Ultrafast Dynamics at Water Interfaces Studied by Vibrational Sum

Apr 5, 2017 - He received his B.S. and M.S. degrees from the Department of Physics, the University of Tokyo, in 1990 and 1992, respectively. He worked...
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Ultrafast Dynamics at Water Interfaces Studied by Vibrational Sum Frequency Generation Spectroscopy Satoshi Nihonyanagi,†,‡ Shoichi Yamaguchi,†,§ and Tahei Tahara*,†,‡ †

Molecular Spectroscopy Laboratory, and ‡Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), RIKEN, Wako, Saitama 351-0198, Japan § Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338-8570, Japan ABSTRACT: We present an overview of studies on the ultrafast dynamics of water at aqueous interfaces carried out by time-resolved vibrational sum frequency generation (VSFG) spectroscopies. This research field has been growing rapidly, stimulated by technical developments achieved recently. In this review, first, the principles and instrumentations are described for conventional VSFG, heterodyne-detected VSFG, and various IR-pump/VSFG-probe techniques, namely, time-resolved conventional VSFG, time-resolved heterodyne-detected VSFG, and their extension to two-dimensional spectroscopy. Second, the applications of these time-resolved VSFG techniques to the study of the femtosecond vibrational dynamics of water at various interfaces are discussed, in the order of silica/water, charged monolayer/water, and the air/water interfaces. These studies demonstrate that there exists water dynamics specific to the interfaces and that time-resolved VSFG spectroscopies can unambiguously detect such unique dynamics in an interface-selective manner. In particular, the most recent time-resolved heterodynedetected VSFG and two-dimensional heterodyne-detected VSFG unveil the inhomogeneity of the hydrogen bond and relevant vibrational dynamics of interfacial water through unambiguous observation of hole-burning in the OH stretch band, as well as the subsequent spectral diffusion in the femtosecond time region. These time-resolved VSFG studies have also left several issues for discussion. We describe not only the obtained conclusive physical insights into interfacial water dynamics but also the points left unclear or controversial. A new type of experiment that utilizes UV excitation is also described briefly. Lastly, the summary and some future perspectives of time-resolved VSFG spectroscopies are given.

CONTENTS 1. Introduction 2. Principle and Instrumentation of VSFG Spectroscopy 2.1. Principle 2.1.1. VSFG and HD-VSFG 2.1.2. TR-VSFG and TR-HD-VSFG 2.2. Instrumentation 2.2.1. TR-VSFG with Homodyne Detection 2.2.2. TR-HD-VSFG 3. Ultrafast Dynamics at Water Interfaces 3.1. Vibrational Dynamics of Water at the Silica/ Water Interface 3.2. Vibrational Dynamics of Water at the Charged Monolayer/Water Interface 3.2.1. Steady-State Spectrum 3.2.2. Dynamics at the Charged Monolayer/ Water Interface Studied by Homodyne TR-VSFG and 2D VSFG 3.2.3. Dynamics at the Charged Aqueous Interface Studied by TR-HD-VSFG and 2D HD-VSFG: Observation of HoleBurning, Spectral Diffusion, and the Effect of Vibrational Coupling © 2017 American Chemical Society

3.2.4. Dynamics at the Charged Aqueous Interface Studied by TR-HD-VSFG and 2D HD-VSFG: Effect of Interaction with Surface Charge on the Water Dynamics 3.3. Vibrational Dynamics of Water at the Air/ Neat Water Interface 3.3.1. Steady-State Spectrum 3.3.2. Dynamics of the Neat Water Surface Studied by Homodyne TR-VSFG and 2D SFG 3.3.3. Dynamics at the Air/Neat Water Interface Studied by TR-HD-VSFG and 2D HD-VSFG 3.4. Reaction Dynamics at Water Interfaces Studied with UV Excitation 4. Conclusion and Perspective Author Information Corresponding Author ORCID Notes

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Special Issue: Ultrafast Processes in Chemistry Received: October 26, 2016 Published: April 5, 2017

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Chemical Reviews Biographies Acknowledgments References

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Vibrational sum frequency generation (VSFG) spectroscopy has intrinsic interface selectivity arising from the principle that the second-order nonlinear optical processes (or even-order nonlinear processes, in general) are forbidden in the medium having inversion symmetry under the dipole approximation.23,24 Therefore, VSFG has very high interface selectivity and only detects molecules in the region where molecules exhibit anisotropy such as specific orientations, which is typically as thin as a few molecular layers at liquid interfaces. It is in sharp contrast to “interface-sensitive” linear spectroscopy, such as attenuated total reflection infrared spectroscopy (ATR-IR), which monitors the molecules in the penetration depth of the evanescent wave of ∼1 μm. In this regard, at the moment, VSFG is the only means to provide true interface selectivity, which is indispensable for obtaining molecular-level information on water interfaces. VSFG has been extensively utilized to investigate water structure at aqueous interfaces.24−29 Moreover, it is possible to perform time-resolved (TR) VSFG measurements to investigate the dynamics of interfacial molecules using the pump−probe scheme: The pump pulse excites the system to start a dynamical process and the VSFG is used to probe the temporal change proceeding at the interface. The first pump/VSFG-probe time-resolved measurement was indeed realized as early as 1990 for adsorbates on clean surfaces under ultrahigh vacuum,30−34 which is soon after the first success of steady-state VSFG measurements at these surfaces. TR-VSFG measurements were extended to ambient interfaces such as air/self-assembled monolayer/glass35 and yttrium aluminum garnet/acetonitrile36 interfaces. However, the weak VSFG signal of the water interface had prevented application of time-resolved measurements to water interfaces for more than a decade after the first report of the steady-state VSFG spectrum of the water interface.37−39 The first TR-VSFG study for the water interface was reported in 2006 by McGuire and Shen, who carried out timeresolved measurements at a silica/water interface using total internal reflection geometry with a silica prism.40 Since then, ultrafast vibrational dynamics of interfacial water has been intensively investigated by TR-VSFG, being promoted by development of new methods in VSFG spectroscopy. So far, vibrational dynamics of interfacial water has been investigated for three water interfaces, i.e., silica/water interface, air/charged monolayer/water interface, and air/water interface. For these water interfaces, IR-pump/VSFG-probe time-resolved measurements have been carried out in the OH stretch region, in which femtosecond IR pulses first excite the OH stretch vibration of water and the change induced at the interface is monitored by TR-VSFG measurements. Single-channel IR-pump/VSFGprobe experiments were performed first, and the vibrational relaxation (T1) time of the OH stretch was evaluated from the temporal change of the VSFG intensity to discuss the difference between the interface and bulk.40 Then, the temporal change of the time-resolved VSFG spectra was measured using broadband IR pulses with multichannel detectors.41 In these TR-VSFG experiments, the temporal intensity change of the VSFG signal was observed. Thus, they do not provide the information on the second-order susceptibility (χ(2)) itself but only give the information about its modulus square (|χ(2)|2). Therefore, it is difficult to interpret the spectral information correctly. Recently, single-channel phase-sensitive VSFG and multiplex heterodyne-detected (HD) VSFG have been developed for steady-state VSFG measurements, which have enabled direct

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1. INTRODUCTION Water is ubiquitous on the earth, and it is the most important liquid and plays an essential role in life.1−3 Despite its abundance, the physicochemical properties of water are very unique and atypical for a liquid, as represented by its high boiling temperature, high surface tension,4 temperature dependences of the density and heat capacitance,5 etc. These anomalous properties of water arise from the strong hydrogenbond network formed among water molecules.6 Because the networks realize and facilitate chemical processes that occur in aqueous solutions, including biological processes in living bodies, molecular-level elucidation of, particularly, the nature of H-bonding is indispensable not only for properly understanding the behavior of water but also for obtaining clues to comprehend the mechanism of various phenomena relating to water, including the mechanism to realize our life. The H-bond network is not static but highly dynamic, i.e., Hbond structure is very rapidly changing all the time. Therefore, elucidation of the dynamic property, in addition to the steadystate property, is essential for understanding water and its Hbonding. Water dynamics in bulk liquid has been intensively studied using a variety of advanced time-resolved spectroscopies, such as time-resolved IR and/or 2D IR,7−15 timeresolved Raman,16 and time-resolved fluorescence spectroscopy using fluorescence probes.17 These time-resolved studies have revealed that the vibrational dynamics of water is extremely fast and that it proceeds on the femto- to picosecond time scale. The memory of the H-bond, i.e., the frequency−frequency correlation function of the OH stretch vibration, decays on the ∼50 fs and ∼1 ps time scales, which have been attributed to the ultrafast fluctuation and rearrangement of H-bonding10,18 and/ or efficient energy transfer between water molecules through dipole−dipole coupling.19 For the population relaxation of the OH stretch vibration of water, it is considered that the bend (overtone) vibration is involved in the main relaxation pathway.9,13,20,21 At the interface, the H-bond network of water is suddenly truncated, and water molecules at the interface are forced to be rearranged to minimize the surface free energy. The optimum interfacial structure is dependent on the nature of the interface, e.g., hydrophobicity, charge, density of H-bonding site, and hence, the dynamics of interfacial water should also vary. The interfacial water plays crucial roles in a variety of phenomena, so it is very desirable to clarify how the unique properties of interfacial water induce, or are related to, these phenomena in a broader sense. For instance, elucidation of the dynamic behavior of water at a hydrophobic interface is important for understanding the hydrophobic effect, which is believed to be important for protein folding.22 Physicochemical properties of water at hydrophilic interfaces are directly related to understanding the lipid membrane interface, which is an essential constituent of cell membranes. In spite of the importance of the interfacial water and its dynamics, our molecular-level understanding is not sufficient, compared to the accumulated knowledge of water in the bulk. In particular, only little has been known about the dynamic properties of interfacial water because the experimental methods for selectively observing the dynamics at the liquid interface are very limited. 10666

