CN Vibrations and Structure in the Langmuir Monolayer of the Long

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Resolving Two Closely Overlapping −CN Vibrations and Structure in the Langmuir Monolayer of the Long-Chain Nonadecanenitrile by Polarization Sum Frequency Generation Vibrational Spectroscopy Zhen Zhang†,‡,§,∥ and Yuan Guo† †

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, and ‡The Graduate School, The Chinese Academy of Sciences, Beijing 100190, China

Zhou Lu,∥ Luis Velarde, and Hong-fei Wang* William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P. O. Box 999, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Polarization sum frequency generation vibrational spectra (SFG-VS) reveals that there are two distinctively different but closely overlapping −CN vibrations at 2244.5 and 2251.1 cm−1, respectively, in the Langmuir monolayer of the long-chain nonadecanenitrile (CH3(CH2)17CN, or C18CN) at the air/water interface. The blue-shifted −CN group at the 2251.1 cm−1 peak is about 1.8 times broader than that of the 2244.5 cm−1. Both the spectral shift and spectral width are consistent with the picture that this blue-shifted peak corresponds to the solvated −CN group, while the 2244.5 cm−1 peak is the signature of the less solvated −CN group. Polarization dependence of these two peaks further suggests that the −CN group corresponding to the 2251.1 cm−1 peak is tilted with an average angle of 50° from interface normal, while that corresponding to the 2244.5 cm−1 peak is tilted with an angle around 67°. The relative population for the −CN groups corresponding to the 2251.1 cm−1 peak is about three times that of the 2244.5 cm−1 peak. These results suggest that the −CN head groups in the C18CN Langmuir monolayer are not aligned uniformly at slightly different depth, in order to avoid the strong repulsive forces between the strong −CN dipoles. The SFG-VS spectra of the O−H stretches at C18CN Langmuir monolayer are similar to those of the 4″-n-pentyl-4-cyano-p-terphenyl (5CT) monolayer, indicating complete exclusion of the water molecules from the C18CN Langmuir monolayer, but significantly different from those of the 4″-n-octyl-4-p-cyanobiphenyl (8CB) monolayer, as well as those of the air/acetonitrile aqueous solution interface. Different from previously held understandings, these results suggest that the structure of the insoluble long-chain C18CN Langmuir monolayer is significantly different from that of the Gibbs adsorption layer of the short-chain soluble acetonitrile or propanenitrile aqueous solution surfaces. These observations not only shed new light on understanding the detailed structure and interactions in the molecular monolayer and films but also suggest the importance of the polarization and spectral resolution in the SFG studies.

1. INTRODUCTION It has been well established that the chemical and physical properties of Langmuir monolayers largely depend on the intermolecular interactions.1−5 For a simple insoluble monolayer film at aqueous interfaces, the interplay of these intermolecular forces can be surprisingly complicated even when the monolayer molecules are chemically identical. Because of the amphiphilic nature of the insoluble surfactant molecules forming the Langmuir monolayer, the dipole−dipole repulsions between the hydrophilic head groups compete with the hydrogen-bonding and van der Waals attractive forces between the water and polar head groups and between the hydrophobic hydrocarbon chains. The balance between these intermolecular forces drives the molecules to adapt to the © 2012 American Chemical Society

interfacial structures possessing the minimized surface energy.6−10 For example, as early as in 1942, Copeland and Harkins studied the surface pressure of the octadecanenitrile Langmuir monolayer at different temperatures and concluded that the remarkable repulsive forces between the aligned −CN head group at the air/water interface caused unusual expansion as showing in its surface pressure vs surface density phase diagram.6 Nevertheless, to obtain the detailed information on the surface structure and intermolecular interactions still remains as Received: October 21, 2011 Revised: December 28, 2011 Published: January 10, 2012 2976

