Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (6): 650-656

The article information

Li-bo Zhang, Hui Fang, Shun-li Chen, Xue-feng Zhu, Wei Gan
张立波, 房辉, 陈顺利, 朱雪峰, 干为
Orientation Angle of Molecules at Hexadecane-Water Interface Studied with Total Internal Reflection Second Harmonic Generation
全内反射构型下分子在十六烷-水界面取向角度的二次谐波测量
Chinese Journal of Chemical Physics, 2016, 29(6): 650-656
化学物理学报, 2016, 29(6): 650-656
http://dx.doi.org/10.1063/1674-0068/29/cjcp1605111

Article history

Received on: May 17, 2016
Accepted on: July 4, 2016
Orientation Angle of Molecules at Hexadecane-Water Interface Studied with Total Internal Reflection Second Harmonic Generation
Li-bo Zhanga,b, Hui Fanga,b, Shun-li Chena, Xue-feng Zhua, Wei Gana     
Dated: Received on May 17, 2016; Accepted on July 4, 2016
a. Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute of Physics & Chemistry, Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China;
b. University of Chinese Academy of Sciences, Beijing 100049, China
*Author to whom correspondence should be addressed. Wei Gan, E-mail:ganwei@ms.xjb.ac.cn, Tel.:+86-991-3677875
Abstract: The orientation angle is an important parameter that reflects the structure of molecules at interfaces. In order to obtain this parameter, second order nonlinear spectroscopic techniques including second harmonic generation (SHG) and sum frequency generation-vibrational spectroscopy (SFG-VS) have been successfully applied through analysis of the nonlinear signal from various polarizations. In some SHG and SFG-VS experiments, total internal reflection (TIR) configuration has been adopted to get enhanced signals. However, the reports on the detailed procedure of the polarization analysis and the calculation of the orientation angle of interfacial molecules under TIR configuration are still very few. In this paper, we measured the orientation angles of two molecules at the hexadecane-water interface under TIR and Non-TIR experimental configurations. The results measured from polarization analysis in TIR configuration consist with those obtained from Non-TIR configuration. This work demonstrates the feasibility and accuracy of polarization analysis in the determination of the orientation angle of molecules at the interfaces under TIR-SHG configuration.
Key words: Second harmonic generation    Total internal reflection    Hyperpolarizability    Polarization    Orientation angle    
Ⅰ. INTRODUCTION

Molecules at interfaces behave differently from those in the bulk phase because of the unbalanced interactions at an environment with the breaking of inversion symmetry. The geometrical arrangement of molecules at an interface, which we refer to as orientation structure, is closely related to the chemical, physical, and biological properties of interfaces. Examples can be the orientation dependent intermolecular energy transfer [1] and the chemical reactions involving DNA at interfaces [2]. So the obtaining of the orientation geometry of molecules at interfaces is very important and will help us learn more about the nature of interfaces.

There are some experimental approaches capable of interface investigations, such as X-ray photoelectron spectroscopy [3], scanning tunnel microscope (STM) [4], X-ray reflectivity study [5], small angle neutron scattering [6], and atomic force microscopy (AFM) [7]. Second Harmonic generation (SHG) and sum frequency generation-vibrational spectroscopy (SFG-VS), as interfacial selective second order nonlinear optical technologies [8-10], have also been widely used in the investigation of interfacial molecules, including air/liquid interface [11-15], air/solid interface [16, 17], liquid/solid interface [18, 19], liquid/liquid interface [20, 21], and even solid/solid interface [22].

As has been demonstrated, SFG-VS can be used to selectively probe the vibrational modes of a functional group and get the orientational structure of the molecular group. For SHG, it is generally used to investigate the orientation of the main axis of certain chromophore in interfacial molecules by probing its electronic response [23]. Since SHG and SFG-VS signals are generated from a thin interfacial layer from second order nonlinear optical process, the emission is so low that weak signal detection technique needs to be used to get decent single-to-noise ratio for data analysis. It has been demonstrated, with proper adjustment of the incident angle of the pumping lasers, second order nonlinear signal from an interface in the TIR experimental geometry can be enhanced by orders magnitudes compared with the Non-TIR experimental geometry [24, 25]. This method can also improve the accurate detecting of weak nonlinear signals and benefit the interfacial investigations. For examples, the Richmond’s and the Girault’s groups used TIR-SHG experimental configuration to analyze the molecular polarizability and adsorption/transfer of ionic species at liquid/liquid interface [26, 27]. The Teramae’s group used TIR-SHG to investigate the association of molecules at liquid/liquid and solid/liquid interfaces [28]. However, polarization analysis and the calculation of the orientation angle of interfacial molecules under TIR configuration are still not clearly demonstrated and verified in previous literatures.