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measurements of χ(2) spectra by determining the phase and amplitude of the VSFG signal.42−49 Because multiplex HDVSFG implements multichannel detection with femtosecond broadband IR pulses, it can be straightforwardly extended to the femtosecond time-resolved VSFG measurement. Actually, time-resolved HD-VSFG (TR-HD-VSFG) as well as 2D HDVSFG experiments were realized at aqueous interfaces very recently.50,51 TR-HD-VSFG enables time-resolved spectral measurements of the imaginary part of χ(2) (Im χ(2)), and the time-resolved Im χ(2) spectra can be directly compared to the time-resolved infrared and/or Raman spectra in the bulk, which correspond to time-resolved Im χ(1) and Im χ(3) spectra, respectively. Measurements of time-resolved Im χ(2) spectra make it possible to distinguish different dynamics occurring at the water interface based on the spectral information. For instance, they enable unambiguous discussion about the inhomogeneity of interfacial water through observation of hole-burning and spectral diffusion. In a sense, from the methodological point of view, we may say that ultrafast interface-selective nonlinear spectroscopy is now reaching the level of ultrafast spectroscopy in solution, at least for water interfaces that have air for the other phase. Ultrafast dynamics at the water interface can now be discussed on the basis of clear time-resolved vibrational data, as we discuss ultrafast dynamics in solution by time-resolved infrared and/or Raman spectroscopy. As described above, TR-VSFG spectroscopy has been drastically developed in the past decade, and our understanding on the ultrafast dynamics at water interfaces is now getting deepened significantly. This situation has brought us to the point where we can overview the past and present TR-VSFG studies carried out for ultrafast dynamics at water interfaces. In this review, we overview experimental efforts and arguments made for elucidating ultrafast dynamics at the three water interfaces, i.e., silica/water, air/charged monolayer/water, and air/water interfaces, in chronological order. Because the study of ultrafast dynamics at the water interface is now in progress and new information is being obtained, some arguments and interpretations have not been established yet, and they need further verification. For such issues, we try to describe the point for discussion clearly. The review consists of the following: In section 2, we describe the principles and instrumentations of TR-VSFG and TR-HD-VSFG spectroscopies. Section 3 is the main body of this paper, and we review TR-VSFG and TR-HDVSFG studies carried out for water interfaces so far. We first describe the works on the silica/water interface and then those on the air/charged monolayer/water interfaces and the studies on the air/water interface. In the last part of section 3, we also mention a new type of TR-HD-VSFG spectroscopy, i.e., the UV-pump/HD-VSFG-probe experiment, which has a high potential for studying reaction dynamics at water interfaces. Finally, we summarize the current understanding and provide some perspectives for the future work in section 4. In this review, we tried to cover all the TR-VSFG and TR-HD-VSFG studies carried out for water at interfaces so far, but we do not include SFG free induction decay (FID) experiments.52,53 This is because information obtainable from SFG FID is equivalent to that obtained by steady-state VSFG experiments.54,55

2. PRINCIPLE AND INSTRUMENTATION OF VSFG SPECTROSCOPY 2.1. Principle

2.1.1. VSFG and HD-VSFG. When a material is irradiated with light, polarization is induced in the material. As long as the light intensity is low, the induced polarization is linear to the electric field of the incident light. However, as the light intensity becomes high, the response of the materials starts deviating from the linear regime and nonlinear polarization is induced, which is represented by the following expansion in a power series of the electric field of the light: P = χ (1) E1 + χ (2) E1E2 + χ (3) E1E2E3 + ...

where P, Ei, and χ(n) are polarization, the electric field of the ith light and the nth-order susceptibility, respectively. The first term represents usual linear polarization, and the second term is the lowest-order nonlinear polarization, i.e., the second-order nonlinear polarization. Because the second-order nonlinear polarization is proportional to the product of E1 and E2, it contains components that have the sum and difference frequencies of the frequencies of E1 (ω1) and E2 (ω2). The former component is the source of the sum-frequency generation (SFG) process, which generates new light having the frequency of ω1 + ω2. The second-order nonlinear polarization (or even-order nonlinear polarization, in general) is generated only in the region where the inversion symmetry is broken. As for liquid samples, the inversion symmetry is practically held in the bulk liquid region, but it is broken at the interface. Therefore, SFG occurs only at the interface, which makes VSFG spectroscopy interface-selective. VSFG spectroscopy is usually performed with the optical scheme shown in Figure 1a: two laser lights having ω1 and ω2

Figure 1. Scheme of (a) homodyne- and (b) heterodyne-detected VSFG.

frequencies are focused onto the sample interface, and the sumfrequency light at ω1 + ω2 is detected in the reflection condition. Infrared light is used for ω2 to realize vibrational resonances, and visible light is used for ω1 light, which “upconverts” the information about the vibrational resonance to the visible region. In the case of this reflection configuration, the electric field of VSFG light, ESFG, is given as44,56 10667

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iωSFG (2) χ E1E2 2cn cos θr

electronic resonance, χ(2) is expressed as follows, assuming the Lorentzian band shape for vibrational resonances23,59

(2-1)

where i is the imaginary unit, E1 and E2 are the electric fields of input visible (ω1) and IR (ω2) pulses, respectively, c is the speed of light in a vacuum, ωSFG stands for the sum frequency (=ω1 + ω2), n is the refractive index of the upper bulk medium for the SFG light, and θr is the reflection angle of the SFG light. Note that the effective second-order nonlinear optical susceptibility of a sample interface is represented by χ(2) that contains Fresnel coefficients. In this formula, the interface selectivity of VSFG is secured by the third-rank tensorial property of χ(2): it vanishes in centrosymmetric bulk phases after averaging over all the orientations of the molecules, but it survives at an interface where molecules have some specific orientation due to the anisotropic nature of the interface. In this review, χ(2), as well as E1 and E2, which are vectors, is treated as scalar for simplicity. Note that the imaginary unit i on the righthand side of eq 2-1 arises from the fact that the phase of the electric field of the VSFG signal is shifted by π/2 from the phase of the relevant second-order nonlinear polarization generated at the interface.57 In conventional VSFG measurements, the intensity of SFG light is simply detected as schematically depicted in Figure 1a. This type of detection is called homodyne detection. In this case, because the light intensity is proportional to the modulus square of the electric field, the detected VSFG signal is proportional to |χ(2)|2: * = |ESFG|2 = ESFGESFG

ωSFG 2 4c 2n2 cos2 θr

|χ (2) |2 |E1E2|2

χ (2) = ANR +

q

iωSFG (2) χ E1E2 2cn cos θr ref

|χ (2) |2 = ANR 2 +

2

|Eref |

=

+

∑ q

+

∑ q≠q′

(2-5)

∑

Aq 2

(ωq − ω2)2 + Γq 2 ANR Aq(ωq − ω2)

(ωq − ω2)2 + Γq 2 AqAq ′ (ωq − ω2 − iΓq)(ωq ′ − ω2 + iΓq ′)

(2-6)

Here, the latter two terms on the right-hand side correspond to the spectral interference. As shown in eq 2-6, the peaks of the |χ(2)|2 spectra do not directly represent the frequencies of the vibrational resonances (ωq), and the spectra are distorted due to the spectral interference. Furthermore, even in the case for which the interference is negligible, |χ(2)|2 spectra cannot provide the information about the sign of the vibrational resonance term (Aq). In principle, the parameters ωq, Aq, and Γq can be determined by analyzing a homodyne-detected VSFG spectrum through fitting with eq 2-6. However, it is not easy to determine them uniquely.60 The above-mentioned drawbacks can be overcome by heterodyne detection, which enables direct acquisition of χ(2) as a complex quantity by determining the phase and amplitude of the electric field of the VSFG signal, i.e., ESFG. Figure 1b shows the scheme of heterodyne detection. In a heterodynedetection experiment,44−46,49 a local oscillator (LO) field, ELO, is separately prepared using E1 and E2, and this ELO is introduced into the detector with ESFG. Then, the intensity detection of the total electric field Etotal (=ESFG + ELO) is carried out:

(2-2)

(2-3)

* + ESFG * E LO |Etotal|2 = |ESFG|2 + |E LO|2 + ESFGE LO

(2-7)

In multiplex heterodyne-detected (HD) VSFG spectroscopy, the third term on the right-hand side of eq 2-7 is separated from the other terms by a Fourier analysis.45,46 This cross-term is proportional to χ(2):

|χ (2) |2 (2) 2 |χref |

ωq − ω2 − i Γq

q

where χ(2) ref represents the effective second-order nonlinear optical susceptibility of the reference interface. (Note that i has to be removed from the right-hand side of eq 2-3 if χ(2) ref is a bulk response.56,58) Using a reference that is nonresonant in the frequency region of interest, χ(2) ref is a constant that depends on neither ω1 nor ω2. Then, the absolute square of χ(2) can be evaluated by normalizing the homodyne-detected VSFG intensity from the sample (eq 2-2) by that from the reference (absolute square of eq 2-3) as follows: |ESFG|2

Aq

where ωq, Aq, and Γq are the frequency, amplitude, and damping constant of the vibrational resonance for a normal mode q, respectively, and ANR is a constant representing the nonresonant background. Note that all these parameters are real. Although eq 2-5 shows each term of vibrational resonances separately, spectral interference between resonant terms and that between the resonant terms and the nonresonant background appear in |χ(2)|2:

This quantity is measured at different IR (ω2) frequencies. Obviously, the detected signal depends on not only χ(2) but also the intensity of the ω2 light at each frequency. To make correction for this frequency dependence of the ω2 light, a reference measurement is necessary. Replacing the sample with a reference, the electric field of VSFG light from a reference interface Eref is given by eq 2-3: Eref =

∑

* = ESFGE LO (2-4)

iωSFG (2) * χ E1E2E LO 2cn cos θr

(2-8)

For the phase and intensity correction, the cross-term for the reference has to be obtained in the same manner as for eq 2-8:

The constancy of χ(2) ref allows the normalized VSFG intensity given by eq 2-4 to represent a |χ(2)|2 spectrum, which is generally called a VSFG spectrum. VSFG spectroscopy is performed to examine the vibrational resonance at the interface. When ω2 is within the mid-IR region associated with the vibrational resonances and ωSFG and ω1 are in the visible or near-infrared region but far below the

* = Eref E LO

iωSFG (2) * χ E1E2E LO 2cn cos θr ref

(2-9)

The normalization of the cross-term for the sample (eq 2-8) by that for the reference (eq 2-9) provides χ(2) as a complex value: 10668

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* ESFGE LO χ (2) = (2) * Eref E LO χref

(2-10)

Note that χ(2) ref has to be not only constant but also real and positive, otherwise eq 2-10 does not give a complex χ(2) spectrum correctly. In HD-VSFG, the reference has to satisfy much more strict conditions than in homodyne-detected VSFG. Also note that E1, E2, and ELO in eqs 2-8 and 2-9 have to be exactly the same in terms of the phase as well as the amplitude, which means that the optical paths must be unchanged for the sample and reference measurements within the accuracy of the phase of the sum-frequency light. It is because the optical-path difference between ESFG and Eref brings about a phase difference with respect to ELO (Figure 1b), resulting in a phase error of χ(2) normalized by χ(2) ref (eq 2-10). In HD-VSFG spectroscopy, the spectral interference is avoided, because χ(2) is directly obtained. Thus, the rigorous absorptive line shape can be obtained from the imaginary part of χ(2) (Im χ(2)). The expression for the Im χ(2) spectrum corresponding to the χ(2) given by eq 2-5 is as follows: Im χ (2) =

∑ q

Aq Γq (ωq − ω2)2 + Γq 2

(2-11)

Each normal mode q exhibits a Lorentzian band with an amplitude AqΓq−1 and a bandwidth (fwhm) 2Γq. The parameters ωq, Aq, and Γq can be determined more readily by analyzing an Im χ(2) spectrum through fitting with eq 2-11. Clearly, there is no spectral interference between resonant terms and between a resonant term and the nonresonant background in an Im χ(2) spectrum. Because Γq is always positive, the sign of Aq is readily determined from Im χ(2), which is related to the up or down orientation of a functional group involved in the normal mode. Moreover, Aq is proportional to the interface density of molecules, which can facilitate quantitative analysis. In the above discussion, we assumed a Lorentzian line shape to simply explain the difference between homodyne detection and heterodyne detection, particularly spectral interference appearing in |χ(2)|2 spectra obtained with homodyne-detected VSFG. However, the assumption of the Lorentzian (or any other) line shape is indeed not required to interpret the data measured with HD-VSFG spectroscopy, because the Im χ(2) spectrum directly shows an “absorptive” line shape. This is a very important advantage of HD-VSFG over conventional VSFG for the study on aqueous systems where the line shape of the water vibrational band is not Lorentzian due to the distribution of hydrogen bonds and the multiple time scales of their dynamics. 2.1.2. TR-VSFG and TR-HD-VSFG. In VSFG measurements, the input visible (ω1) and IR (ω2) are femtosecond or picosecond pulses, and therefore, extension to time-resolved experiments is straightforward by additional use of a pump (ωpump) pulse. In TR-VSFG experiments, the sample is vibrationally (or electronically) excited by irradiation of the IR (or UV−visible) ωpump pulse. The pulse sequence of a homodyne-detected TR-VSFG experiment is depicted in Figure 2a, where the femtosecond IR ω2 pulse is implemented. (This figure also provides a time-domain picture of the VSFG process.) In this experiment, the sample is first excited by the femtosecond ωpump pulse, and after a delay time (ΔT), the narrow-band picosecond visible (ω1) and femtosecond broadband IR (ω2) pulses probe the interface. When the ω2 pulse

Figure 2. Schematic of the pulse sequence in time-resolved (a) homodyne- and (b) heterodyne-detected VSFG. Red dashed waves represent the vibrational coherence created by the ω2 pulse. (c) The energy ladder diagram for IR-excitation and subsequent VSFG probe processes.

achieves vibrational resonance, the vibrational coherence is generated, and it is up-converted by mixing with the visible ω1 pulse to generate the VSFG signal at (ω1 + ω2). To up-convert the vibrational coherence into the visible region without distortion, the temporal duration of the visible ω1 pulse needs to be long enough compared with the dephasing time of the vibrational coherence. It means in the frequency domain that the bandwidth of the ω1 pulse needs to be sufficiently narrow compared to the intrinsic vibrational bandwidth. As readily seen in Figure 2a, the delay time of the measurement is defined by the time interval between the ωpump pulse and femtosecond IR ω2 pulse (ΔT), and therefore, the time-resolution is determined by the duration of these two pulses, although the picosecond visible ω1 pulse is used for the VSFG probing process. It is also noteworthy that the photoexcitation process is not interfaceselective, although the VSFG probe process is interfaceselective. In the pump process, the electric field of the pump pulse interacts with the sample twice (as depicted by E and E*) 10669

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Figure 3. Schematic of a femtosecond time-resolved homodyne-detected VSFG setup. This figure is reprinted with permission from ref 63. Copyright 2008 American Institute of Physics.

selectivity can be discussed in the same manner as for timeresolved homodyne VSFG. In this case, however, a ΔIm χ(2) spectrum is directly obtained as a time-resolved HD-VSFG spectrum by subtracting the Im χ(2) spectrum observed without pump (i.e., steady-state Im χ(2) spectrum) from that with pump:

to generate the population of the transient state, and the electric fields of ω1 and ω2 pulses interact with the sample in the VSFG probe process to generate the second-order nonlinear polarization at the interface. Therefore, time-resolved VSFG is a χ(4) process as a whole (Figure 2c). Nevertheless, as long as the effect of the coherence generated in the pump process is not an issue, time-resolved VSFG can be practically considered as χ(2) spectroscopy for the transient state prepared by the pump irradiation. In the time-resolved homodyne-detected VSFG measurements, the pump-induced change in |χ(2)|2 was measured. In other words, the time-resolved VSFG signal (Δ|χ(2)|2) at ΔT is obtained as a difference spectrum between the |χ(2)|2 spectra measured with and without pump irradiation:61,62

ΔIm χ (2) = (Im χ (2) + ΔIm χ (2) ) − Im χ (2)

The temporal change of ΔIm χ spectra can be analyzed in the same manner as we do for bulk transient absorption (e.g., global fitting) to extract time constants associated with spectral diffusion, vibrational relaxation and thermalization, etc. Heterodyne detection is far more important in time-resolved measurements than in steady-state measurements because of three reasons: First, the spectral distortion due to the interference in Δ|χ(2)|2 becomes more serious and problematic in time-resolved measurements because a time-resolved spectrum has contribution from many more species appearing transiently than a steady-state spectrum. Second, a bleaching band and a hot band are of opposite sign inherently, which are readily recognized in a ΔIm χ(2) spectrum but not in Δ|χ(2)|2. Third, transient species with low concentration can contribute linearly to ΔIm χ(2) but more complicatedly to Δ|χ(2)|2, meaning that HD-VSFG has higher sensitivity than homodyne VSFG, especially for minor species. Therefore, it is highly desirable to carry out time-resolved VSFG spectroscopy with heterodyne detection.

