Hong-fei Wang

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow 3693 cm(-1) and broad 3550 cm(-1) peaks belong to C(infinityv) symmetry, while the broad 3250 and 3450 cm(-1) peaks belong to the symmetric stretching modes with C2v symmetry. Thus, the 3693 cm(-1) peak is assigned to the free OH, the 3550 cm(-1) peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and 3450 cm(-1) peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.

Quantitative analysis of orientational order in the molecular monolayer by surface second harmonic generation

We have developed a systematic approach to quantitatively determine the orientational order of molecular monolayer based on surface second harmonic generation (SHG) measurement. Traditionally, the experimentally obtainable orientational parameter D=〈cosu200aθ〉/〈cos3u200aθ〉 has been used to determine the averaged orientational angle θ with SHG. Also, it has been widely accepted that without the help from other techniques, SHG was insufficient for determination of both the orientational angle θ0 and its distribution Δθ. Here, we derived an explicit and general functional form, R(θ)=|〈cosu200aθ〉−cu2009*u2009〈cos3u200aθ〉|2, which is orientationally dependent and is proportional to the SH intensity measured from the molecular monolayer. We further showed that R(θ) contains the full information of the orientational order of the molecular monolayer, and the orientational parameter D represents only a special case of R(θ). Through orientationally sensitive R(θ) and D, both the orientational angle and its distribution could be uniquely a...

Microscopic molecular optics theory of surface second harmonic generation and sum-frequency generation spectroscopy based on the discrete dipole lattice model

The aim of this work is to present a coherent description of the microscopic theory of surface nonlinear optical spectroscopy for solving practical problems in understanding the details of the molecular interface. We review the related issues and show that by following microscopic molecular optics theory, second harmonic generation (SHG), as well as sum-frequency generation vibrational spectroscopy (SFG-VS), radiation from a monolayer or submonolayer can be rigorously treated as the radiation from an induced dipole lattice at the interface. In this approach, the introduction of an infinitesimally thin polarization sheet as in macroscopic theory with the Maxwell equations and the boundary conditions is no longer necessary. As a direct consequence, the ambiguity of the unaccounted dielectric constant of the interfacial layer is no longer an issue. Moreover, the anisotropic two-dimensional microscopic local field factors can be explicitly defined with the linear polarizability tensors of the interfacial molecules. Based on the planewise dipole sum rule in the molecular monolayer, experimental tests of this microscopic treatment with SHG and SFG-VS are discussed.In the currently accepted models of the nonlinear optics, the nonlinear radiation was treated as the result of an infinitesimally thin polarization sheet layer, and a three layer model was generally employed. The direct consequence of this approach is that an apriori dielectric constant, which still does not have a clear definition, has to be assigned to this polarization layer. Because the Second Harmonic Generation (SHG) and the Sum-Frequency Generation vibrational Spectroscopy (SFG-VS) have been proven as the sensitive probes for interfaces with the submonolayer coverage, the treatment based on the more realistic discrete induced dipole model needs to be developed. Here we show that following the molecular optics theory approach the SHG, as well as the SFG-VS, radiation from the monolayer or submonolayer at an interface can be rigorously treated as the radiation from an induced dipole lattice at the interface. In this approach, the introduction of the polarization sheet is no longer necessary. Therefore, the ambiguity of the unaccounted dielectric constant of the polarization layer is no longer an issue. Moreover, the anisotropic two dimensional microscopic local field factors can be explicitly expressed with the linear polarizability tensors of the interfacial molecules. Based on the planewise dipole sum rule in the molecular monolayer, crucial experimental tests of this microscopic treatment with SHG and SFG-VS are discussed. Many puzzles in the literature of surface SHG and SFG spectroscopy studies can also be understood or resolved in this framework. This new treatment may provide a solid basis for the quantitative analysis in the surface SHG and SFG studies.

Reconsideration of second-harmonic generation from isotropic liquid interface: Broken Kleinman symmetry of neat air/water interface from dipolar contribution

It has been generally accepted that there are significant quadrupolar and bulk contributions to the second-harmonic generation (SHG) reflected from the neat air/water interface, as well as common liquid interfaces. Because there has been no general methodology to determine the quadrupolar and bulk contributions to the SHG signal from a liquid interface, this conclusion was reached based on the following two experimental phenomena: the breaking of the macroscopic Kleinman symmetry and the significant temperature dependence of the SHG signal from the neat air/water interface. However, because the sum frequency generation vibrational spectroscopy (SFG-VS) measurement of the neat air/water interface observed no apparent temperature dependence, the temperature dependence in the SHG measurement has been reexamined and proven to be an experimental artifact. Here we present a complete microscopic analysis of the susceptibility tensors of the air/water interface, and show that dipolar contribution alone can be used to address the issue of the breaking of the macroscopic Kleinman symmetry at the neat air/water interface. Using this analysis, the orientation of the water molecules at the interface can be obtained, and it is consistent with the measurement from SFG-VS. Therefore, the key rationales to conclude significantly quadrupolar and bulk contributions to the SHG signal of the neat air/water interface can no longer be considered as valid as before. This new understanding of the air/water interface can shed light on our understanding of the nonlinear optical responses from other molecular interfaces as well.