In this work, we investigated the orientation structures of two dye molecules at the hexadecane-water interface under TIR and Non-TIR experimental geometries. To get decent signals for the polarization analysis at both experimental geometries, we used two molecules with relatively high hy-perpolarizabilities, namely, 4-(4-diethylaminostyryl)-1-methylpyridinium-iodide (D289) and 4′-(n-butyloxyl)-azobenzene-4-sulphonic acid (AZO), which both have similar rod-like structures (Fig. 1). We found the results measured from polarization analysis in TIR configuration were essentially the same as those obtained from Non-TIR configuration. We also found the adsorbed D289 and AZO molecules have very stable orientation angles for solution with various concentrations, 57±2° for D289 and 50±2° for AZO, respectively.

FIG. 1 Molecular structures of D289 and AZO.
Ⅱ. POLARIZATION ANALYSIS OF THE SHG INTENSITY

The basic theory of SHG and SFG-VS has been described in many previous reports [23, 29-35]. It is known that the second order nonlinear light scattered from the interface comes from nonlinear polarization of the interface which is induced by the electromagnetic field of the pumping laser. When the fundamental laser with frequency ω interacts with interface, a second order nonlinear polarization P (2)(2ω) is generated [32, 36].

$P^{(2)} (2\omega)=\chi_{\textrm{eff}}^{(2)} (\omega):E(\omega)E(\omega)$ (1)

here $\chi^{(2)}_{\textrm{eff}}$ ( $\omega$ ) is the effective second-order nonlinear susceptibility of the interface. $E$ ( $\omega$ ) is the electric field of the pumping laser. Under the electric-dipole approximation, the interfacial polarization sheet is the dominating source of the second harmonic radiation in the reflected direction. The SHG intensity $I$ (2 $\omega$ ) is given by [9]:

$I(2\omega)=\frac{32\pi^3 \textrm{sec}^2 \beta}{c_0^3 n_1 (\omega) n_1 (\omega) n_1 (2\omega)} |\chi_{\textrm{eff}}^{(2)} |^2 I^2 (\omega)$ (2)

where $n_1$ ( $\omega$ ) and $n_1$ (2 $\omega$ ) are refractive indexes of medium 1 at frequency $\omega$ and 2 $\omega$ , respectively, $c_0$ is the speed of light in vacuum, $\beta$ is the incident angle of the input laser (Fig. 2(a)), $I$ ( $\omega$ ) is the intensity of the fundamental laser. The effective nonlinear susceptibility $\chi^{(2)}_{\textrm{eff}}$ ( $\omega$ ) can be express as [23, 32]:

$\chi_{\textrm{eff}}^{(2)}\hspace{-0.1cm}=\hspace{-0.1cm}[\hat e(2\omega)\hspace{-0.05cm}\cdot \hspace{-0.05cm} \textbf{L}(2\omega)]\cdot \chi^{(2)}\hspace{-0.05cm}:\hspace{-0.05cm}[\hat e(\omega)\cdot \textbf{L}(\omega)][\hat e (\omega)\hspace{-0.05cm}\cdot\hspace{-0.05cm} \textbf{L}(\omega)]$ (3)
FIG. 2 Diagrammatic sketches of (a) the SHG emission and (b) Euler transformation. x, y, z are the lab coordinates and a, b, c are the molecular coordinates.

with $\hat e$ ( $\Omega$ ) being the unit polarization vector at frequency $\Omega$ , $\chi^{(2)}$ being the macroscopic susceptibility of the interfacial molecules at a certain polarization component. Here $\chi^{(2)}$ is a third rank tenser with 27 elements, which reflects the response of molecules to a electromagnetic stimulation. L( $\Omega$ ) are the Fresnel factors at frequency $\Omega$ , which are formulated as [29, 36]

${{\boldsymbol{L}}_{xx}}(\Omega ) = \frac{{2{n_1}(\Omega ){\text{cos}}\gamma }}{{{n_1}(\Omega ){\text{cos}}\gamma + {n_2}(\Omega ){\text{cos}}\beta }}$ (4)
${{\boldsymbol{L}}_{yy}}(\Omega ) = \frac{{2{n_1}(\Omega ){\text{cos}}\beta }}{{{n_1}(\Omega ){\text{cos}}\beta + {n_2}(\Omega ){\text{cos}}\gamma }}$ (5)
${{\boldsymbol{L}}_{zz}}(\Omega ) = \frac{{2{n_1}{{(\Omega )}^2}{n_2}(\Omega ){\text{cos}}\beta }}{{{n_1}(\Omega ){\text{cos}}\gamma + {n_2}(\Omega ){\text{cos}}\beta }}\frac{1}{{n'{{(\Omega )}^2}}}$ (6)

In above equations, n'( $\Omega$ ) is called as the refractive index of interfacial layer at frequency of $\Omega$ . $\gamma$ is the refractive angle of the pumping laser shown in Fig. 2(a).