Δ |χ (2) |2 = |χ (2) + Δχ (2) |2 − |χ (2) |2 = χ (2) Δχ (2) * + χ (2) * Δχ (2) + |Δχ (2) |2

(2-13)

(2)

(2-12)

Strictly speaking, the analysis of the time-resolved Δ|χ(2)|2 data, e.g., fitting, deriving time constants, needs to be performed on the basis of eq 2-12. In the case of infrared-pumped timeresolved homodyne-detected VSFG experiments, bleaching in the vibrational ground state for a normal mode q can be approximately represented by a change in Aq (i.e., ΔAq). Nevertheless, it is also necessary to consider Δωq or ΔΓq, depending on how the ωpump pulse changes the microenvironment of the interface. A hot band corresponding to the v = 1 → 2 (v is the vibrational quantum number) transition can be treated as the appearance of a “new” mode, because its frequency is lower than that of the v = 0 → 1 fundamental transition due to vibrational anharmonicity. The Δ|χ(2)|2 spectrum observed with IR ωpump pulses can contain all of these contributions (ΔAq, Δωq, ΔΓq, hot band), which in principle bring rich information on the vibrational dynamics of interfacial molecules. However, it is extremely difficult, though not impossible, to unambiguously determine these quantities by fitting analysis based on eq 2-12. Figure 2b shows the pulse sequence of a time-resolved HDVSFG experiment, where the time resolution and interface

2.2. Instrumentation

Time-resolved VSFG experiments have been performed so far three different ways: pump/VSFG-probe with a single-channel detector, pump/multiplex (homodyne-detected) VSFG-probe with a multichannel detector, and pump/multiplex HD-VSFGprobe with a multichannel detector. The pump/single-channel VSFG-probe experiment is sometimes useful for measuring the temporal change of the VSFG signal, but it can be performed with a setup for pump/multiplex VSFG-probe experiments by narrowing the bandwidth of the broadband IR pulse and replacing a multichannel detector with a single channel detector. Therefore, we only describe the instrumentation of multiplex TR-VSFG and TR-HD-VSFG in this section. 10670

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2.2.1. TR-VSFG with Homodyne Detection. Figure 3 shows the time-resolved homodyne-detected VSFG setup built by the Bonn group,63 which is probably the most advanced homodyne setup to date. Using this setup as an example, the instrumentation of time-resolved homodyne-detected VSFG spectroscopy is outlined here. A Ti:sapphire regenerative multipass amplifier seeded by a Ti:sapphire oscillator produces a femtosecond pulse (center wavelength, 800 nm; pulse energy, 3.5 mJ; repetition rate, 1 kHz; pulse width, 100 fs; bandwidth, 12 nm). A part of the 800 nm output pumps an optical parametric amplifier to generate signal (∼1250 nm) and idler (∼2200 nm) pulses. Difference frequency generation (DFG) between the 800 nm pulse and the frequency-doubled idler pulse produces an IR pulse tunable from ∼2800 to ∼3500 cm−1 (pulse energy, 80−100 μJ; bandwidth, 200 cm−1). The bandwidth of this IR pulse can be narrowed down to 20 cm−1 by a Fabry−Pérot etalon and used as the ωpump pulse (“IR pump” in Figure 3). The residual 800 nm and frequencydoubled idler pulses after DFG are again difference-frequency mixed to generate an IR pulse (pulse energy, ∼25 μJ; bandwidth, 150 cm−1) that is used as the ω2 pulse (“IR probe” in Figure 3). The residual 800 nm pulse after the optical parametric amplifier is spectrally filtered by a pulse shaper and used as the ω1 pulse (bandwidth, 10 cm−1; “Vis” in Figure 3). The ω1, ω2, and ωpump pulses are focused onto one spot of the sample interface (“Langmuir−Blodgett monolayer” in Figure 3) held in a trough. The ω1 and ω2 pulses are temporally overlapped, and their time delay after the ωpump pulse can be varied arbitrarily. The sum frequency (ωSFG) light (“pump−probe SFG” in Figure 3) generated at the sample interface is introduced into a polychromator and detected by a CCD or directly into a photomultiplier tube (PMT). The SFG spectra with and without the ωpump pulse are simultaneously recorded on vertically shifted pixels of the CCD to obtain the Δ|χ(2)|2 spectrum at each time delay, which is realized by a galvanometric scanning mirror (placed between the sample and polychromator, not shown in Figure 3) synchronized with a chopper on the ωpump optical path. The PMT is used to more rapidly record spectrally integrated Δ|χ(2)|2 as a function of time delay. The trough is rotated at ∼5 rpm to reduce cumulative heating. The height of the sample interface is feedbackcontrolled by a motorized labjack with respect to the vertical position of spectrally dispersed SFG signals on the CCD. 2.2.2. TR-HD-VSFG. Figure 4 shows a typical setup for time-resolved HD-VSFG spectroscopy developed by our group.62 This setup is based on a high-power Ti:sapphire regenerative amplifier (center wavelength, 795 nm; pulse energy, 5.0 mJ; repetition rate, 1 kHz; pulse width, 80 fs; bandwidth, 22 nm) seeded by a femtosecond Ti:sapphire oscillator. About 2 W of the output from the regenerative amplifier is used for the excitation of an optical parametric amplifier to generate signal (1257 nm) and idler (2163 nm) pulses. A collinear DFG between the signal and idler pulses in a 0.4 mm thick AgGaS2 crystal yields broadband IR (ω2, center wavelength, 3000 nm; pulse energy, 3−13 μJ depending on the center wavelength as well as the DFG crystal; bandwidth, ∼300 cm−1). This ω2 beam is used as broadband IR light for the HDVSFG probe. The broadband ω2 covers the whole OH stretch region without scanning the IR frequency. About 1 W of the regenerative amplifier output passes through a narrow-band filter [center wavelength, 795 nm; bandwidth, 1.5 nm (24 cm−1)], and it is used as the narrow-band visible (ω1) light for HD-VSFG. For time-resolved measurements, another IR

Figure 4. Schematic of a femtosecond time-resolved heterodynedetected VSFG setup. This figure is reprinted with permission from ref 62. Copyright 2015 American Institute of Physics.

excitation pulse (ωpump) is generated for the pump process by DFG between the frequency-doubled idler beam and the rest of the regenerative amplifier output using a KTP crystal. The bandwidth of the IR generated in this process is narrowed down to ca. 120 cm−1 by the use of a 3 mm thick KTP crystal, and the pulse energy is about 35 μJ. The frequency of the ωpump light is tuned by rotating the KTP crystal. The intensities of the ωpump light at different wavelengths are kept constant by adjusting the power of the fundamental beam by a variable neutral density filter. The heterodyne scheme employed in Figure 4 generates LO with a transmission configuration prior to VSFG at the sample (or reference) interface. The LO is generated by focusing the ω1 and ω2 beams into a y-cut quartz crystal (thickness, 10 μm) that has large bulk χ(2) as well as a high optical damage threshold. The ω1 and ω2 pulses that transmit through the y-cut quartz are refocused by a concave mirror onto the sample surface. The height of the sample surface is monitored by a displacement sensor and kept constant with the accuracy of 1 μm. The LO, on the other hand, is passed through a 2 mm thick silica plate to delay it by ∼3.3 ps, and then it is refocused by the concave mirror onto the sample surface. The reflected LO and the sample (or reference) VSFG pulses collinearly propagate, and the sum of them is dispersed by a polychromator and detected by a liquid-nitrogen-cooled CCD. The temporal delay of the LO pulse with respect to the sample (or reference) VSFG pulse gives rise to fringes in a detected raw spectrum. The fringes correspond to the interference between ELO and ESFG, i.e., the third and fourth terms in the right-hand side of eq 2-7. The Fourier analysis of the fringes in the raw spectrum yields a complex spectrum corresponding to the third term (ESFGE*LO or ErefE*LO). Then, an Im χ(2) spectrum is obtained based on eq 2-10. The difference of Im χ(2) spectra with and without the ωpump pulse gives a ΔIm 10671

