4P067
Study of the Water Bending Mode at the Air/Water Interface by Heterodyne-DetectedVibrational Sum Frequency Generation(Mol. Spectrosc. Lab., RIKEN) KUNDU, Achintya; NIHONYANAGI, Satoshi; YAMAGUCHI,Shoichi; TAHARA, Tahei

Introduction
The air/water interface is important for many processes occurring in the atmosphere. To
understand these processes, it is important to know the structure and dynamics of water at theair/water interface. Water structure at the air/water interface has been studied extensively bymeasuring the OH stretch vibration of water at the air/water interface.1 Since the OH stretch
frequency is close to the frequency of the overtone of the HOH bend, it is important to study theHOH bend in order to understand the vibrational energy relaxation pathway of the OH stretch.
Vibrational spectra of the bending mode of H2O at the air/ H2O interface was obtained previously by
conventional VSFG2,3 and theoretical calculation.3 Here we report for the first time an experimentallyobtained heterodyne-detected vibrational sum frequency generation spectrum (HD-VSFG) of theair/water interface in the bend region.

ExperimentalIn our HD-VSFG setup, a narrow-band ω1 pulse (795 nm) and a broad-band ω2 pulse (1500cm-1 ~ 1900 cm-1) were focused onto the sample interface. When the ω1 and ω2 pulses weretemporally and spatially overlapped, the SF (ω1 + ω2) was generated at the sample interface. The ω1,
ω2, and SF pulses were again focused by a concave mirror onto a GaAs(110) surface to generate SF
once more which acts as local oscillator (LO). The SF from the sample passes through a 2 mm thicksilica plate located in between the sample and the concave mirror, which delays the SF pulse relative
to the ω1 and ω2 pulses. This delay resulted in the time difference between the SF from the sample
and that from GaAs. The two SF beams are introduced together into a polychromator and thendetected by charge coupled device (CCD).
Results and Discussion
The

spectra of the air/water interface in the water bend region were compared in theFigure 1a. Blue line in Figure 1a is ourspectrum of the air/water interface in the bend region
calculated from the complexspectrum. Black and green curves were taken from papers by
Nagata et al.3 and Vinaykin et al.2, respectively, which were obtained by conventional VSFG
measurements. The overall shape of ourspectrum resembles thespectra reported bythem.
Figure 1b shows the HD-VSFG spectrum of H2O at the air/H2O interface. The Im[χ(2)]spectrum of H2O in the bend region has an resonance absorption peak at ~1660 cm-1 and the Re[χ(2)]spectrum shows a dispersive band shape with the nonresonant back ground. The peak frequency inthe Im[χ(2)] spectrum is ~30 cm-1 lower than that obtained by conventional VSFG measurement. The
resonant amplitude is smaller than the nonresonant background in the bend region and hence thespectrum is significantly distorted by the nonresonant background.

1.2(2) 2| | (arb. units)

1.0

SSP

(a)

0.8
0.6

Nagata et al.
Vinaykin et al.
Our

0.4
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

(2)

(arb. units)

0.0
0.5

(b)

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Imag
Real

-1.0
-1.5
1550

1600

1650

1700

1750

1800

-1

Wavenumber (cm )Figure 1. (a) Conventional VSFG spectra of H2O at the air/H2O interface. (b) HD-VSFG spectra ofH2O at the air/H2O interface. Red and black lines represent the Im[χ(2)] and Re[χ(2)] spectra,
respectively.
Nagata et al. reported that the calculated Im[χ(2)] spectrum of the bend mode at the air/ H2Ointerface has a negative peak at ~1650 cm-1 and a positive peak at ~1730 cm-1. The Im[χ(2)] spectrum
reported here is different from their calculated Im[χ(2)] spectrum. Our Im[χ(2)] spectrum shows only
one positive peak at ~1660 cm-1. However, we note that, in our experimental configuration,correction of the complex reflection coefficient of the ω2 pulse at the air/water interface is necessary.
This correction is underway.

References(1) Nihonyanagi, S.; Ishiyama, T.; Lee, T.-k.; Yamaguchi, S.; Bonn, M.; Morita, A.; Tahara, T. J. Am.
Chem. Soc. 2011, 133, 16875. (2) Vinaykin, M.; Benderskii, A. V. J. Phys. Chem. Lett. 2012, 3,3348. (3) Nagata, Y.; Hsieh, C.-S.; Hasegawa, T.; Voll, J.; Backus, E. H. G.; Bonn, M. J. Phys. Chem.
Lett. 2013, 4, 1872.