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triphenyl and biphenyl groups in the 5CT and 8CB/5CB Langmuir monolayers controlled the phase behavior, as well as the water molecule structure in contact with the Langmuir monolayer.8 These studies revealed the complexities of the interactions in the Langmuir monolayer. With the advancement on the SFG-VS instrumentation in the past decades, especially with the commercial picosecond scanning SFG spectrometer, the broad-band SFG-VS spectrometer, and even the Fourier-transform SFG-VS,53,63−67 the spectral and time resolution, the quality, and the spectral reproducibility of the SFG-VS spectra have been significantly improved. These developments heralded the era of quantitative measurement and analysis of the SFG-VS spectra for detailed understanding of the structure and interaction of the interfacial molecules and chemical species.35,39 These developments also led to reexamination of the molecular theory of the surface SHG and SFG,68−70 based on which more detailed molecular level interpretation of the SHG and SFG-VS data is also possible. Recent SFG studies showed that different polarization configurations employed in the SFG-VS experiments can yield more detailed spectroscopic information.39,40 Using this concept, a systematic polarization and incident angle dependent SFG measurement were carried out on the long-chain nonadecanenitrile (C18CN, CH3 (CH2 ) 17CN) Langmuir monolayer at the air/water interface. From these polarization data, a 6.6 cm−1 spectral splitting of the −CN stretching vibrational mode was observed for the C18CN Langmuir monolayer at the air/water interface, implying the structural differences among the seemingly identical cyano head groups in the uniform Langmuir monolayer phase. The blue-shifted spectral peak has a full width at half-maximum (fwhm) that is about 1.8 times that of the red-shifted peak, consistent with the picture that the −CN group corresponding to the blue-shifted peak is more solvated by water molecules, while the other −CN group is in a more hydrophobic environment. The relative intensity of the two peaks also suggests that the population of the blue-shifted −CN group is about three times that of the red-shifted −CN groups. That is, in every four −CN groups, one is in a more hydrophobic environment and the other three are in a more hydrophilic environment. The average tilt angles of the −CN groups corresponding to the two spectral features are determined to be 50° and 67°, repectively, from the interface normal, using the polarization dependent SFG spectral intensities. This −CN vibrational spectral splitting occurs throughout the entire phase diagram regime of the C18CN monolayer, indicating the coexistence of the distinct intermolecular interactions in the homogeneous liquid-phase region of the Langmuir monolayer film. These observations not only shed new light on understanding the detailed structure and interactions in the molecular monolayer and films but also suggest the importance of the polarization and spectral resolution in the SFG studies.