For an azimuthally isotropic interface, there are seven non-vanishing components of $\chi^{(2)}$ , while only three of them are independent for SHG experiment. With the lab coordinates chosen such that $z$ is along the interface normal and $x$ in the incidence plane (Fig. 2(b)), they are $\chi_{zzz}$ , $\chi_{zxx}$ = $\chi_{zyy}$ and $\chi_{xzx}$ = $\chi_{yzy}$ = $\chi_{xxz}$ = $\chi_{yyz}$ . These three independent components can be deduced by measuring the SHG signal at three different input and output polarization combinations, namely sp, pp and 45°s (here s indicates the polarization of the light's electric field perpendicular to the incidental plane, i.e. the $xz$ plane, while p parallel to it. 45° is the polarization rotated clockwise from p direction when facing the coming laser). The effective nonlinear susceptibilities under these three polarization combinations can be expressed as [33, 37]:

$\chi _{{\text{eff}}, {\text{sp}}}^{(2)} = {L_{zz}}(2\omega )L_{yy}^2(\omega ){\text{sin}}\beta {\chi _{zxx}}$ (7)
$\chi _{\text{eff},{{45}^{{}^\circ }}\text{s}}^{(2)}={{L}_{yy}}(2\omega ){{L}_{zz}}(\omega ){{L}_{yy}}(\omega )\text{sin}\beta {{\chi }_{xzx}}$ (8)
$\begin{gathered} \chi _{{\text{eff}}, {\text{pp}}}^{(2)} = {L_{zz}}(2\omega )L_{xx}^2(\omega ){\text{sin}}\beta {\text{co}}{{\text{s}}^2}\beta {\chi _{zxx}} - \hfill \\ 2{L_{xx}}(2\omega ){L_{zz}}(\omega ){L_{xx}}(\omega ){\text{sin}}\beta {\text{co}}{{\text{s}}^2}\beta {\chi _{xzx}} + \hfill \\ {L_{zz}}(2\omega )L_{zz}^2(\omega ){\text{si}}{{\text{n}}^3}\beta {\chi _{zzz}} \hfill \\ \end{gathered} $ (9)

The orientation information of interfacial molecules can be expressed based on the relationship between the lab coordinates and molecular coordinates, through an Euler transformation as shown in Fig. 2(b) [38, 39]. As to the rod-like molecules used in this work, only $\alpha^{(2)}_{ccc}$ (the molecular polarizability along $c$ -direction, Fig. 2(b)) needs to be considered [40, 41]. With the orientation angles of molecules at the hexadecane-water interface expressed as the tilt angle $\theta$ , it can be related to three independent non-vanishing components $\chi_{zzz}$ , $\chi_{zxx}$ and $\chi_{xzx}$ as [42]:

${\chi _{zxx}} = {\chi _{zyy}} = \frac{1}{2}{N_s}\alpha _{ccc}^{(2)}(\langle {\text{cos}}\theta \rangle - \langle {\text{co}}{{\text{s}}^3}\theta \rangle )$ (10)
$\begin{gathered} {\chi _{yyz}} = {\chi _{xxz}} = {\chi _{yzy}} = {\chi _{xzx}} \hfill \\ = \frac{1}{2}{N_s}\alpha _{ccc}^{(2)}(\langle {\text{cos}}\theta \rangle - \langle {\text{co}}{{\text{s}}^3}\theta \rangle ) \hfill \\ \end{gathered} $ (11)
${\chi _{zzz}} = {N_s}\alpha _{ccc}^{(2)}\langle {\text{co}}{{\text{s}}^3}\theta \rangle $ (12)

Considering a $\delta$ -function distribution for $\theta$ , we then get the orientation angle of molecules with the form of

$\theta= \frac{180}{\pi} \textrm{arccos}\sqrt{\frac{\chi_{zzz}/\chi_{xzx}}{2+\chi_{zzz}/\chi_{xzx}}}$ (13)

It has been noticed that if other orientational distribution functions, such as the Gaussion distribution, were applied, the calculated average orientation angle of interfacial molecular groups would change with the changed orientational distribution, as detailed by the "magic angle" plot [10, 43]. We also demonstrated that in some cases, the average orientation angle can be deduced with higher accuracy when the orientational distribution of interfacial molecules is relatively narrow, based on the polarization analysis of single or multiple vibrational modes within interfacial molecules. In the absence of other information, a $\delta$ -function distribution can be used in calculating the orientation angle of interfacial molecules [56].

Ⅲ. EXPERIMENTS A. Materials

Deionized water (DI water, 18.25 M $\Omega$ $\cdot$ cm) was prepared from a water purification system (Water Purifier, WP-UP-UV-20, Sichuan Water Technology Development Co. Ltd., China). Hexadecane (99%, Sigma-Aldrich) was purified by six passes through basic alumina columns with procedures described in previous reports [44-46]. D289 ( $\geq$ 97%, Sigma-Aldrich) was used as received. AZO was synthesized as reported [47, 48]. $^1$ H NMR (400 Hz, DMSO-d $_6$ , $\delta$ /ppm): 0.90 (t, 3H), 1.44 (m, 2H, CH $_2$ ), 1.71 (m, 2H, CH $_2$ ), 4.06 (t, 2H, CH $_2$ ), 7.09 (d, 2H, CH), 7.74 (m, 4H, CH), 7.85 (d, 2H, CH). ESI-MS: $m$ / $z$ =332.75 (M-1).

