Novel method for accurate determination of the orientational angle of interfacial chemical groups

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1 Chinese Science Bulletin 00 Vol. 48 No Novel method for accurate determination of the orientational angle of interfacial chemical groups LÜ Rong,,GANWei, & WANG Hongfei. State Key Laboratory of Molecular Reaction Dynamics, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 00080, China;. Graduate School of the Chinese Academy of Sciences, Beijing 0009, China Correspondence should be addressed to Wang Hongfei ( Abstract The common practice for determination of orientational angle of interfacial molecular groups with the interfacial sum frequency generation vibrational spectroscopy (SFG-VS) is to measure the intensity ratio between two SFG intensities at a certain vibrational frequency with different polarization directions. Because sometimes the SFG at one polarization direction is too weak to be measured accurately, this ratio usually has big uncertainty, and consequently it is impossible to obtain accurate orientation parameters. Thus the corresponding orientation angle was not accurately calculated. Through analyzing the basic relationship between the orientation angle and SFG intensity, we found that the null angle method would be suitable for such cases. The null angle measurement is simple, precise and easy to realize, therefore can greatly improve the accuracy of orientation angle. Using this method, we determined the orientation angle of the methyl group of methanol orients about.8 ±.4 from the surface normal at the air/methanol interface, much more accurate than the previously reported value of 4. The null angle method provides a reliable experimental analytical tool for studying the orientation of the chemical group or chemical bond at interfaces. Keywords: sum frequency generation vibrational spectra, null angle, orientation angle, methyl, methanol, orientation parameter. DOI: 0.60/0wb007 Interface is the entity separates two bulk phases. It is usually one to a few molecular layers thick, usually not more than nm. Interfaces exist ubiquitously in the heterogeneous system in nature. Because the molecules at he interface layer experience uneven interactions from the two bulk phases, they usually take on a certain alignment or orientation. This makes the interface a unique entity with different physicochemical properties from those of the two bulks. With the advances of modern experimental techniques and theoretical tools for interface studies, research on the interfacial molecular systems has gradually become a very broad and important interdisciplinary field. Surface Second Harmonic Generation (SHG) and Sum Frequency Generation (SFG) have developed into effective optical techniques for interfacial analysis in the past 0 years [ 7]. SFG is the second-order nonlinear optical process that two light fields with two different frequencies simultaneously interact with a medium to generate an optical radiation at the sum of these two frequencies. SHG is the special case with two degenerate frequencies for SFG. In an isotropic medium, there is no effective second-order nonlinear susceptibility because of its central symmetry due to randomly oriented molecules; while in the interfacial layer, because the interfacial molecules bear asymmetry force from the two bulk media, this central symmetry is broken and the second-order nonlinear effect is allowed [ 7]. As a result, SFG possesses unique interfacial selectivity and sensitivity. In SFG experiments, a visible (Vis) pulse and a tunable infrared (IR) pulse are shined onto the interface. When the IR frequency is close to a vibrational frequency of the interfacial chemical group, the SFG intensity would be spectroscopically enhanced. A SF spectrum, which is similar to an IR or Raman vibrational spectrum, would be obtained when the IR frequency is scanned. Through analysing the polarization dependence of the spectral peaks and their intensities, such information as the interfacial molecular orientation, structure and ordering will be obtained. The instrumentation of SFG-VS has developed very quickly since the 990s [5 7]. SFG-VS not only has brought important progress in studies on the fundamental and simple interfacial systems [8 4], but also has been widely applied to studying