) in the same manner as performed in 2D IR spectroscopy.23 These black lines are fits for black solid markers that indicate the peak ω2 wavenumbers of vertical cuts at each ωpump wavenumber. The slope decays from 0.067 at 0.0 ps to −0.001 at 0.8 ps (Figure 3), which accords well with the temporal change of the bandwidth ratio. The temporal change of these two quantities shown in Figure 3 reveals that spectral diffusion in the hydrogen-bonded OH stretch at the air/water interface proceeds in a few hundred femtoseconds. This timescale is faster than the spectral diffusion of the OD stretch at the D2 O surface that was reported in the conventional 2D VSFG study.16 The 2D HD-VSFG spectrum at 0.0 ps shows not only diagonal peaks but also off-diagonal peaks. These off-diagonal peaks provide information about the vibrational coupling among the three OH stretch oscillators at the air/water inter-

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Time Delay (ps) FIG. 3. Temporal changes of the slope of the black solid lines in the 2D HD-VSFG spectra in Figure 1(b)–1(e) (black markers) and the ratio of the bandwidth of the diagonal peak in the diagonal cut to that in the anti-diagonal cut (red markers).

face. We discuss them one by one using anti-diagonal cuts along relevant lines in the following. Figure 2(b) depicts the anti-diagonal cut along the line 1 in Figure 1(b). This anti-diagonal cut exhibits a sharp negative peak at (ωpump , ω2 ) = (3450 cm−1 , 3700 cm−1 ) that indicates the depletion of the free OH stretch induced by the excitation of the negative hydrogen-bonded OH stretch band. Instantaneous appearance of this off-diagonal peak manifests vibrational coupling between the hydrogen-bonded OH and the free OH.24 We can think of the following two possible coupling mechanisms: One is inter- or intra-molecular vibrational energy transfer to populate the vibrationally excited state of the free OH stretch. The other is the anharmonic coupling that makes the free OH stretch frequency shifted with the vibrational excitation of the hydrogen-bonded OH stretch. We note that the instantaneous appearance of the cross peak between the free OD stretch and the high-frequency hydrogenbonded OD stretch was recognized also in the previous conventional 2D VSFG experiment.16 The anti-diagonal cut also shows a positive peak around (ωpump , ω2 ) = (3700 cm−1 , 3450 cm−1 ). This peak is attributable to the depletion of the negative hydrogen-bonded OH band induced by the excitation of the free OH stretch, which also arises from coupling between the two OH stretch oscillators. For this peak, we think that anharmonic coupling is the predominant mechanism because the intrinsic bandwidth of the free OH stretch is as narrow as 10 cm−1 .25 The 10-cm−1 bandwidth approximately corresponds to the dephasing time of 1 ps. It is unlikely that this cross peak appears by energy transfer from the free OH to the hydrogen-bonded OH, because the energy transfer within the time resolution of 0.2 ps must dephase the vibrational coherence of the free OH stretch simultaneously. The anti-diagonal cut along the line 2 (Figure 2(c)) shows two negative peaks at (ωpump , ω2 ) = (3200 cm−1 , 3700 cm−1 ) and (3700 cm−1 , 3200 cm−1 ). They correspond to the depletion of the free OH stretch induced by the excitation of the low-frequency positive band, and the depletion of the lowfrequency positive band induced by the excitation of the free OH stretch, respectively. It is noteworthy that the cross peak between the free OD and the low-frequency hydrogen-bonded OD is not recognized in the conventional 2D VSFG spectrum of the D2 O surface at 0 fs.16 These peaks indicate coupling

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between the two OH oscillators, which may be explainable in the same manner as the peak at (ωpump , ω2 ) = (3450 cm−1 , 3700 cm−1 ). As for the peak around (3700 cm−1 , 3200 cm−1 ), however, we also need to take account of a contribution of the anharmonically redshifted 1–2 transition (so-called “hot band”) of the negative OH stretch band. Actually, the corresponding negative off-diagonal signal around (ωpump , ω2 ) = (3700 cm−1 , 3200 cm−1 ) rapidly disappears in a few hundred femtoseconds, as shown in Figures 1(b)–1(g). We note that this rapid decay is compatible with the faster decay of the 1–2 absorption than 0–1 bleaching which has been observed in 2D IR spectroscopy of bulk water.2 Because of the possible significant contribution of the hot band, it is safe to ascribe the off-diagonal signal around (ωpump , ω2 ) = (3700 cm−1 , 3200 cm−1 ) not only to the vibrational coupling but also to the hot band. The anti-diagonal cut along the line 3 (Figure 2(d)) shows a positive peak around (ωpump , ω2 ) = (3200 cm−1 , 3450 cm−1 ) and negative peak around (ωpump , ω2 ) = (3450 cm−1 , 3200 cm−1 ). This discloses the coupling between the two hydrogen-bonded OH oscillators, although the contribution of the hot band needs to be taken into account for the peak at (ωpump , ω2 ) = (3450 cm−1 , 3200 cm−1 ). The above-described analysis of the cross peaks in 2D HD-VSFG spectra at 0 ps reveals that the three OH oscillators are vibrationally coupled with each other, and any delayed energy flow and/or chemical exchange is not recognized with the present time resolution of 0.2 ps. Skinner and coworkers theoretically predicted that the cross peaks between the isolated OD oscillators at the interface gradually appear on the timescale of a few picoseconds,20 owing to the hydrogenbond rearrangement at the interface. This difference is highly likely due to the difference in the system, i.e., HOD in D2 O and neat H2 O, because it is well known that vibrational dynamics in neat water is much faster than that in isotopically diluted water due to the intra- and inter-molecular vibrational couplings. To address this issue, 2D HD-VSFG of the isotopically diluted air/water interface is highly desirable, although it is experimentally very challenging because of much smaller signal than the neat H2 O interface. In summary, we report the 2D HD-VSFG spectra of the air/water interface for the first time. The 2D HD-VSFG spectra provide full information about Imχ (2) , and they are direct interfacial counterparts to 2D IR spectra in the bulk. The obtained data show that the hydrogen-bonded OH stretch band is partly broadened by inhomogeneity, and that the spectral diffusion occurs in a few hundred femtoseconds. Moreover, the 2D spectra show that the three distinct OH oscillators at the air/water interface are vibrationally coupled

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with each other. Compared with 2D IR, which has already grown into an established method for studying ultrafast vibrational dynamics of bulk water, 2D HD-VSFG in the present study still has room for improvement in terms of the time and frequency resolutions. 2D HD-VSFG will be improved to the level of 2D IR in the near future, and in combination with complementary computations it will enable us to obtain complete understanding of the ultrafast vibrational dynamics of the water interface. This work was supported by a grant-in-aid for Scientific Research (A) (No. 24245006) and (B) (No. 25288014) from Japan Society for the Promotion of Science (JSPS). 1 E.

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Communication: Ultrafast vibrational dynamics of hydrogen bond network terminated at the air∕water interface: a two-dimensional heterodyne-detected vibrational sum frequency generation study.

Ultrafast vibrational dynamics of hydrogen bond network at the air∕water interface is revealed by two-dimensional heterodyne-detected vibrational sum ...
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