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χ(2) spectrum. The tunability of the ωpump pulse of this setup allows recording a two-dimensional (2D) HD-VSFG spectrum, which shows the ωpump dependence of the time-resolved ΔIm χ(2) spectrum at a certain delay time. In the 2D spectrum, the horizontal axis represents ω2, and the vertical axis represents ωpump. (Note that there are also opposite cases.) Thus, the slice along the horizontal axis corresponds to the time-resolved ΔIm χ(2) spectrum measured with the IR ωpump pulse whose frequency corresponds to the value in the vertical axis. So far, all the 2D spectra at the water interfaces have been measured with this pump−probe scheme. Zanni and co-workers reported an advanced scheme of 2D HD-VSFG spectroscopy. They scan the interval between two phase-locked broadband IR pump pulses in the time domain to measure the ωpump dependence of ΔIm χ(2), instead of scanning narrow-band IR ωpump pulses in the frequency domain.64 Figure 5 shows their setup for 2D HD-VSFG spectroscopy. This setup

Fourier transformation. This time-domain 2D HD-VSFG method originates from 2D IR spectroscopy65 in the bulk, for which the pulse shaper and handling of the time-domain data were developed. The data collection and processing are much more straightforward with the frequency-domain method (Figure 4), but the time-domain method (Figure 5) is able to simultaneously provide optimum time and frequency resolutions in 2D VSFG measurements, as already demonstrated in 2D IR.66

3. ULTRAFAST DYNAMICS AT WATER INTERFACES 3.1. Vibrational Dynamics of Water at the Silica/Water Interface

The silica/water interface is the first aqueous interface to which TR-VSFG was applied by McGuire and Shen.40 Silica is one of the most widely spread and important oxides, and it has been a subject of extensive research because of its importance in geochemistry and in mineral science.67−70 In this regard, the silica/water interface can be considered the most fundamental solid/water interface. In addition to this scientific importance, a reason for choosing the silica/water interface for the first TRSFG study is that a silica prism can be used for the bulk silica phase, which allows us to use the total internal reflection geometry and obtain the VSFG signal with high intensity. In these first experiments, two femtosecond infrared pulses were generated at different frequencies with relatively narrow bandwidth (70−85 cm−1), and they were used for the IR pump and the ω2 pulse in two-color TR-VSFG experiments. The ω2 pulse and the femtosecond ω1 pulse simultaneously hit the interface after a certain delay time respective to the IR pump to generate the VSFG signal. The temporal change of the VSFG intensity was monitored with the single-channel detection, and the time-resolved spectral change was also examined by changing the frequency of the ω2 pulse in a pointby-point fashion. Figure 6 shows the time-resolved VSFG (Δ|χ(2)|2) spectra at the silica/water interface, as well as the steady-state VSFG (|χ(2)|2) spectra, which were reported in this study.40 With IR excitation, the VSFG signal initially exhibited an intensity decrease as the overall change of |χ(2)|2, and then the decreased VSFG intensity recovered toward that of a quasisteady state. This quasisteady state is the temperatureincreased state, which appears as the IR-pump energy is finally converted to the thermal energy and increases the local temperature of the interface region. The time-resolved spectra observed with two different IR pump frequencies (3200 and 3400 cm−1) are noticeably different immediately after IR excitation, although they look indistinguishable at the delay time later than 0.6 ps: The time-resolved spectrum at 0.2 ps observed with 3200 cm−1 excitation exhibits a much narrower spectral shape than that observed with 3400 cm−1 excitation. This difference indicates the inhomogeneity of the OH stretch band at the silica/water interface, so it was called the “holeburning” of the OH stretch band of interfacial water in this paper, although the change of |χ(2)|2 arises from not only the bleaching of the OH stretch band (v = 1 ← v = 0 transition) but also the appearance of the hot band due to the transiently populated v = 1 state (v = 2 ← v = 1 transition). The temporal change of the VSFG signal was well-fit by the sum of the decay of the initial transient state and the rise of the thermalized state,71,72 and the vibrational relaxation time (T1 of the v = 1 level) and thermalization time (Tth) of the H-bonded OH vibration of interfacial water was evaluated to be 300 and 700 fs,

Figure 5. Schematic of a time-domain 2D HD-VSFG setup: AWG, arbitrary waveform generator; Ge-AOM, germanium acoustic optical modulator; PM, plane mirror; CM, curved mirror; L, lens; BS, beamsplitter; WP, waveplate; WD, ZnSe wedge; S, sample; CP, cube polarizer; ST, stage; LN, 5% Mg:LiNbO3. This figure is adapted with permission from ref 64. Copyright 2011 National Academy of Sciences.

is based on a Ti:sapphire regenerative amplifier (center wavelength, 800 nm; pulse energy, 0.8 mJ; repetition rate, 1 kHz; pulse width, 100 fs). A part of the output from the regenerative amplifier is used for excitation of an optical parametric amplifier to generate a mid-IR pulse (center wavelength, 4800 nm; pulse energy, 4 μJ). The 5% of the mid-IR pulse is used as the ω2 pulse, and 90% is sent through the pulse shaper (the box containing Ge-AOM and AWG in Figure 5) to generate the ωpump pulse pair. The remaining regenerative amplifier output passes through a narrow-band filter (bandwidth, 1.5 nm), and it is used as the narrow-band visible (ω1) pulse. The remaining 5% of the mid-IR pulse and the ω1 pulse are mixed in a Mg:LiNbO3 crystal (LN in Figure 5) to generate the LO. The ω1 pulse that transmits through LN and the ω2 pulse are refocused by a concave mirror onto the sample surface. The LO is delayed by a pair of wedges and is refocused by a lens onto the sample surface. The reflected LO and the sample VSFG pulses collinearly propagate, and the sum of them is dispersed by a polychromator and detected by a CCD. An Im χ(2) spectrum is obtained in the same manner as with the setup in Figure 4. By “scanning” the pulse shaper, the ωpump dependence of the ΔIm χ(2) spectrum is obtained in the time domain as a function of the time interval between the two phase-locked ωpump pulses. Then, this ωpump dependence in the time domain is converted to that in the frequency domain by 10672

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respectively. Since these time constants are similar to those of bulk water, it was concluded that the vibrational dynamics of the OH stretch are rather insensitive to more ordering and termination of the H-bond network at the interface and that the dynamics at the water interface is very similar to that in the bulk. The vibrational dynamics of interfacial water at the octadecyltrichlorosilane (OTS)-coated silica was also examined, which exhibits free OH at the hydrophobic wall at 3680 cm−1. The T1 time of the free OH was estimated to be 1.3 ± 0.1 ps, much longer than that of H-bonded OH. This difference accorded with the general proposition that the vibrational relaxation of the OH stretch in the condensed phase is accelerated by H-bonding. (It is noteworthy that the mechanism of the vibrational relaxation of the free OH at the OTS/water interface was further investigated very recently and it has been proposed that the rotation (jump) of the free OH to the H-bond configuration is the dominant relaxation pathway of the excited free OH.73) The first TR-VSFG experiment described above successfully grasped the essential features and the time scale of the vibrational dynamics of interfacial water. However, the main finding, i.e., the vibrational dynamics of H-bonded OH of interfacial water is very similar to that of bulk water, was further examined in the following studies, and it was shown that the dynamics of water in a few layers at the interface is in fact significantly different from that of the bulk, as described below. At the silica/water interface, the charge density at the silica surface can be readily controlled by changing the pH of the aqueous phase because silica has a number of hydroxyl groups (Si−OH) that can be deprotonated in contact with water due

Figure 6. First TR-VSFG experiment on the silica/water interface. (A) The steady-state VSFG (|χ(2)|2) spectrum of the silica/H2O interface obtained with a narrow-band picosecond SFG setup (solid curve) and the spectral profiles of the IR-pump pulse at 3200 cm−1 (dash dotted curve) and 3400 cm−1 (dashed curve). (B and C) The Δ|χ(2)|2 spectra obtained by two-color IR-pump/homodyne-detected VSFG-probe measurements with 3200 and 3400 cm−1 IR pump at three pump− probe delay times (0.2, 0.6, and 2.0 ps). The polarizations of ωSFG, ω1, ω2, and ωpump beams were s, s, p, and p. Distilled water (pH 5.7) was used in this experiment. The third-order cross-correlation width between IR-pump and SFG-probe pulses was ∼170 fs. Figure reprinted with permission from ref 40. Copyright 2006 American Association for the Advancement of Science.

