Ken Ichi Inoue

Bio: Ken Ichi Inoue is an academic researcher from Tohoku University. The author has contributed to research in topics: Sum frequency generation spectroscopy & Sum-frequency generation. The author has an hindex of 12, co-authored 28 publications receiving 527 citations. Previous affiliations of Ken Ichi Inoue include Kyoto University.

Accurate determination of complex χ(2) spectrum of the air/water interface

Abstract: Discussion on the structure of the water surface relies on accurate determination of the χ(2) spectrum. For obtaining accurate χ(2) spectrum of the air/water interface in the OH stretch region, we performed heterodyne-detected vibrational sum-frequency generation measurements with a high phase accuracy, and also examined the validity of the phase and amplitude calibration using different non-resonant materials. In contrast to the previous reports, it was concluded that the imaginary part of the χ(2) spectrum of the air/water interface does not exhibit noticeable positive resonance in the low frequency region within the experimental error. This result urges us to reconsider the structure of the air/water interface based on the accurate χ(2) spectrum.

2D heterodyne-detected sum frequency generation study on the ultrafast vibrational dynamics of H2O and HOD water at charged interfaces.

Abstract: Two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy is applied to study the ultrafast vibrational dynamics of water at positively charged aqueous interfaces, and 2D HD-VSFG spectra of cetyltrimethylammonium bromide (CTAB)/water interfaces in the whole hydrogen-bonded OH stretch region (3000 cm−1 ≤ ωpump ≤ 3600 cm−1) are measured. 2D HD-VSFG spectrum of the CTAB/isotopically diluted water (HOD-D2O) interface exhibits a diagonally elongated bleaching lobe immediately after excitation, which becomes round with a time constant of ∼0.3 ps due to spectral diffusion. In contrast, 2D HD-VSFG spectrum of the CTAB/H2O interface at 0.0 ps clearly shows two diagonal peaks and their cross peaks in the bleaching region, corresponding to the double peaks observed at 3230 cm−1 and 3420 cm−1 in the steady-state HD-VSFG spectrum. Horizontal slices of the 2D spectrum show that the relative intensity of the two peaks of the bleaching at the CTAB/H2O interface gradually change wi...

Femtosecond Hydrogen Bond Dynamics of Bulk-like and Bound Water at Positively and Negatively Charged Lipid Interfaces Revealed by 2D HD-VSFG Spectroscopy.

Abstract: Interfacial water in the vicinity of lipids plays an important role in many biological processes, such as drug delivery, ion transportation, and lipid fusion. Hence, molecular-level elucidation of the properties of water at lipid interfaces is of the utmost importance. We report the two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) study of the OH stretch of HOD at charged lipid interfaces, which shows that the hydrogen bond dynamics of interfacial water differ drastically, depending on the lipids. The data indicate that the spectral diffusion of the OH stretch at a positively charged lipid interface is dominated by the ultrafast (<∼100 fs) component, followed by the minor sub-picosecond slow dynamics, while the dynamics at a negatively charged lipid interface exhibit sub-picosecond dynamics almost exclusively, implying that fast hydrogen bond fluctuation is prohibited. These results reveal that the ultrafast hydrogen bond dynamics at the positively charged lipid-water interface are attributable to the bulk-like property of interfacial water, whereas the slow dynamics at the negatively charged lipid interface are due to bound water, which is hydrogen-bonded to the hydrophilic head group.

Bend Vibration of Surface Water Investigated by Heterodyne-Detected Sum Frequency Generation and Theoretical Study: Dominant Role of Quadrupole.

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.

Efficient Spectral Diffusion at the Air/Water Interface Revealed by Femtosecond Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract: Femtosecond vibrational dynamics at the air/water interface is investigated by time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy and molecular dynamics (MD) simulation. The low- and high-frequency sides of the hydrogen-bonded (HB) OH stretch band at the interface are selectively excited with special attention to the bandwidth and energy of the pump pulses. Narrow bleach is observed immediately after excitation of the high-frequency side of the HB OH band at ∼3500 cm–1, compared to the broad bleach observed with excitation of the low-frequency side at ∼3300 cm–1. However, the time-resolved spectra observed with the two different excitations become very similar at 0.5 ps and almost indistinguishable by 1.0 ps. This reveals that efficient spectral diffusion occurs regardless of the difference of the pump frequency. The experimental observations are well-reproduced by complementary MD simulation. There is no experimental and theoretical evidence that supports ext...

Vibrational Spectroscopy and Dynamics of Water

Abstract: We present an overview of recent static and time-resolved vibrational spectroscopic studies of liquid water from ambient conditions to the supercooled state, as well as of crystalline and amorphous ice forms. The structure and dynamics of the complex hydrogen-bond network formed by water molecules in the bulk and interphases are discussed, as well as the dissipation mechanism of vibrational energy throughout this network. A broad range of water investigations are addressed, from conventional infrared and Raman spectroscopy to femtosecond pump–probe, photon-echo, optical Kerr effect, sum-frequency generation, and two-dimensional infrared spectroscopic studies. Additionally, we discuss novel approaches, such as two-dimensional sum-frequency generation, three-dimensional infrared, and two-dimensional Raman terahertz spectroscopy. By comparison of the complementary aspects probed by various linear and nonlinear spectroscopic techniques, a coherent picture of water dynamics and energetics emerges. Furthermore,...

Structure and Dynamics of Interfacial Water Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation

Abstract: Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility [χ(2)]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ(2) [Imχ(2)] obtained with this interferometric technique is the direct counterpart to the infrared [Imχ(1)] and Raman [Imχ(3)] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ(2)|2] spectra. The Imχ(2) signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation...

Second-order spectral lineshapes from charged interfaces

Abstract: Second-order nonlinear spectroscopy has proven to be a powerful tool in elucidating key chemical and structural characteristics at a variety of interfaces. However, the presence of interfacial potentials may lead to complications regarding the interpretation of second harmonic and vibrational sum frequency generation responses from charged interfaces due to mixing of absorptive and dispersive contributions. Here, we examine by means of mathematical modeling how this interaction influences second-order spectral lineshapes. We discuss our findings in the context of reported nonlinear optical spectra obtained from charged water/air and solid/liquid interfaces and demonstrate the importance of accounting for the interfacial potential-dependent χ (3) term in interpreting lineshapes when seeking molecular information from charged interfaces using second-order spectroscopy.

Real-Time Observation of Surface Bond Breaking with an X-ray Laser

Abstract: We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells—chemisorbed and precursor state separated by an entropy barrier—explain the anomalously high prefactors often observed in desorption of molecules from metals.