4B16
Ultrafast Vibrational Dynamics of Water at a Charged Interface
Revealed by Two Dimensional Heterodyne-Detected Vibrational
Sum-Frequency Generation (2D HD-VSFG) Spectroscopy(Molecular Spectroscopy Laboratory, RIKEN) Prashant Chandra Singh, Satoshi
Nihonyanagi, Shoichi Yamaguchi, Tahei Tahara[Introduction]
The water dynamics in the bulk has been intensively studied by various time-resolved(TR-) spectroscopies, and the ultrafast dynamics relating to the spectral diffusion,
inhomogeneity, energy transfer etc. have been observed. Especially, two-dimensional
infrared (2D-IR) spectroscopy is a powerful tool and it has been playing a key role.1 On
the other hand, the dynamics of water at interfaces is much less understood because aninterface-selective spectroscopy is needed to probe only interfacial water in the presenceof bulk water behind. The 2nd-order nonlinear spectroscopy, in particular vibrational
sum-frequency generation (VSFG), has intrinsic interface specificity and has been
widely utilized to study steady-state properties of water at various interfaces. Recently,
TR-VSFG measurements (i.e., pump-probe and 2D-VSFG) have been realized ataqueous interfaces. 2 Since heterodyne detection (HD-) enables us to directly measure(2) spectra, it is very crucial to realize heterodyne detection in TR-VSFG measurements
to elucidate true vibrational dynamics of interfacial water.3 Here, we report theextension of our TR-HD-VSFG method to two-dimensional spectroscopy, whichenabled us to observe 2D-HD-VSFG spectra at an aqueous interface for the first time.
[Experiamental Setup]
Figure 1 shows the TR-HD-VSFG setup which isthe combination of IR pump and HD-VSFG
probe. A narrow band visible (1) and broadband
IR (2) beams are used for the HD-VSFG probe.
In the TR-HD-VSFG setup, a pump IR pulse

Figure 1: The optical Scheme of(pump) was obtained by splitting ca. 80% of the energy
the 2D HD-VSFG setup.
from the IR output of the different frequency generator.

The pump pulse was passed through a band pass filter to narrow the bandwidth down to~100 cm-1. In order to make 2D HD-VSFG, the pump dependence of Im(2) wasmeasured for the five different pump whose center wavenumber was at 3300, 3340,3400, 3475, and 3500 cm-1. The SF, 1, 2, pump beams were s-, s-, p-, and p-polarized,respectively.

[Results and Discussion]
Figure 2 shows the 2D
HD-VSFG spectra in theOH stretch region of HOD
in D2O at an interface
positivelychargedbycetyltrimethylammonium
bromide (CTAB) measured
at the delay times of 0, 100,and 300 fs after IR
photoexcitation. In the2D HD-VSFG spectra, thehorizontal and vertical axes

Figure 2. 2D HD-VSFG spectra of the OH stretch of HOD in
D2O at the CTAB interface for 0, 100, and 300 fs delay times
after photoexcitation. The concentration of CTAB is 0.5 mM
and water isotope ratio is H2O: HOD: D2O = 1:8:16. In the 2D

represent pump and probe

HD-VSFG spectra red color represent bleaching of the 0–1

wavenumbers, respectively.

transition, whereas negative peaks blue color indicate the hot

The red lobe of the spectra

band due to the 1–2 transition. The color scales are shown atrepresents positive Im(2)the top of the figures.

associated with ground-state bleaching of the 0–1 transition, and the blue represents
negative Im(2) associated with the 1–2 transition. The 2D HD-VSFG spectra clearly
represent the overall feature of ultrafast vibrational dynamics in the OH stretch region at
the charged water interface. In fact, the spectrum at 0 fs is diagonally elongated, whichindicates the memory of the pump wavenumber and the initial inhomogeneous
frequency distribution on the 0–1 and 1–2 transitions. At 100 fs, the elongation becomesless prominent, because the initial frequency distribution is getting randomized byspectral diffusion. At 300 fs, the elongation almost disappears, and the slope of the node
separating the 0–1 and 1–2 transitions becomes nearly parallel with the horizontal axis.
This demonstrates that the memory of the pump wavenumber is nearly lost and thespectral diffusion is almost completed at 300 fs. Inhomogeneity and spectral diffusion ofwater at the charged interface is clearly exhibited by the 2D HD-VSFG technique,which shows the novelty of this technique.
[References]
1. E. T. J. Nibbering, and T. Elsaesser, Chem. Rev. 104, 1887 (2004).
2. J. A. McGuire, and Y. R. Shen, Science 313, 1945 (2006).
3. S. Nihonyanagi, P. C. Singh, S. Yamaguchi, and T. Tahara, Bull. Chem. Soc. Jpn. 85,
758 (2012).