Glassware were cleaned with piranha solutions (H $_2$ O $_2$ :H $_2$ SO $_4$ with a volume ratio of 3:7), which are strongly oxidized, then thoroughly rinsed with DI water and dried before each experiment.

B. SHG setup

The setup for SHG measurements has been described elsewhere [14, 21, 45]. A broadband Ti:sapphire oscillator laser (Coherent, Mira-900f) with a pulse width of approximately 130 fs and a repetition rate of 76 MHz was used. The centered wavelength of the fundamental laser was 810 nm. The room temperature was 22±1° during the experiment.

A neutral density filter was used to control the energy of the incidental laser. The polarization of the incidental laser was controlled by a polarizing cube beam splitter (Thorlabs, PBS202, 620-1000 nm, extinction ration 1:1000) and a half-wave plate (Thorlabs, WPH05M-808, 808 nm). The half-wave plate was fixed in a rotation stage controlled by a T-Cube DC Servo Motor Controller (Thorlabs, TDC 001) to perform the polarization dependent measurements. A high pass filter ( $>$ 750 nm) was used to eliminate the second harmonic light produced in the laser system or other former optics. The reflected SHG signal from the interfaces at 405 nm} wavelength was directed through a filter (Thorlabs, BG-39, 300-600 nm) to remove the residual light at 810 nm and another polarizing cube beam splitter (Thorlabs, PBS201, 420-680 nm, extinction ration 1:1800 at 405 nm). The signal was focused into a monochromator (Andor SR-500I) and detected by a photomultiplier tube (PMT, Hamamatsu R-1527p). The output of the PMT was amplified by a factor of five with a preamplifier (Stanford Research, SR450A) and recorded by a photon counter (Stanford Research, SR400).

The power of incidental laser was 50 mW for D289 experiments and 100 mW for AZO experiments under both TIR and Non-TIR configurations. The PMT voltages were set as -1000 V in all experiments. The typical dark noise level was 1-2 counts per second.

C. TIR and Non-TIR configurations in SHG measurements

For SHG experiment with 810 nm fundamental laser, the total internal reflection angle for the hexadecane-water interface is 69.4°. In our lab, a square cell made from fused quartz (9.4 cm×5.2 cm×3.4 cm, with 25 mL water as the lower phase and 45 mL hexadecane as the higher oil phase) shown in Fig. 3(a) was used to form a TIR geometry for measuring the SHG emission from the hexadecane-water interface. The incident angle of the fundamental laser was approximately 70°, slightly larger than the total internal reflection angle for this interface. For Non-TIR SHG experiment, a cylindrical cell (radium 2.1 cm, length 4.3 cm, filled with 25 mL water and 25 mL hexadecane) as shown in Fig. 3(b) was used. The incident angle of the fundamental laser was then 60±1°.

FIG. 3 Side view of the SHG experimental configures. (a) TIR geometry and (b) Non-TIR geometry.
Ⅳ. RESULTS AND DISCUSSION A. Determination of the relative phase of SHG fields emitted from various interfaces and various polarization combinations

In SHG experiments, D289 and AZO molecules were dissolved in DI water and adsorbed at the hexadecane-water interface. Experiments were conducted after the addition of probe molecules in the water phase and a gentle mixing of the water solution. Typically a waiting time beyond 0.5 h was needed for the molecules to adsorb at the interface and reach a balance, which was observed as a stable SHG emission.

In order to get the orientation angles of the adsorbed molecules, the phase of the effective nonlinear susceptibilities of the interface at various polarization directions needs to be obtained from polarization analysis. The relative phase of the effective nonlinear susceptibilities of the interfaces with adsorbed probe molecules can be determined with the 45° detecting approach [10, 35]. The 45° detected SHG signals can be expressed as:

$\begin{align} & {{I}_{\alpha \text{-in},{{45}^{{}^\circ }}\text{-out}}}(2\omega )\propto |{{\chi }_{\text{eff},{{45}^{{}^\circ }}}}{{|}^{2}} \\ & =\frac{1}{2}|{{\chi }_{\text{eff},\text{pp}}}\text{co}{{\text{s}}^{2}}\alpha +{{\chi }_{\text{eff},\text{sp}}}\text{si}{{\text{n}}^{2}}\alpha + \\ & {{\chi }_{\text{eff},{{45}^{{}^\circ }}\text{s}}}\text{sin}2\alpha {{|}^{2}} \\ \end{align}$ (14)

here $\alpha$ is the polarization angle of the incidental laser, 0° corresponds to the direction of p polarization. $\chi_{\textrm{eff, pp}}$ , $\chi_{\textrm{eff, sp}}$ and $\chi_{\textrm{eff}, 45^{\circ}\textrm{s}}$ are macroscopic susceptibilities, which are proportional to the SHG signals at pp, sp, and 45°s, respectively. At the same time, we could deduce from Eq.(14) [42, 49]:

${{\left( \frac{\partial {{I}_{\alpha \text{-in},{{45}^{{}^\circ }}\text{-out}}}(2\omega )}{\partial \alpha } \right)}_{\alpha ={{0}^{{}^\circ }}}}\propto {{\chi }_{\text{eff},{{45}^{{}^\circ }}\text{s}}}{{\chi }_{\text{eff},\text{pp}}}$ (15)
${{\left( \frac{\partial {{I}_{\alpha \text{-in},{{45}^{{}^\circ }}\text{-out}}}(2\omega )}{\partial \alpha } \right)}_{\alpha ={{90}^{{}^\circ }}}}\propto -{{\chi }_{\text{eff},{{45}^{{}^\circ }}\text{s}}}{{\chi }_{\text{eff},\text{sp}}}$ (16)

So, by fitting the 45° detected curve, and deciding the slopes at incident polarization at $\alpha$ =0° and 90°, we could know the phase information between $\chi_{\textrm{eff, pp}}$ , $\chi_{\textrm{eff, sp}}$ and $\chi_{\textrm{eff}, 45^{\circ}\textrm{s}}$ . The results from the SHG measurements are shown in Fig. 4.

FIG. 4 Incident laser polarization dependence measurements of the SHG signal at 45° polarization from the hexadecane-water interface with (a) D289 adsorption and (b) AZO adsorption. The curves were measured under NonTIR configuration.

According to Eq.(15) and Eq.(16), we know that, for the interface with D289 adsorption and the interface with AZO adsorption, their respective three effective nonlinear susceptibilities, $\chi_{\textrm{eff, pp}}$ , $\chi_{\textrm{eff, sp}}$ and $\chi_{\textrm{eff}, 45^{\circ}\textrm{s}}$ , have the same signs.

B. Polarization measurements of the SHG signals from interfaces under TIR and Non-TIR experimental geometries

To get the relative strengths of nonlinear susceptibility tensors $\chi_{\textrm{eff, pp}}$ , $\chi_{\textrm{eff, sp}}$ and $\chi_{\textrm{eff}, 45^{\circ}\textrm{s}}$ of the interface with D289 adsorption and those for the interface with AZO adsorption, we set the detected polarization directions as p and s, respectively, and changed the polarization directions of the incidental laser from 0° to 360°. The obtained data were fitted with the following equations [49, 50] and plotted in Fig. 5.

${I_{\alpha {\text{ - in}}, {\text{p - out}}}}(2\omega ) \propto |{\chi _{{\text{eff}}, {\text{p}}}}{|^2} = |{\chi _{{\text{eff}}, {\text{pp}}}}{\text{co}}{{\text{s}}^2}\alpha + {\chi _{{\text{eff}}, {\text{sp}}}}{\text{si}}{{\text{n}}^2}\alpha {|^2}$ (17)
${{I}_{\alpha \text{-in},\text{s-out}}}(2\omega )\propto |{{\chi }_{\text{eff},\text{s}}}{{|}^{2}}=|{{\chi }_{\text{eff},{{45}^{{}^\circ }}\text{s}}}\text{sin}2\alpha {{|}^{2}}$ (18)
FIG. 5 The incident laser polarization dependence of the SHG intensity detected at s (red) and p (blue) polarization directions from various interfaces under TIR (a, c) and Non-TIR (b, d) geometries. The signals are emitted from the interface with D289 adsorption (a, b) and AZO adsorption (c, d), respectively. The concentrations for D289 and AZO in water phase were 50 and 200 µmol/L, respectively.

Compared with the interfaces adsorbed with D289 or AZO molecules, the SHG signals obtained before D289 or AZO adsorption were so small ( < 1%) that it can be ignored. Based on the p and s measurement shown above, the relative strength of the adsorbed D289 interface and the adsorbed AZO interface can be deduced as listed in Table Ⅰ.

Table Ⅰ Fitted values of the three nonlinear susceptibility tensors from Fig. 5.
C. The orientation analysis of interfacial molecules with data from TIR and Non-TIR geometries

The refractive indices of water at 400 and 800 nm wavelengths are 1.34 and 1.33 [51], respectively. These values are used for the orientation analysis in this work because we found that the small changes in the refractive index at slightly different wavelengths barely change the deduced orientation angle of interfacial molecules. Also, for hexadecane, the value of 1.43 at 589 nm was used in our analysis [52]. For the TIR configuration with $\beta$ =70°, from $n_1$ sin $\beta$ = $n_2$ sin $\gamma$ , we have sin $\gamma$ =1.014, cos $\gamma$ =0.17i. So the Fresnel factors for TIR configuration are imaginary numbers. For Non-TIR configuration in our experiment, $\beta$ =60°, sin $\gamma$ =0.935, cos $\gamma$ =0.355, the Fresnel factors are real numbers. Here, the modulus of the Fresnel factors are used for the orientation analysis as has been suggested [24].