a challenge, partly due to the lack of tools which can effectively probe the structural detail of the interfacial molecules in the Langmuir film.5 Traditional surface measurement techniques, such as surface pressure and Brewster angle microscopy, can provide macroscopic information on the interactions in the Langmuir monolayer. Spectroscopic tools are ideal for providing information on structure and interaction at the molecular detail. Linear spectroscopic methods, such as infrared internal reflection absorption spectroscopy (IRRAS) and polarization modulated external infrared reflection absorption spectroscopy (PM-IRRAS),11−20 Raman spectroscopy,21−23 UV−visible spectroscopy,24,25 and fluorescence,26−28 have been applied in molecular monolayer studies. However, they are not interface selective, and the information inferred from these experiments is often limited by the lack of sensitivity at the submonolayer level as well as by the spectral resolution. Recently, surface selective and submonolayer sensitive nonlinear spectroscopic techniques, including second harmonic generation (SHG)29−32 and sum frequency generation (SFG),31,33 have been proven as versatile tools for studying the molecular structure and chemical reactions on various surfaces and interfaces with submonolayer sensitivity. Particularly, recent studies have demonstrated that the polarizationresolved SFG-VS is able to provide detailed knowledge on the vibrational spectroscopic and structural details of the molecules at various interfaces.34−42 There have been many studies on the Langmuir monolayer using the SFG-VS technique to directly probe the vibrational spectra of the head and tail groups of the amphiphilic surfactant molecule, as well as that of the substrate water molecule.5,7,8,35,43−48 The interfacial vibrational spectra of the −CN group at air/liquid7,39,49−54 and solid55−57 interfaces have also been a subject of extensive studies using the SFG-VS. Most of these studies on the −CN group focused on the interactions between water and acetonitrile or propanenitrile, the smallest organic nitrile molecules at the air/liquid interface.49−54 For the SFG vibrational spectra of the −CN group in the Langmuir monolayer, there are two studies on the 4″-n-pentyl-4-cyano-p-terphenyl [5CT, or CH3(CH2)4(C6H4)3CN] monolayer by the Shen group35 and on the terminally deuterated 1-cyanoeicosane [C20CN, or CD3(CH2)19CN] monolayer by the Eisenthal group.7 The latter study closely resonated with the studies of Copeland and Harkins in 1942,6 in showing that the consequences of the strong repulsive interactions between the aligned −CN groups might cause reorientation transition. By measuring the SFG vibrational spectra of the −CN group at different surface densities, it was found that the strong repulsion between the aligned −CN head group in the C20CN Langmuir monolayer resulted a ∼10 cm−1 red shift of the −CN spectral peak position and a sharp reorientation of the −CN group at higher surface density around 27 Å2/molecule when the Langmuir monolayer was believed to undergo an orientational phase transition.7 The spectral shift was in good agreement with the −CN spectral shift from a hydrophilic to a hydrophobic environment as observed previously,58−60 and this fact on the −CN functional group has been widely used as the peptide local environment indicator or molecular probe in the biophysical spectroscopic studies.61,62 A recent SFG study on the 5CT, 4″-n-octyl-4-pcyanobiphenyl (8CB) and 4″-n-pentyl-4-p-cyanobiphenyl (5CB) Langmuir monolayers also indicated that the detailed balance between the repulsive interactions between the aligned −CN groups and the attractive interactions between the aligned

2. EXPERIMENT In SFG-VS experiments, two laser beams, one in visible (ω1) and the other in infrared (ω2) frequency regions, are focused on a common spot of a molecular interface with broken inversion-symmetry, generating SFG signals with the frequency of ω = ω1 + ω2. When the infrared (IR) frequency ω2 is tuned across the molecular vibration frequencies, the SFG response reveals the vibrational spectrum of the interface. 2977

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The SFG spectrometer (EKSPLA) used in this work has been described in detail elsewhere.8,71 The 10 Hz and 23 ps visible and IR laser beams were in a copropagation configuration, with the incident angles of 63° ± 1° or 45° ± 1° for the visible and 50° ± 1° or 55° ± 1° for the IR. The wavelength of the visible beam was fixed at 532 nm, and the full range of the IR was tunable from 1000 to 4300 cm−1. The energy of the visible beam was typically less than 300 μJ and that of the IR beam less than 150 μJ around 2200 and 2300 cm−1, and less than 100 μJ in the region in between 3000 and 3800 cm−1. The SFG signal was collected around 61.4° or 46.0° at the reflection geometry, within a small range of about 0.3° which varies depending on the corresponding IR wavelength range in the SFG spectra. To improve the detection efficiency for weak SFG signals, a high-gain low-noise photomultiplier (Hamamatsu, PMT-R585) and a two-channel boxcar average system (Stanford Research Systems) were integrated into the EKSPLA system. The voltage of the R585 was set at 1100 V for the measurement of the cyano vibrational spectra and 1300 V for the measurement of the hydrogen-bonding vibrational spectra. Spectra were recorded with a 2 cm−1 increment and were averaged for 300 laser pulses per point. Each spectrum was repeated at least several times and normalized against the SFG signals from the z-cut α-quartz. The detailed procedure for (2) 2 nomalization and calculation of the |χijk,eff | values was 40,67,69 established in the literature. The spectral resolution was 95%) and chloroform (analytical grade, >99.0% with 0.3−1.0% ethanol) were purchased and used as received from TCI and Beijing Chemical Reagent Inc., respectively. The C18CN monolayer was prepared in a cylindrical Teflon cell (Φ90 × 5 mm) by spreading C18CN/ chloroform solutions (0.1 mM) on the ultrapure water using a microsyringe. Liquid water was the double-distilled water purified with a Millipore Simplicity 185 (18.2 MΩ·cm). All measurements were carried out at controlled room temperature (22.0 ± 0.5 °C) and humidity (40%). To reduce the air flow, the whole experimental setup on the optical table was covered in a plastic housing. The surface pressure vs surface area (π−A) isotherms were acquired by a paper Wilhelmy plate and an electromagnetic sensor (PS4, Nima) in a temperature-controlled Langmuir trough (250 × 65 × 5 mm) made of Teflon. A compression speed of 0.2 mm·s−1 was used in the measurement.