polymeric, biological films and other functional materials [5 ]. ThecommonpracticeinSFG-VSistomeasurethe intensity ratio of the same vibrational peak at two different polarization combinations in order to derive the orientation parameter D cos θ / cos θ of the corresponding molecular group. By assuming the orientation distribution function as a δ function, the orientation angle could be readily obtained. Because usually for some polarization combinations the SF signal is almost undetectable, the experimental error of the required intensity ratio is usually big, thus the accuracy of the D and θ values from the SF experiment is unsatisfactory. For methanol molecule, the reported orientation angle thus obtained is θ 4 [0,].In this report, we propose a different approach for obtaining D through measurement of the SFG polarization null angle. Thus we are able to obtain a very accurate value for θ.8 ±.4 for the methyl group at the air/methanol interface. Theoretical background A typical SFG experiment is illustrated in Fig.. Two incident beams, ω (visible) and ω (infrared), simultaneously overlap at the interface from medium with the input angles β and β, respectively, and generate SFG signal at the frequency ω s ω + ω from the interface Chinese Science Bulletin Vol. 48 No. 0 October 00 8

2 Fig.. Geometry of interfacial SFG. with angle β s. γ, γ and γ s are the corresponding transmission angles in medium, respectively. The refraction index of frequency ω i (i,,s) in medium, medium and at interface are n (ω i), n (ω i) andn (ω i). The plane of incidence is the p plane, and s direction is perpendicular to the p plane. The polarization angle of the three optical electric fields denotes as Ω i (i,,s), counterwise away from p plane. Therefore, the SFG-VS signal intensity from the interface layer is [4 7] : ωs sec 8π βs I( ω s ) eff I ( ω) I ( ω ), () c n( ωs ) n( ω ) n( ω ) where the effective second-order nonlinear susceptibility eff, s s i eff [ L( ω ): e ] :[ L( ω ): e ][ L( ω ): e ], is proportional to SFG spectra signal, which contains all measurable information of second-order nonlinear optics, and it also includes all spectral and molecular orientational information. In eq. (), I (ω )andi (ω )arethe fundamental frequency light, and I(ω s ) is the SFG signal intensity. In eq., is the macroscopic second-order nonlinear susceptibility, and is a third-order tensor. ijk (i, j, kx, y, z), the element of third order tensor,has DD 7 elements. For interface, which is isotropy in the interface plane, there are only seven non-zero tensor yyz yzy zyy elements terms [,] :,,,,, zxx and zzz. In eq., the unit optical field is e i B cosω i cosβ i x +sinω i y +cosω i sinβ i z (i,,s).l(ω i )(i,, s) is the different Fresnel factor tensor which is a D second-order tensor. After diagonalizing, the nonzero diagonal elements of L(ω i )are [4 7] : n ( ω )cos i γ i L ( ω ), xx i (-) n ( ω )cos ( )cos i γ i + n ω i βi n ( ωi)cos βi L ( ω ), yy i (-) n ( ω )cos β + n ( ω )cosγ i i i i L n ( ω i)cos βi n ( ωi) ( ω ). zz i n( ω i)cos γ i + n( ω i)cos β i n ( ω i ) (-) e i, L(ω i ) should be known values for a given SFG experiment. Then the eff can be calculated with eq.. Here, symbol: denotes the operation of tensor multiplication, the result is a vector; while symbol C denotes the operation of vector multiplication, then one scalar quantity was obtained. In SFG experiment, usually only s and p polarization directions, which are vertical to each other, are considered. Then, four sets of polarization combinations are left. If eff,ijk s footnotes i, j and k are index for the polarization states (s or p) of SF, visible and infrared light, then [4 7] yy yy zz yyz eff, ssp L ( ω) L ( ω ) L ( ω )sin β, (4-) eff, sps Lyy ( ω) Lzz ( ω) Lyy ( ω)sin βyzy, (4-) eff, pss Lzz ( ω) Lyy ( ω) Lyy ( ω)sin βzyy, (4-) L ( ω) L ( ω ) L ( ω )cosβ cosβ sinβ eff, ppp xx xx zz xx ( ω) zz ( ω) xx ( ω)cosβsinβsinβ zz ( ω) xx( ω) xx ( ω)sinβ cosβcosβ zxx zz ( ω) zz ( ω) zz ( ω)sinβsinβsin βzzz. αijk L L L + L L L + L L L (4-4) In the molecular coordinate system (x, y, z ), a chemical group s second-order nonlinear polarizability αijk also is a third-order tensor with 7 terms. For molecule groups belong to different symmetry types, their has different nonzero terms. CH group belongs to C v symmetry, and its symmetric stretching vibrational mode has two independent nonzero terms [5,7,4,5] : α z z z, α x x z α y y z. Here the symmetric axis of CH group is z, and the angle between z and the surface normal z in the laboratory coordinates is defined as the orientational angle θ. Therefore, the relation between the macroscopic tensor polarizability ijk of the C v molecular group at any rotationally isotropic interface, and its microscopic polarizability αijk is [5,7] : yyz Nsαzzz [ cos θ ( + r) cos θ ( r)], (5-) zxx yzy zyy Nsαzzz [cos θ cos θ ]( r), (5-) zzz s z z z N α [ r cosθ + cos θ ( r)]. (5-) 84 Chinese Science Bulletin Vol. 48 No. 0 October 00