Figure 7. Left: One-color IR-pump/homodyne-detected VSFG-probe trace of the silica/H2O solution interface for ω2 = ωpump = 3200 cm−1 at pH 6 with various NaCl concentrations. The solid lines represent the fitting curves based on a four-level model. The polarizations of ωSFG, ω1, ω2, and ωpump beams were p, p, p, and p, respectively. Right: NaCl concentration dependence of the vibrational relaxation (T1) times of the H-bonded OH stretch. The T1 times were extracted from the data shown in the left panel (red circles) and the corresponding data collected with s, s, p, p polarization combination (blue triangles). The top axis of the right panel represents Debye length. Figure reprinted with permission from ref 76. Copyright 2011 American Chemical Society. 10673

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to an equilibrium reaction: SiOH ⇄ SiO− + H+. Therefore, the silica interface is negatively charged at moderate and high pH, while it becomes neutral at low pH. Thus, the silica/water interface is a good system to study the effect of the charge on interfacial water.67−69,74 The effect of the surface charge on the vibrational dynamics of water at the silica/aqueous interfaces was investigated by the Borguet group with one-color IRpump/VSFG-probe experiments,75 and the vibrational relaxation (T1) time of H-bonded OH of interfacial water was evaluated as a function of pH at two frequencies (3200 and 3450 cm−1). The T1 time at pH 12 was determined to be 255 ± 25 fs, which is close to the bulk value, as concluded in the first study of the Shen group. However, the T1 time at pH 2 was 570 ± 30 fs, which is significantly different from the bulk value. They interpreted this change of the T1 time in terms of the difference in the depth monitored by VSFG. At pH 12, the silica surface is fully negatively charged by the deprotonated silanolate, and the surface electric field induces water orientation (or polarization) more into the bulk. Thus, the SFG probing depth is extended so that more water having bulklike character is monitored. On the other hand, the silica surface is neutral at pH 2, so the water orientation is induced only in the close vicinity of the interface and hence VSFG selectively monitors such water molecules having a character specific to the interface region. This conclusion was further confirmed by their experiments on the ionic strength dependence of the vibrational dynamics at silica/H2O at pH 6, where the silica surface is negatively charged.76 Figure 7 shows the temporal trace of the timeresolved VSFG data as well as the plot of the salt concentration dependence of the vibrational relaxation (T1) time evaluated by pumping and probing at 3200 cm−1. At the low salt concentration (10−3 mol dm−3 or below), the T1 time of the H-bonded OH (∼200 fs) is similar to the value obtained in the absence of the salt and that in the bulk. However, the T1 time increases at salt concentration >10−3 mol dm−3 and reaches a plateau (∼700 fs) at a concentration at 10−2 mol dm−3. As the ionic strength becomes high with the addition of excess salt, the electric field at the interface is screened and the penetration of the electric field into the bulk is reduced. Therefore, the dynamics of the water molecules close to the silica surface is monitored at the high ionic strength. In other words, the long T1 time observed at the high ionic strength is the T1 time of the “true” interfacial water. The shorter vibrational dynamics similar to that of the bulk was observed at the charged interface because of the large penetration of the electric field into the bulk region and consequent sampling of a bulklike water response. This experiment showed that the vibrational dynamics of water in the vicinity of the interface is significantly different also in the case of the charged interface. The data also suggested that the screening effect for the surface electric field is much more efficient than the prediction from the classical Gouy−Chapman theory. Actually, the theory provided the thickness of the electric double layer as 3 nm at the ionic strength of 10−2 mol dm−3. If this prediction is correct, bulklike dynamics should be observed even at an ionic strength of 10−2 mol dm−3, because 3 nm corresponds to ∼10 layers of water. This prediction contradicted the experimental observation, and it was concluded that VSFG only monitors the first few water layers at an ionic strength higher than 10−2 mol dm−3, where the T1 time shows a plateau. These TR-VSFG experiments on pH and ionic strength dependence of the vibrational relaxation (T1) time at the silica/

water (H2O) interface showed that the water in the first few interface layers is substantially different from that in the bulk, at both neutral and charged interfaces. In addition, it was observed in the experiments on the pH dependence that the vibrational dynamics at two different frequencies in the H-bonded OH region was indistinguishable at each pH.75 However, TR-VSFG experiments using isotopically diluted water (H2O:HOD:D2O = 1:11:32) showed that the vibrational dynamics exhibits significant frequency dependence as HOD becomes the predominant species monitored in the OH stretch region.77 The shorter T1 time was evaluated for the red side of the Hbonded OH stretch band. Because the inter- and intramolecular couplings are removed in the isotopically diluted water (HOD−D2O) interface, the inhomogeneous character of the OH stretch band can be observed clearly with isotopic dilution. As a possibility for the mechanism of this frequency dependence, the frequency gap between the OH stretch and bend overtone of HOD was pointed out. The frequency of the bend overtone is ∼2900 cm−1, and the red side of the Hbonded OH is closer to this frequency. This causes larger coupling with the bend overtone that acts as an efficient relaxation channel, which results in faster vibrational relaxation. TR-VSFG experiments on the silica/water interface described in this section successfully showed that the vibrational dynamics of the water in close vicinity (i.e., in a few layers) of the interface is different from the dynamics of water in the bulk, in particular for the vibrational relaxation (T1) time of the OH stretch vibrations of water. The silica/ water interface also enabled us to study both charged and neutral water interfaces by changing pH. However, the silica/ water interface is a “buried” interface, which has limited TRVSFG experiments to the single-channel homodyne detection. At the moment, it is difficult to perform more advanced methods such as heterodyne detection in TR-VSFG experiments for the silica/water interface. In fact, even in the steadystate VSFG, it is still technically demanding to determine the complex phase of χ(2) for buried interfaces.78 Thus, the dynamics of interfacial water has been investigated more rigorously with advanced VSFG spectroscopy for charged and neutral interfaces that have the air for one side of the interface, as described in the following two sections. 3.2. Vibrational Dynamics of Water at the Charged Monolayer/Water Interface

3.2.1. Steady-State Spectrum. Langmuir monolayer and Gibbs monolayer adsorbed on a water surface are good model systems for studying cell membrane/water interfaces as well as the electric double layer formed at charged interfaces (Figure 8). The structure of water at these monolayer interfaces has been intensively studied with conventional homodyne-detected VSFG79−82 and HD-VSFG.46,83−93 The conventional steadystate VSFG spectra of water at typical aqueous interfaces, including charged monolayer interfaces, exhibit a broad Hbonded OH band with a doublet-shaped peak at around 3200 and 3400 cm−1.37−39,94 (See the steady-state VSFG spectrum at the silica/water interface in Figure 6A, for example.) It is known that the intensity ratio of the two peaks varies depending on the condition of the interface, e.g., monolayer density and ionic strength of the water phase,79 and this double-peak feature of the OH stretch band was believed to indicate the existence of two distinct water species at the interfaces, namely, icelike and liquidlike water, for a long time in the community of interface-selective nonlinear spectrosco10674

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detected TR-VSFG technique.99 In this study, they performed one-color IR-pump/VSFG-probe measurements at four different frequencies in the OH stretch region, at the interface between water and the monolayer of the negatively charged lipid DMPS, i.e., the air/DMPS/H2O interface. (The chemical structure of DMPS, as well as other lipids and surfactants discussed in this section, is given in Figure 10). The observed time-resolved traces were analyzed with a three-state model that includes the v = 0 and v = 1 OH stretch vibrational states and the heated v = 0* state (thermalized state) generated with vibrational relaxation. It was claimed that the vibrational relaxation (T1) time of the OH stretch vibration of interfacial water is highly frequency-dependent: T1 < 100, =130, =430, and =570 fs at 3200, 3300, 3400, and 3500 cm−1, respectively.99 Because such drastic frequency dependence was not observed for the bulk water, they concluded that membrane-bound interfacial water observed by VSFG was energetically decoupled from the bulk, and it was located in the headgroup region. However, this conclusion was revised in a subsequent study by the same group (Figure 11). They carried out one-color IRpump/VSFG-probe measurements for four different lipids [one negative (DMPS), one positive (DPTAP), two zwitterionic lipids (DPPC, DPPE)],100 and the data were analyzed with the more widely used four-level model in which one more intermediate state was added in the relaxation path to the three-level model adopted in the first study. Then, it was concluded that the T1 times of the interfacial water at all four lipid interfaces are actually frequency-independent for the 3300−3500 cm−1 region and that the T1 time at these charged lipid interfaces is essentially the same as that in the bulk. For the low-frequency region (3200 cm−1), however, they kept arguing that the T1 time is much shorter (3300 cm−1) decreases and becomes negative after 0.3 ps. The spectrum in later time (>1 ps) shows a typical “thermalized” spectral feature, i.e., the positive (3200 cm−1) features

Figure 11. One-color IR-pump/homodyne-detected VSFG-probe trace of the various air/lipid monolayer/H2O interfaces. Pump/ probe IR frequencies in cm−1 are shown in the figure. The polarizations of ωSFG, ω1, ω2, and ωpump beams were s, s, p, and p, respectively. Solid lines are the model fit. Figure reprinted with permission from ref 100. Copyright 2010 American Chemical Society.