Figure 1. π−A phase diagrams of the C18CN monolayer at the air/ water interface at a room temperature of 22 °C. The arrows indicate the surface densities at which the SFG spectra were measured in this work.

other and the monolayer transits into the liquid-condensed phase region. The molecular densities within the condensed islands in the coexistence region are 35 Å2/molecule, the same as that where the liquid-condensed phase region starts. When the monolayer is further compressed, the surface pressure rises steeply throughout the liquid-phase region before the monolayer collapse at about 17 Å2/molecule. The liquidcondensed phase region between the surface densities of 35 and 17 Å2/molecule has been postulated to be a homogeneous monolayer in previous studies.72 However, different molecular structures can coexist in this seemingly homogeneous film and be resolved by the SFG-VS as described below. 3.2. Resolving the Two Different −CN Spectral Features. The arrows in Figure 1 indicate the surface densities, i.e., 19.0, 22.2, 26.6, 30.2, 34.6, 38.8, and 53.2 Å2/ molecule, at which the SFG-VS spectra of the C≡N vibrations in the C18CN monolayer at air/water interfaces were taken. The SFG data in the following two sets of incident angles are shown in Figure 2, i.e., set I, VIS = 45° and IR = 55° (left panel) and set II, VIS = 63° and IR = 55° (right panel). Three polarization configurations in each set of incident angle were obtained during the measurement, including ssp (S-polarized electric fields for the SFG signals and the visible laser beam, Ppolarized electric field for the IR laser beam), ppp, and sps. Here the S-polarization is defined as the electric field of the light parallel to the sample surface, while the P-polarization is in the plane perpendicular to the surface. As shown below and discussed previously,37 the reason to perform SFG-VS measurement in two sets of incident angles is that the strong incident angle dependence in the ppp spectra can help unambiguously determine the overlapping spectral features. (2) 2 The SFG spectra intensity is proportional to sec2β|χeff |, where β is the angle between the SFG signal direction and the interface normal.35,39,67 Therefore, for set I, β = 61.4°, and for set II, β = 46.0°. In order to compare and calculate the SFG spectra from different experiments and in different experimental configurations, SFG spectra can be presented in absolute values using proper normalization procedures. There are three different ways to present the SFG spectra in absolute (2) 2 (2) 2 (2) 2 values, i.e., sec2β|χeff | , |χeff | , and occasionally |χijk | . Their differences and meanings were discussed in the literature.67 (2) 2 Here all the spectra are presented with their |χeff | values as normalized and calculated using the standard z-cut quartz

3. RESULTS AND DISCUSSION 3.1. Pressure−Area Isotherm of C18CN Langmuir Monolayer. In order to understand the spectral and structural details in a Langmuir monolayer, SFG-VS spectral measurements have to be carried out at different surface densities or coverages, covering both the gas−liquid coexistence and condensed-liquid-phase regions. The π−A phase diagram of the C18CN Langmuir monolayer at air/water interfaces is illustrated in Figure 1. The curve is similar to those measured by Copeland and Harkins for octadecanenitrile [CH3(CH2)16CN]6 and by the Eisenthal group for CD3(CH2)19CN.7 At the area greater than 35 Å2/ molecule, the surface pressure remains constant, corresponding to the gas−liquid coexistence region. In this region, the surface is inhomogeneous owing to the coexistence of isolated C18CN molecules and condensed islands.7 Upon compression, the sizes of these condensed islands gradually grow until the surface density of 35 Å2/molecule where all the islands merge into each 2978