An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra

Knowledge of the ratios between different polarizability betaijk tensor elements of a chemical group in a molecule is crucial for quantitative interpretation and polarization analysis of its sum frequency generation vibrational spectroscopy (SFG-VS) spectrum at interface. The bond additivity model (BAM) or the hyperpolarizability derivative model along with experimentally obtained Raman depolarization ratios has been widely used to obtain such tensor ratios for the CH3, CH2, and CH groups. Successfully, such treatment can quantitatively reproduce the intensity polarization dependence in SFG-VS spectra for the symmetric (SS) and asymmetric (AS) stretching modes of CH3 and CH2 groups, respectively. However, the relative intensities between the SS and AS modes usually do not agree with each other within this model even for some of the simplest molecular systems, such as the air/methanol interface. This fact certainly has cast uncertainties on the effectiveness and conclusions based on the BAM. One of such examples is that the AS mode of CH3 group has never been observed in SFG-VS spectra from the air/methanol interface, while this AS mode is usually very strong for SFG-VS spectra from the air/ethanol interface, other short chain alcohol, as well as long chain surfactants. In order to answer these questions, an empirical approach from known Raman and IR spectra is used to make corrections to the BAM. With the corrected ratios between the betaijk tensor elements of the SS and AS modes, all features in the SFG-VS spectra of the air/methanol and air/ethanol interfaces can be quantitatively interpreted. This empirical approach not only provides new understandings of the effectiveness and limitations of the bond additivity model but also provides a practical way for its application in SFG-VS studies of molecular interfaces.

Quantitative measurement and interpretation of optical second harmonic generation from molecular interfaces

Second harmonic generation (SHG) has been proven a uniquely effective technique in the investigation of molecular structure and conformations, as well as dynamics of molecular interfaces. The ability to apply SHG to molecular interface studies depends on the ability to abstract quantitative information from the measurable quantities in the actual SHG experiments. In this review, we try to assess recent developments in the SHG experimental methodologies towards quantitative analysis of the nonlinear optical properties of the achiral molecular interfaces with rotational isotropy along the interface normal. These developments include the methodology for orientational analysis of the SHG experimental data, the experimental approaches for more accurate SHG measurements, and a novel treatment of the symmetry properties of the molecular polarizability tensors in association with the experimentally measurable quantities. In the end, the recent developments on the problem of surface versus bulk contribution in SHG surface studies is discussed. These developments can put SHG on a more solid foundation for molecular interface studies, and to pave the way for better understanding and application of SHG surface studies in general.

Phase measurement in nonlinear optics of molecules at air/water interface with femtosecond laser pulses

There are less than a handful of papers in the literature on the phase measurement of surface Second Harmonic Generation (SHG) from liquid interfaces, in sharp contrast to tens of the works on such measurement of films on solid substrate surfaces. Even though the SHG phase measurement is very important for obtaining structural and spectroscopic information, such as molecular orientation, at the liquid interfaces, experimentally there are many difficulties due to the intrinsically weak SH signal of liquid interfaces. Using a femtosecond pulsed laser and photon counting system, we have demonstrate the ability to measure the SH phase for the non-resonant neat air/water interface, which is clearly among the systems with the smallest surface SH susceptibility, and is well below the detection limit of experimental setups in most laboratories. After clarifying some confusions about the reference standards for absolute SH phase measurement using the α-quartz crystal, we used air/water interface as an intrinsic SH phase reference standard to measure the SH phases of some air/pure-liquid interfaces, and adsorbed molecular layers at the air/water interface. These results demonstrate the effectiveness of this very sensitive technique in obtaining molecular structural and spectroscopic information of liquid interfaces.

Quantitative Surface Chirality Detection with Sum Frequency Generation Vibrational Spectroscopy: Twin Polarization Angle Approach

Here we report a novel twin polarization angle (TPA) approach in the quantitative chirality detection with the surface sum-frequency generation vibrational spectroscopy (SFG-VS). Generally, the achiral contribution dominates the surface SFG-VS signal, and the pure chiral signal is usually two or three orders of magnitude smaller. Therefore, it has been difficult to make quantitative detection and analysis of the chiral contributions to the surface SFG-VS signal. In the TPA method, by varying together the polarization angles of the incoming visible light and the sum frequency signal at fixed s or p polarization of the incoming infrared beam, the polarization dependent SFG signal can give not only direct signature of the chiral contribution in the total SFG-VS signal, but also the accurate measurement of the chiral and achiral components in the surface SFG signal. The general description of the TPA method is presented and the experiment test of the TPA approach is also presented for the SFG-VS from the S- and R-limonene chiral liquid surfaces. The most accurate degree of chiral excess values thus obtained for the 2878 cm1 spectral peak of the S- and R-limonene liquid surfaces are (23.7 0.4)% and (25.4 1.3)%, respectively.

Observation of the Interference between the Intramolecular IR−Visible and Visible−IR Processes in the Doubly Resonant Sum Frequency Generation Vibrational Spectroscopy of Rhodamine 6G Adsorbed at the Air/Water Interface

Using the picosecond visible light at 532.1 nm and infrared light at 2800-3100 cm(-1), we observed the interference between the intramolecular IR-visible and visible-IR processes in the doubly resonant sum frequency generation vibrational spectroscopy of Rhodamine 6G adsorbed at the air/water interface. The interference phenomenon exists for both the C-H stretching vibrations in the 2800-3100 cm(-1) region and the skeleton vibrations in the 1450-1700 cm(-1) region. The relative strength of the visible-IR process at different wavelengths is the result of the electronic structure of the molecule. This is the first direct observation of the visible-IR sum frequency generation process in the electronically excited state of a model molecular system.

Spectroscopic evidence for the specific Na+ and K+ interactions with the hydrogen-bonded water molecules at the electrolyte aqueous solution surfaces

Sum frequency generation vibrational spectra of the water molecules at the NaF and KF aqueous solution surfaces showed significantly different spectral features and different concentration dependence. This result is the first direct observation of the cation effects of the simple alkali cations, which have been believed to be depleted from the aqueous surface, on the hydrogen bonding structure of the water molecules at the electrolyte solution surfaces. These observations may provide important clues to understand the fundamental phenomenon of ions at the air/water interface.