It is also expected that the vibrational coupling affects the vibrational dynamics. In particular, the Fermi resonance should cause instantaneous spectral broadening over the region of the split two bands because the depletion of the ground-state population bleaches the two transitions that appear with the Fermi resonance. These problems are solved by utilizing the 10676

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spectral spread/diffusion is rationalized by the intra- and/or intermolecular vibrational couplings, which induce instantaneous and ultrafast spectral spreading, respectively. The ΔIm χ(2) spectra obtained by TR-HD-VSFG unveil the complex temporal evolution of the interfacial vibrational spectrum in the OH stretch region, which exhibits the appearance of a bleach and hot band having opposite signs, instantaneous/ultrafast spectral spreading due to intra/intermolecular vibrational couplings, and subsequent spectral evolution to the thermalized spectrum representing the increase of the local temperature. The observation of the spectral hole-burning and subsequent spectral diffusion is the key to elucidate the inhomogeneity and fluctuation of H-bonds at the water interfaces. In the case of H2O, a clear spectral hole and spectral diffusion are not observed even in the ΔIm χ(2) spectra because of the spectral broadening due to the vibrational coupling. This complication can be removed by isotopic dilution, as briefly mentioned in the previous section. When H2O is mixed with D2O, water becomes a mixture of H2O, HOD, and D2O by H/D exchange, and if the amount of D2O is much larger than that of H2O, HOD becomes the predominant species giving rise to the OH stretch band. (We refer to such H2O diluted by a larger amount of D2O as “HOD−D2O” in this review.) Because HOD has only one OH group, there is no splitting arising from the coupling between two OH bonds in H2O. Simultaneously, the HOD bending frequency is located at 1460 cm−1, so the Fermi resonance between the OH stretch fundamental and bending overtone is not effective.109 Furthermore, when the HOD is sufficiently diluted and isolated in D2O, the OH oscillator cannot be coupled with other OH by intermolecular vibrational coupling. Therefore, isotopic dilution can suppress both intraand intermolecular couplings of water. The drawback of the isotopic dilution is the low signal intensity due to “dilution”, but the high intensity of the VSFG signal of charged water interfaces allows us to carry out time-resolved measurements on HOD−D2O.51 Figure 13 shows the time-resolved ΔIm χ(2) spectra of the air/CTAB/HOD−D2O interface. At 0 ps, the ΔIm χ(2) spectrum clearly shows a narrow bleach (i.e., hole) that well matches the ωpump spectrum.51 (A hot band also appears in the lower-frequency region.) The spectral hole gradually broadens in a few hundred femtoseconds as the excitation energy spectrally diffuses over other subensembles in the OH stretch region. This is an unambiguous observation of the hole-

Figure 12. Time-resolved ΔIm χ(2) spectra of the air/CTAB/H2O interface at the various delay times in the OH stretch region. The pump wavenumber is centered at 3400 cm−1 and the pump spectrum is shown at the bottom (black, arbitrarily scaled). The corresponding steady-state Im χ(2) spectrum is shown at the top. The polarizations of ωSFG, ω1, ω2, and ωpump beams were s, s, p, and p, respectively. The fwhm of the system response function determined by third-order cross-correlation was 180 fs. Figure reprinted with permission from ref 50. Copyright 2012 Chemical Society of Japan.

corresponding to the blue shift of the ground-state OH stretch band. As well-known in bulk studies,108 IR excitation finally causes the temperature increase of the sample, which results in the weakening of the hydrogen bond and the blue shift of the OH stretch band. The most prominent feature in the ΔIm χ(2) spectra is the extremely broad bleach at 0 ps. This broad bleach covers almost the entire v = 0 → 1 OH stretch region from 3100 to 3600 cm−1, despite the narrow bandwidth of the excitation pulse centered at 3400 cm−1. This very broad bleach observed immediately after excitation indicates that this broadening of the bleach feature occurs much faster than the time resolution of the measurement (ca. 180 fs). Such a fast

Figure 13. Time-resolved ΔIm χ(2) spectra of the air/CTAB/HOD−D2O interface. Delay times after photoexcitation are 0, 100, and 300 fs and the pump wavenumbers are 3300 cm−1 (a), 3400 cm−1 (b), and 3500 cm−1 (c). The isotope concentration of the aqueous phase is H2O:HOD:D2O = 1:8:16. The polarizations of ωSFG, ω1, ω2, and ωpump beams were s, s, p, and p, respectively. The lines shaded with red (positive) and blue (negative) represent the experimental data, and black lines represent the fits. Figure reprinted with permission from ref 51. Copyright 2012 American Institute of Physics. 10677

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burning, which discloses that the OH stretch band at the charged HOD−D2O interface is indeed inhomogeneously broadened. TR-HD-VSFG combined with isotopic dilution enabled clear observation of the hole-burning and subsequent spectral diffusion at the water interface. The spectral diffusion at the HOD−D 2O interface is slow compared with the instrumental response (∼200 fs), and hence, it is clearly observed with TR-HD-VSFG experiments. These TR-HD-VSFG experiments demonstrated the advantage of heterodyne detection in time-resolved measurements explicitly. Figure 14b shows the magnitude-squared [Δ|χ(2)|2 ≡

Figure 14. Comparison between ΔIm χ(2) and Δ|χ(2)|2 spectra. (a) Steady-state Im χ(2) and time-resolved ΔIm χ(2) spectra of the air/ CTAB/HOD−D2O interface. The steady-state Im χ(2) spectrum is shown with the negative sign up. The ΔIm χ(2) spectra are identical to Figure 13b, which was measured with IR pump at 3400 cm−1 (bottom, scaled arbitrarily). The delay time for each time-resolved spectrum is shown in the figure. (b) The magnitude-squared spectra corresponding to part a.

|χ(2)(t)|2 − |χ(2)steady|2] spectra, which were calculated from the imaginary and real parts of the time-resolved χ(2) spectra shown in Figure 13b. These spectra are equivalent to the time-resolved spectra obtained with homodyne TR-VSFG measurements. The time-resolved Δ|χ(2)|2 spectrum at 0 ps only exhibits a very broad bleach, and neither narrow spectral hole nor hot bands can be recognized. Although some temporal change of the OH stretch band is recognized for the Δ|χ(2)|2 spectra, the gradual band broadening corresponding to spectral diffusion is not clear as compared to the unambiguous spectral change observed in the ΔIm χ(2) spectra. This demonstrates the necessity of heterodyne detection for clarifying the true temporal spectral change of interfacial water. It is noted that the steady-state Im χ(2) and |χ(2)|2 spectra at this interface are not drastically different from each other, in contrast to the significantly different time-resolved spectra. This comparison negates a recent claim that spectral distortion in homodyne TR-VSFG (or homodyne 2D VSFG) should be minor if the steady-state Im χ(2) and |χ(2)|2 spectra look similar to each other.103 As shown in Figure 14, spectral distortion in Δ|χ(2)|2 is fatally substantial. The measurement of the pump-frequency dependence of the ΔIm χ(2) spectra can generate 2D HD-VSFG spectra, which correspond to 2D IR spectra measured with the pump−probe scheme.110 In principle, the 2D HD-VSFG spectrum contains all the information that is obtainable with time-resolved HDVSFG measurements. Figure 15 shows the 2D HD-VSFG spectra of the air/CTAB/H2O and air/CTAB/HOD−D2O interfaces at a delay range from 0 to 0.4 ps.62 These spectra

Figure 15. 2D HD-VSFG spectra of the air/CTAB/water interfaces. (a, c) Steady-state Im χ(2) spectrum and 2D HD-VSFG spectra of the air/CTAB/H2O interfaces at 0.0−0.4 ps and (b, d) the corresponding Im χ(2) and 2D HD-VSFG spectra of the air/CTAB/HOD−D2O interface. The isotope concentration of the aqueous phase is H2O:HOD:D2O = 1:8:16. The polarizations of ωSFG, ω1, ω2, and ωpump beams were s, s, p, and p, respectively. The horizontal and vertical axes represent the ω2 (probe) and ωpump (pump) frequencies, respectively. Figure reprinted with permission from ref 62. Copyright 2015 American Institute of Physics.

were constructed from the TR HD-VSFG data measured with seven different pump (ωpump) frequencies at 3000, 3100, 3200, 3300, 3400, 3500, and 3600 cm−1 with the aid of interpolation. In these 2D spectra, horizontal and vertical axes represent ω2 and ωpump, respectively. Thus, a horizontal slice at a certain ωpump represents the ΔIm χ(2) spectrum measured with the pump frequency at ωpump. The 2D spectra immediately after IR 10678