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Figure 2. SFG-VS spectra of CH3(CH2)17CN (C18CN) monolayer at the following surface densities, 19.0, 22.2, 26.6, 30.2, 34.6, 38.8, and 53.2 Å2/ molecule, in the −CN spectral region with two sets of incident angles of VIS = 45° and IR = 55° (left panel) and VIS = 63° and IR = 50° (right panel), respectively. The data for the 30.2, 34.6 Å2/molecule in VIS = 63° and IR = 50° followed the general trend and are not shown. The solid lines are the global fit results using two Lorentzian peaks, whose positions are shown by the two vertical dashed lines. All spectra were normalized to z-cut 2 2 | were calculated accordingly.40,67,69. quartz crystal, and the absolute values for |χeff

crystal susceptibility values. The procedure was first established (2) 2 | values thus by Shen and co-workers.35,39,67 The spectra |χeff obtained as shown in Figure 2 are all consistent with each other. This shall enable quantitative analysis of the spectral intensity and the calculation of the orientation (tilt angles) of the molecular groups. One can see in Figure 2 that the ssp and sps spectra have almost identical spectral shapes in the two sets of incident angles at each surface density, while the ppp spectra are drastically different. Such strong incident angle dependence of the line shapes in the ppp spectra, as well as the no incident angle dependence in the ssp and sps spectra, was systematically investigated in recent literature and well understood.36,37,67 The difference in the ppp spectra in the two sets of incident angles holds the key for resolving the two different −CN groups in the C18CN Langmuir monolayer. This shall be discussed in detail below. After the two −CN peaks are identified, the following Lorentzian line shapes can be used to fit all the spectra as shown in Figure 2. The fitting curves are shown as the solid lines for each spectra. The two peak positions thus obtained are ω1 = 2244.5 ± 0.3 cm−1, Γ1 = 4.8 ± 0.8 cm−1 and ω2 = 2251.1 ± 0.4 cm−1, Γ1 = 8.6 ± 0.4 cm−1.

2

|χijk|2 = χNR +

∑ q

Aq ω IR − ωq + i Γq

(1)

Before discussing what these two different −CN groups suggest about structure and interactions on the C18CN Langmuir monolayer, we need to discuss how the two different −CN groups were identified. Here, the discussion is focused on the SFG data at 19.0 Å2/ molecule, since at this surface density the SFG signals are the highest and are also with the best signal-to-noise ratio. In addition, one can also see in Figure 2 that at each polarization and experimental configuration, the SFG spectra increase as the surface density increases with almost identical trend. This fact suggests that there is little dependence on the surface density for the spectral features as well as the molecular orientation structure in the C18CN monolayer. This fact also suggests that limiting the discussion on the 19.0 Å2/molecule SFG data is adequate to draw conclusions about the two different −CN groups. The SFG data and the various fitting results for the 19.0 Å2/ molecule surface density are shown in Figure 3 and in Table 1. On the left panel in Figure 3, the spectra in ssp, ppp, and sps polarization combinations in the two sets of incident angles are plotted together for comparison. Again, the ssp and sps data 2979

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Figure 3. Comparison of the SFG-VS spectra and fitting results for the surface density of 19.0 Å2/molecule. It is evident in the left panel that while the ssp and sps spectra in the two sets of incident angles have nearly identical shapes, the ppp spectra are drastically different. Such strong incident angle dependence in the ppp spectra, as well as the no incident angle dependence in the ssp and sps spectra, was systematically investigated in the literature.36,37,67 In the right panel, different types of fitting curves for the SFG spectra in ssp, ppp, and sps polarization combinations are shown. The red arrows indicate the spectral region where the single-peak fitting results have significant errors.