Another important parameter in the orientation analysis is the reflective index n', which is relatively hard to determine [10, 33, 39, 53, 54]. The refractive index of one of the bulk phases, or values from certain experimental estimations have been used [30, 55, 56]. Some groups also developed a self-consistent approach to calculate the value of n' [57]. As discussed in previous reviews [23, 33], this parameter should not be simply treated as the refractive index of the interfacial layer. On the other hand, it reflects the ratio of the local field factors at the interface [10, 23, 33]. For this reason, we chose to use the value of 1.45, which has been used for molecular layers containing chromospheres with two or three phenyl groups [33]. The influence of different n' values is further discussed latter.

The relative ratios of $\chi_{zzz}$ , $\chi_{zxx}$ , and $\chi_{xzx}$ deduced from Table Ⅰ and Eqs.(7)-(9) was listed in Table Ⅱ. From Table Ⅱ and Eq.(13), the orientation angles for interfacial D289 molecules measured from TIR geometry and Non-TIR geometry are 58±2° and 56±2°, respectively; the orientation angles for interfacial AZO molecules measured from TIR geometry and Non-TIR geometry are 51±3° and 48±3°, respectively. It is very clear that the orientation angle of interfacial molecules measured from TIR geometry and Non-TIR geometry are very close to each other. This proves the feasibility of polarization analysis of the SHG signals at TIR experimental configuration. Based on those results, the detailed procedure applied in this work can be readily applied for orientation analysis of molecules with much lower nonlinear efficiency at liquid-liquid interface or solid-liquid interface with the choosing of TIR experimental geometry when the signal from experiments at Non-TIR geometry is too low to be detected.

Table Ⅱ Ratio of the three independent non-vanishing components $\chi_{zzz}$ , $\chi_{zxx}$ , and $\chi_{xzx}$ of the interface with D289 adsorption and the interface with AZO adsorption.

We also performed above experiments at the hexadecane-water interface with the adsorption of the two probe molecules at other concentrations (10 and 30 μmol/L for D289, and 50 and 400 μmol/L for AZO). It was found that the orientation angle of the two molecules only has a minor difference (less than 3°) at various concentrations. It shows that the two probe molecules have very stable orientation structures at the hexadecane-water interface.

In our analysis only the dominant hyperpolarizability tenser $\alpha^{(2)}_{ccc}$ of the molecules was considered. We also included $\alpha^{(2)}_{caa}$ and calculated the orientation angles, as discussed in previous reports [31]. It was found that the changes in the orientation angles induced by considering $\alpha^{(2)}_{caa}$ are within the experimental uncertainties for both molecules. It is also known that the value of interfacial refractive index n' influences the calculated orientation angle. We used values close to 1.45 and tested the obtained results. It was found that although the absolute values of the orientation angles are subjected to change at various n' values, the results obtained from the TIR SHG experiments are all consistent with that obtained from the Non-TIR experiments. This confirms the reliability of the polarization analysis at both TIR and Non-TIR experimental geometries.

Ⅴ. CONCLUSION

The orientation angles of two molecules, namely, 4-(4-diethylaminostyryl)-1-methy methylpyridinium-iodide (D289) and 4'-(n-butyloxyl)-azobenzene-4-sulphonic (AZO) adsorbed at the hexadecane-water interface were measured with second harmonic generation at both TIR geometry and Non-TIR geometry. It was found that the results obtained from the two sets of experimental geometries consisted with each other. The validity and accuracy of the polarization analysis and molecular orientation analysis under TIR geometry were demonstrated. The orientation angles of the two molecules adsorbed at the interface were found to be independent of their concentrations in the water phase within the ranges studied.

Ⅵ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.21273277 and No.21403293), the 1000 Talent Program (the Recruitment Program of Global Experts), and the Young Creative Sci-Tech Talents Cultivation Project of Xinjiang Uyghur Autonomous Region (No.2013711016).