DOI: 10.1021/acs.chemrev.6b00728 Chem. Rev. 2017, 117, 10665−10693

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excitation clearly exhibit a positive bleach lobe in the higher ω2 region and a negative hot-band lobe in the lower ω2 region. The bleach lobe of 2D HD-VSFG spectra at the HOD−D2O interface (Figure 15d) exhibits a diagonally elongated narrow bleach lobe at 0.0 ps, corresponding to the different hole positions created by different ωpump frequencies. This bleach lobe becomes round and broad at ∼0.4 ps due to spectral diffusion. The center line slope analysis of the 2D spectra provides the time constant of this spectral diffusion process as 0.3 ± 0.1 ps. In the 2D HD-VSFG spectra at the air/CTAB/ HOD−D2O interface, the bleach and hot band lobes are clearly recognized, even at 0.4 ps, implying that the population relaxation process is not completed at this delay time. This means that the spectral diffusion proceeds faster than the population decay of the vibrationally excited v = 1 state at the air/CTAB/HOD−D2O interface. The observed 2D HD-VSFG spectra do not support the presence of the distinct, energetically isolated interfacial water that was claimed on the basis of homodyne TR-VSFG measurements at the charged monolayer/water interfaces.100,101,103 The 2D HD-VSFG spectrum of the air/CTAB/H2O interface at 0.0 ps shows two diagonal peaks as well as their cross-peaks in the bleaching region (Figure 15c). This feature at the H2O interface reflects the double peaks observed at 3230 and 3420 cm−1 in the steady-state Im χ(2) spectrum. These two diagonal peaks and their cross-peaks are not attributable to two different water species and their exchange, because they are completely lacking in the 2D spectra of the corresponding HOD−D2O interface. Instead, they are attributed to inter- and intramolecular coupling. Figure 16 shows that the bleach lobe simulated by a model function taking account of the Fermi resonance as well as inhomogeneity of the OH stretch vibration at the H2O interface.62 The simulated spectra well-reproduced the essential features of the bleach lobe of the 2D HD-VSFG spectra of the CTAB/H2O interface. The OH stretch vibration having a different frequency is mixed with the bend overtone with a different efficiency, and it exhibits the Fermi doublet, showing two peaks with different relative intensity. Thus, the hole-burning of the OH stretch vibration exhibits a bleach feature that shows a doublet with a different intensity ratio, depending on the frequency difference between the OH stretch and bend overtone. Accordingly, the horizontal cut of the bleach lobe exhibits a doublet feature having a different relative intensity of the two peaks, depending on ωpump. This results in the two diagonal peaks and their cross-peaks in the 2D HDVSFG spectra at the H2O interface. The 2D HD-VSFG spectra of the air/CTAB/water interfaces demonstrated that isotopic dilution is necessary for clearly observing the hole-burning and subsequent spectral diffusion. Isotopic dilution is also effective to distinguish the spectral feature due to the vibrational coupling from a feature arising from the structural subensembles because it suppresses the intra- and intermolecular vibrational couplings that heavily affect the spectral response of the H2O interfaces. TR-HD-VSFG also allows us to investigate the time-resolved spectral change with the change of the ionic strength. For charged interfaces, VSFG probes rather thick water layers at a low ionic strength. However, it probes water layers only in the vicinity of the interface at a high ionic strength due to the screening of the electric field by the counterions,83,86 as already described in section 3.1. Under the high ionic strength condition, the ultrafast vibrational dynamics of the interfacial water in the vicinity of charged interface was examined by TR-

Figure 16. Simulation for the beach lobes in 2D HD-VSFG spectra: (a) steady-state spectrum, (c) 2D HD-VSFG, and (e) horizontal slices of the bleach in the presence of the Fermi resonance. Spectra b, d, and f correspond to spectra a, c, and e in the absence of the Fermi resonance. The horizontal and vertical axes represent the ω2 (probe) and ωpump (pump) frequencies, respectively. Figure reprinted with permission from ref 62. Copyright 2015 American Institute of Physics.

HD-VSFG.111 Figure 17a shows the steady-state Im χ(2) spectra of the air/CTAB/water interfaces measured under high ionic strength by adding different excess salts. The intensity of the OH stretch band drastically decreases with addition of the excess salts, reflecting that the thickness of the probed water layer is significantly reduced.111 Furthermore, it was found that the shape of the OH stretch band changes with the change of the salts: Fluoride salts do not noticeably affect spectral features, whereas the chloride and bromide salts induce significant blue shifts of the average OH stretch frequency. Figure 17b shows time-resolved ΔIm χ(2) spectra at 0.0 ps observed with the excess salts. The ΔIm χ(2) spectrum obtained with fluoride salts exhibits a very broad bleach, even at 0.0 ps, as observed without excess salts, while chloride and bromide salts give rise to narrow spectral holes.111 These results indicate that water structure in the vicinity of the charged interface is substantially affected by chloride and bromide ions when they are highly concentrated, whereas the effect of fluoride ion is negligible. It was concluded that the high excess chloride and bromide ions strongly interacts with interfacial water and causes the blue shift of the OH stretch band in the vicinity of the interface. Then, the resultant large frequency mismatch between the OH stretch and bend overtone suppresses the 10679

DOI: 10.1021/acs.chemrev.6b00728 Chem. Rev. 2017, 117, 10665−10693

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Figure 18. Steady-state HD-VSFG and 2D HD-VSFG data of the air/ charged lipid/isotopically diluted water interfaces. (a, c) Im χ(2) and 2D HD-VSFG spectra at 0.0 ps of the air/DPTAP/HOD−D2O interface and (b, d) those of the air/DPPG/HOD−D2O interfaces. The horizontal and vertical axes represent the ω2 (probe) and ωpump (pump) frequencies, respectively. The isotope concentration of the aqueous phase is H2O:HOD:D2O = 1:8:16. The polarizations of ωSFG, ω1, ω2, and ωpump beams were s, s, p, and p, respectively. (e) The plot of peak line slope for DPTAP (red) and DPPG (blue) interfaces as a function of delay time. Schematic models for DPTAP (left) and DPPG (right) interfaces are also shown. Reproduced with permission from ref 98, DOI: 10.1002/anie.201603676. Copyright 2016 Wiley-VCH.

Figure 17. Steady-state and time-resolved HD-VSFG data of the air/ CTAB/H2O solution interfaces with and without addition of different excess salts. (a) Steady-state Im χ(2) spectra and (b) time-resolved ΔIm χ(2) spectra. The salt concentrations in the solution phase are shown in the figure. The time delay is at 0.0 ps and the pump wavenumber is 3500 cm−1. The polarizations of ωSFG, ω1, ω2, and ωpump beams were s, s, p, and p, respectively. Figure adopted with permission from ref 111. Copyright 2014 American Institute of Physics.

signs of the charge of the head groups.98 Except for this difference in the sign, the spectral features, including the peak frequencies, look more or less similar to each other, and the difference in the property (e.g., time-averaged H-bond strength) between the two interfaces is not obvious from the steady-state spectra. In sharp contrast, however, the vibrational dynamics of the interfacial water at the two charged interfaces is remarkably different. Figures 18c,d shows the 2D HD-VSFG spectra in the OH stretch region of these air/charged lipid/ HOD−D2O interfaces at 0.0 ps.98 The bleach lobe for the negatively charged DPPG interface is almost perfectly diagonally elongated, clearly indicating the inhomogeneity of the hydrogen bonds of interfacial water. It implies that the inhomogeneity is well captured because the spectral diffusion does not noticeably proceed within the time-resolution of the measurement (∼200 fs). On the other hand, the bleach lobe of the positively charged DPTAP interface is elongated toward much more vertical direction even at 0.0 ps, indicating that the memory of the excitation frequency is largely lost very rapidly. This suggests the existence of ultrafast dynamics that is not time-resolved with the instrumental response of the measurement. A former bulk 2D IR study has shown that the major part

intramolecular coupling (i.e., Fermi resonance) between them, which narrows the spectral hole. We note that the effect of halide ions on the vibrational dynamics of water has been studied also at air/electrolyte solution interfaces using 2DVSFG.112 3.2.4. Dynamics at the Charged Aqueous Interface Studied by TR-HD-VSFG and 2D HD-VSFG: Effect of Interaction with Surface Charge on the Water Dynamics. One of the central interests of the time-resolved studies at the charged aqueous interface is how the dynamics of interfacial water is affected by the interaction with the charge and property of the interface. This question was addressed by comparing the interfacial water dynamics at negatively and positively charged lipid monolayer interfaces, DPPG and DPTAP (see Figure 10 for the molecular structures of these lipids) using 2D HDVSFG.98 Parts a and b of Figure 18 show the steady-state Im χ(2) spectra of HOD−D2O at the air/DPPG/HOD−D2O and air/DPTAP/HOD−D2O interfaces, respectively, which exhibit the OH stretch band having the opposite signs, reflecting the opposite net orientation of interfacial water due to the opposite 10680

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of the frequency−frequency correlation of bulk HOD−D2O decays substantially within 50 fs due to ultrafast fluctuation of the H-bond, which is followed by ∼1 ps decay due to the rearrangement of the H-bond. 10 The former ultrafast component is not detectable in this 2D HD-VSFG experiment because of the limited time-resolution, and it should lead to the incomplete diagonal elongation at 0.0 ps as well as a smaller initial value of the peak line slope. On the basis of this consideration, the 2D spectrum of the DPTAP interface at 0.0 ps, as well as the initial peak line slope of the bleach lobe, was interpreted to indicate the (invisible) ultrafast (