Table 1. Fitting Results Using Eq 1 for the SFG-VS Spectra of the −CN Frequency Region at Surface Density of 19.0 Å2/ Molecule fit type global double 45°/55°

63°/55°

global single 45°/55°

63°/55°

individual single 45°/55°

63°/55°

polarization

χNR

ω1 (cm−1)

A1

Γ1 (cm−1)

ssp ppp sps ssp ppp sps

1.47 0.39 0.96 0.97 1.15 0.69

± ± ± ± ± ±

0.06 0.08 0.08 0.06 0.05 0.09

10.4 −7.2 4.3 7.5 −3.0 2.8

± ± ± ± ± ±

4.3 2.6 1.8 3.1 1.6 1.9

2244.5 2244.5 2244.5 2244.5 2244.5 2244.5

± ± ± ± ± ±

0.3 0.3 0.3 0.3 0.3 0.3

4.8 4.8 4.8 4.8 4.8 4.8

± ± ± ± ± ±

0.8 0.8 0.8 0.8 0.8 0.8

ssp ppp sps ssp ppp sps

1.40 0.52 0.90 0.91 1.15 0.71

± ± ± ± ± ±

0.04 0.07 0.07 0.05 0.06 0.08

60.7 −27.5 21.3 40.8 −6.3 21.3

± ± ± ± ± ±

0.09 0.7 1.0 0.7 1.5 1.0

2249.1 2249.1 2249.1 2249.1 2249.1 2249.1

± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.2 0.2

9.8 9.8 9.8 9.8 9.8 9.8

± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.2 0.2

ssp ppp sps ssp ppp sps

1.43 −0.10 0.99 0.78 1.15 0.62

± ± ± ± ± ±

0.08 0.11 0.06 0.08 0.01 0.04

59.4 −32.0 19.5 45.6 −5.8 22.1

± ± ± ± ± ±

1.7 2.0 1.7 1.6 0.4 0.7

2249.0 2244.3 2248.1 2249.7 2245.2 2250.2

± ± ± ± ± ±

0.3 1.0 0.6 0.5 0.4 0.3

9.5 11.1 9.3 11.1 7.4 9.8

± ± ± ± ± ±

0.3 0.9 0.8 0.4 0.6 0.3

ω2 (cm−1)

A2 43.9 −17.2 14.0 28.8 −2.2 16.7

± ± ± ± ± ±

5.8 3.0 2.8 4.1 2.1 2.8

2251.1 2251.1 2251.1 2251.1 2251.1 2251.1

± ± ± ± ± ±

0.4 0.4 0.4 0.4 0.4 0.4

Γ2 (cm−1) 8.6 8.6 8.6 8.6 8.6 8.6

± ± ± ± ± ±

0.4 0.4 0.4 0.4 0.4 0.4

However, while the ppp spectra in VIS = 45° and IR = 55° clearly show a peak around 2244 cm−1, the ppp spectra in VIS = 63° and IR = 50° do not show a clear peak. Since all the SFG

show that the line shape is identical for the two sets of incident angles, while the ppp spectra are drastically different. Here, both the ssp and sps spectra are peaked at about 2252 cm−1. 2980