Reference
[1] S. Saini, G. Srinivas, and B. Bagchi, J. Phys. Chem. B 113 , 1817 (2009). DOI:10.1021/jp806536w
[2] J. I. Dadap, and K. B. Eisenthal, J. Phys. Chem. B 118 , 14366 (2014). DOI:10.1021/jp507834s
[3] S. Ghosal, J. C. Hemminger, H. Bluhm, B. S. Mun, E. L. D. Hebenstreit, G. Ketteler, D. F. Ogletree, F. G. Requejo, and M. Salmeron, Science. 307 , 563 (2005). DOI:10.1126/science.1106525
[4] Y. Wang, H. Xu, H. Wang, S. Li, W. Gan, and Q. Yuan, RSC Adv. 4 , 20256 (2014). DOI:10.1039/c3ra46651e
[5] D. M. Mitrinovic, Z. Zhang, S. M. Williams, Z. Huang, and M. L. Schlossman, J. Phys. Chem. B 103 , 1779 (1999). DOI:10.1021/jp984640o
[6] G. Alvarez, J. Jestin, J. F. Argillier, and D. Langevin, Langmuir 25 , 3985 (2009). DOI:10.1021/la802736c
[7] J. Zhang, P. Chen, B. Yuan, W. Ji, Z. Cheng, and X. Qiu, Science 342 , 611 (2013). DOI:10.1126/science.1242603
[8] G. L. Richmond, Chem. Rev. 102 , 2693 (2002). DOI:10.1021/cr0006876
[9] K. B. Eisenthal, Chem. Rev. 106 , 1462 (2006). DOI:10.1021/cr0403685
[10] H. F. Wang, L. Velarde, W. Gan, and L. Fu, Annu. Rev. Phys. Chem. 66 , 189 (2015). DOI:10.1146/annurev-physchem-040214-121322
[11] D. Zhang, J. Gutow, and K. B. Eisenthal, J. Phys. Chem. 98 , 13729 (1994). DOI:10.1021/j100102a045
[12] D. E. Gragson, B. M. McCarty, and G. L. Richmond, J. Am. Chem. Soc. 119 , 6144 (1997). DOI:10.1021/ja962277y
[13] S. Sun, C. Tian, and Y. R. Shen, Proc. Natl. Acad. Sci. 112 , 5883 (2015). DOI:10.1073/pnas.1505438112
[14] Y. Niu, K. Tian, W. Gan, and S. Ye, J. Mol. Liq. 219 , 111 (2016). DOI:10.1016/j.molliq.2016.03.025
[15] L. Fu, S. L. Chen, W. Gan, and H. F. Wang, Chin. J. Chem. Phys. 29 , 70 (2016). DOI:10.1063/1674-0068/29/cjcp1512248
[16] G. J. Holinga, R. L. York, R. M. Onorato, C. M. Thompson, N. E. Webb, A. P. Yoon, and G. A. Somorjai, J. Am. Chem. Soc. 133 , 6243 (2011). DOI:10.1021/ja1101954
[17] H. Zhang, F. Li, Q. Xiao, and H. Lin, J. Phys. Chem. Lett. (2015).
[18] M. S. Yeganeh, S. M. Dougal, and H. S. Pink, Phys. Rev. Lett. 83 , 1179 (1999). DOI:10.1103/PhysRevLett.83.1179
[19] W. Gan, G. Gonella, M. Zhang, and H. L. Dai, J. Chem. Phys. 134 , 041104 (2011). DOI:10.1063/1.3548668
[20] M. C. Messmer, J. C. Conboy, and G. L. Richmond, J. Am. Chem. Soc. 117 , 8039 (1995). DOI:10.1021/ja00135a032
[21] W. Wu, H. Fang, F. Yang, S. Chen, X. Zhu, Q. Yuan, and W. Gan, J. Phys. Chem. C 120 , 6515 (2016).
[22] D. A. Beattie, R. Fraenkel, S. A. Winget, A. Petersen, and C. D. Bain, J. Phys. Chem. B 110 , 2278 (2006). DOI:10.1021/jp056204p
[23] X. Zhuang, P. B. Miranda, D. Kim, and Y. R. Shen, Phys. Rev. B 59 , 12632 (1999). DOI:10.1103/PhysRevB.59.12632
[24] N. Bloembergen, and C. H. Lee, Phys. Rev. Lett. 19 , 835 (1967). DOI:10.1103/PhysRevLett.19.835
[25] J. Conboy, J. Daschbach, and G. Richmond, J. Phys. Chem. 98 , 9688 (1994). DOI:10.1021/j100090a600
[26] J. C. Conboy, and G. L. Richmond, Electrochim. Acta 40 , 2881 (1995). DOI:10.1016/0013-4686(95)00217-3
[27] H. Nagatani, D. J. Fermn, and H. H. Girault, J. Phys. Chem. B 105 , 9463 (2001). DOI:10.1021/jp010732t
[28] T. Uchida, A. Yamaguchi, T. Ina, and N. Teramae, J. Phys. Chem. B 104 , 12091 (2000). DOI:10.1021/jp0034832
[29] Y. R. Shen, the Principles of Nonlinear Optics, Hoboken, NJ:Wiley-Interscience (1984).