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spectra peaks interfere with the nonresonant background χNR as in eq 1, the apparent peak positions may not be the actual peak positions. In order to identify the peak positions, fitting the spectra using proper line shape equations, such as in eq 1, is necessary. However, it has been known that critical analysis of the fitting results is often needed. Especially with multiple peaks and spectral interferences, the multiple peak fitting may not generate unique fitting parameters.73 In the right panel in Figure 3, one can see that the fittings for the ssp, ppp, and sps data in the two sets of incident angles with two peaks (labeled as “global with double peaks” or “global double” in Table 1) are evidently better, even though fitting with single peak (labeled as “global with single peak” or “global single” in Table 1) seems also capture the main features of the spectra reasonably well. In most cases, even for the ppp spectra in VIS = 45° and IR = 55° where an apparent peak is different from those of the ssp and sps spectra, the single-peak global fit with ω1 = 2249.1 ± 0.2 cm−1, Γ1 = 9.8 ± 0.2 cm−1 may pass as a reasonably good fit, given the fact that the ppp intensity is relatively small and the data points are with experimental errors. These fitting parameters are listed in Table 1. However, close examination reveals a different story. One important fact is that fitting each of the spectra separately with single peak (labeled as “with single peak” or “individual single” in Table 1) revealed that the two ppp spectra consistently resulted in peak positions around ω1 = 2244−2245 cm−1, while the ssp and sps spectra resulted in peak positions around ω1 = 2248−2250 cm−1, and the half-width value Γ varies in the range of 7.4−11.1 cm−1. Without the ppp spectra, one would have easily concluded from the ssp or sps spectra that there is only one spectral peak around 2249 cm−1. However, the incident angle dependence of the ppp spectra provides convincing evidence otherwise. Not only do the VIS = 45° and IR = 55° ppp spectra exhibit a distinctive peak position other than the apparent ssp and sps peaks position, the fitting of both ppp spectra in two different sets of incident angles resulted in a robust peak position which is about 5 cm−1 from the ssp and sps peak position. The close comparison of the different fittings for the ppp spectra is shown in Figure 4. It is evident that the simplest single-peak assumption fails and the second simplest two peak assumption gives better fit and consistent fitting results, as shown in Table 1. Therefore, we conclude that there are actually two closely overlapped spectral peaks from the polarization and incident angle dependent SFG data. The global double-fitting results give the following two peaks values, i.e., ω1 = 2244.5 ± 0.3 cm−1, Γ1 = 4.8 ± 0.8 cm−1 and ω2 = 2251.1 ± 0.4 cm−1, Γ1 = 8.6 ± 0.4 cm−1. Because the difference between the two peak positions is only about 6 cm−1, while the spectral full width at halfmaximum (fwhm = 2Γ) is two or more times of this splitting, if only a single spectra is considered (usually a single ssp spectrum for the dielectric interfaces or a single ppp spectra for the metal surfaces), one would not be able to identify the existence of two peaks and to resolve the two peaks, given the uncertainty and nonuniqueness of the fitting parameters in multiple-peak fitting.73 Therefore, multiple polarization and incident angle measurement is crucial for resolving the details in SFG spectral features. In the early SFG-VS studies of the −CN group spectra of the CD3(CH2)19CN (d1-C20CN) Langmuir monolayer,7 only one spectral feature for the −CN group was considered. Even though the d1-C20CN −CN spectrum in principle would be similar to that of C18CN, without performing the same

Figure 4. Comparison of the single-peak and double-peak fittings of the ppp spectra in incident angles of VIS = 45°, IR = 55° and VIS = 63°, IR = 55°. The global fit of the six spectra, as in Figure 3, with single peak (bottom panel) failed significantly to capture the line shape and peaks in the experimental data. The single-peak fit of individual ppp spectra (middle panel) is better than the global fit with single peaks. However, as shown in Table 1, the ppp spectra in both incident angles showed a 5 cm−1 peak position shift from the ssp and sps spectra fitting results. These evidence strongly suggest the failure of the single-peak fittings. The bigger errors in the global single-peak fitting evidently came from forcing the different spectra to generate the same peak position and width. In comparison, the global fitting with two peaks not only captures the line shape and the peaks in the ppp spectra but also provides consistent and stable fitting parameters as shown in the Table 1. The blue and red arrows are drawn in the figure to indicate the spectral region with significant errors between the fitting curves and the SFG data (shown as connected dots).

polarization and incident angle measurement on the d1C20CN Langmuir monolayer, one cannot make the conclusion that the d1-C20CN SFG spectral would definitely have the same spectral splitting features. However, recently Ding et al. showed that the −CH3 SFG spectral of the air/acetonitrile interface exhibited two distinctive peak positions separated by about 14 cm−1 in the ssp and ppp SFG spectra, while the spectral resolution in this report was 14−18 cm−1.54 In comparison, in our experiment the spectral resolution is known as