[30] G. Berkovic, T. Rasing, and Y. R. Shen, J. Opt. Soc. Am. B 4 , 945 (1987).
[31] R. M. Corn, and D. A. Higgins, Chem. Rev. 94 , 107 (1994). DOI:10.1021/cr00025a004
[32] K. B. Eisenthal, Chem. Rev. 96 , 1343 (1996). DOI:10.1021/cr9502211
[33] H. F. Wang, W. Gan, R. Lu, Y. Rao, and B. H. Wu, Int. Rev. Phys. Chem. 24 , 191 (2005). DOI:10.1080/01442350500225894
[34] W. Gan, B. H. Wu, Z. Zhang, Y. Guo, and H. F. Wang, J. Phys. Chem. C 111 , 8716 (2007). DOI:10.1021/jp067062h
[35] D. S. Zheng, Y. Wang, A. A. Liu, and H. F. Wang, Int. Rev. Phys. Chem. 27 , 629 (2008). DOI:10.1080/01442350802343981
[36] Y. R. Shen, Annu. Rev. Phys. Chem. 40 , 327 (1989). DOI:10.1146/annurev.pc.40.100189.001551
[37] H. T. Bian, R. R. Feng, Y. Y. Xu, Y. Guo, and H. F. Wang, Phys. Chem. Chem. Phys. 10 , 4920 (2008). DOI:10.1039/b806362a
[38] C. Hirose, N. Akamatsu, and K. Domen, Appl. Spectrosc. 46 , 1051 (1992). DOI:10.1366/0003702924124385
[39] W. Gan, B. H. Wu, H. Chen, Y. Guo, and H. F. Wang, Chem. Phys. Lett. 406 , 467 (2005). DOI:10.1016/j.cplett.2005.03.043
[40] M. A. Van Der, V. K. Van Der Veen, and T. Verbiest, D. E. De Vos, Langmuir 25 , 4256 (2009). DOI:10.1021/la8039785
[41] A. Liu, L. Lin, Y. Lin, and Y. Guo, J. Phys. Chem. C 117 , 1392 (2013). DOI:10.1021/jp310569v
[42] H. T. Bian, Ph. D Dissertation, Beijing:Institute of Chemistry, Chinese Academy of Science, No.200518003208132. (2005).
[43] G. J. Simpson, and K. L. Rowlen, J. Am. Chem. Soc. 121 , 2635 (1999). DOI:10.1021/ja983683f
[44] A. Goebel, and K. Lunkenheimer, Langmuir 13 , 369 (1997). DOI:10.1021/la960800g
[45] H. Fang, W. Wu, Y. Sang, S. Chen, X. Zhu, L. Zhang, Y. Niu, and W. Gan, RSC Adv. 5 , 23578 (2015). DOI:10.1039/C4RA15401K
[46] Y. Sang, F. Yang, S. Chen, H. Xu, S. Zhang, Q. Yuan, and W. Gan, J. Chem. Phys. 142 , 224704 (2015). DOI:10.1063/1.4922304
[47] T. Nagasaki, S. Tamagaki, and K. Ogino, Chem. Lett. 717(1997).
[48] F. Vera, J. Barber, P. Romero, J. L. Serrano, M. B. Ros, and T. Sierra, Angew. Chem. 122 , 5030 (2010). DOI:10.1002/ange.v122:29
[49] W. K. Zhang, H. F. Wang, and D. S. Zheng, Phys. Chem. Chem. Phys. 8 , 4041 (2006). DOI:10.1039/b608005g
[50] G. J. Simpson, and K. L. Rowlen, Anal. Chem. 72 , 3399 (2000). DOI:10.1021/ac000346s
[51] G. M. Hale, and M. R. Querry, Appl. Opt. 12 , 555 (1973).
[52] J. Jasny, B. Nickel, and P. Borowicz, JOSA B 21 , 729 (2004). DOI:10.1364/JOSAB.21.000729
[53] G. Cnossen, K. E. Drabe, and D. A. Wiersma, J. Chem. Phys. 97 , 4512 (1992). DOI:10.1063/1.463895
[54] R. M. Plocinik, and G. J. Simpson, Anal. Chim. Acta. 496 , 133 (2003). DOI:10.1016/S0003-2670(03)00994-2
[55] T. F. Heinz, H. W. K. Tom, and Y. R. Shen, Phys. Rev. A 28 , 1883 (1983). DOI:10.1103/PhysRevA.28.1883
[56] T. G. Zhang, C. H. Zhang, and G. K. Wong, J. Opt. Soc. Am. B 7 , 902 (1990). DOI:10.1364/JOSAB.7.000902
[57] R. W. Munn, J. Chem. Phys. 113 , 8774 (2000). DOI:10.1063/1.1318903
全内反射构型下分子在十六烷-水界面取向角度的二次谐波测量
张立波a,b, 房辉a,b, 陈顺利a, 朱雪峰a, 干为a     
a. 中国科学院新疆理化技术研究所, 中国科学院特殊环境功能材料与器件重点实验室, 环境科学与技术实验室, 乌鲁木齐 830011;
b. 中国科学院大学, 北京 100049
摘要: 测量了在全内反射条件及非全内反射条件下,两种探针分子在十六烷-水界面上的取向角度.通过详细的偏振分析,发现探针分子在两种构型下的取向角度测量结果一致.表明全内反射条件下,取向角度的测量和偏振分析同样是准确且可行的.
关键词: 二次谐波    全内反射    超极化率    偏振    取向角度