3 Here denotes the ensemble average over the interfacial molecular orientational distribution r α x x z / α z z z, is the hyperpolarizability ratio. For the CH group symmetric stretching vibration mode of methanol molecule, r.7 [6]. In previous literature, the method to obtain θ is to directly calculate the / values with eqs. (4-) and (4-), then using eq. (5) to calculate D cos θ / cos θ,thatis r r eff, sps / [ cos θ ( + ) cos θ ( r)]. (6) [cosθ cos θ ]( ) In many actual experiments, is often very small, which makes the ratio value very inaccurate and also cannot get precision D. In addition, there will be one positive and one negative / from the same SFG intensity ratio, which would generate two different D values. Therefore, additional information is needed to judge which value is reasonable, so that sometimes more complex situation might occur [8]. Previous report has / > 0 for methyl at the air/methanol interface [0]. When the ratio is 0, D > 0.7, which has no clear physical meaning; when the ratio is 0, D <.8. Assuming a δ distribution function, it would give θ < 4. Literature reported that the result of ethanol is the same as methanol s results []. We found that the null angle method is very precise in SHG, but it has not been used in SFG study [7].Measuring the changes of SHG [8,9] or SFG [0] null angle at the different interfacial conditions has been employed to study interfacial molecular group orientational changes under those conditions. But so far as we know, this method has not been used in SFG to measure interfacial molecular orientation angle. In eq., the right side is the square of a scalar quantity through point multiplying of the measurement vector [L(ω s ):e s ] with the polarizability vector P :[L(ω ):e ][L(ω ):e ]. Then, the measured null angle Ωs of the total SFG signal should satisfy I ( Ωs) A sin( Ωs Ωs ) 0 i, (7) where A includes all of the experimental constants. When other conditions keep fixed, through tuning detective polarizer to change the SF polarization angle Ω s,itiseasyto fit accurate value for Ω s. Inserting the known parameters (input angle, reflected angle and refractive angle), refractive index, vector expressions and eqs. (), (5) into eq.,weget L s es L e L e eff [ ( ω ): ] :[ ( ω ): ][ ( ω ): ] 0. The orientational parameter D would be solved from this equation, and then the orientational angle θ can be obtained. Since the null angle can be accurately measured, the orientation angle will be accurate. In addition, for Ω s the SF intensity is essentially zero, therefore, the problem of choosing the positive and negative values would disappear, thus the complication of getting two D values is avoided. Experimental Our picosecond SFG vibrational spectroscopy laser system was manufactured by EKSPLA (Lithuania). Since 000, similar systems have been tested by a few other groups [7,,]. Our system has a repetition rate of 0 Hz, and the pulse width is about ps. The visible light at ω 5 nm is fixed at an incident angle of β 6 ;the infrared wavelength was tunable from 000 cm to 4000 cm, with line width less than 6 cm and incident angle β 5. These two lights are simultaneously focused onto the sample interface with a spot diameter about 0.5 mm. In this experiment, the IR frequency was tuned in cm then the corresponding SFG wavelength is ω s nm, and the detection angle β s 60.5 ± 0.. The IR frequency was scanned at cm per step, and the accumulating time is 00 pulses each step. The visible is typically about 00 µj per pulse, while the infrared is 00 µj per pulse. Such intensities will not cause the damage of the interface or cause any photochemical reaction. The SFG signal is normalized to the laser intensities. The methanol solvent is from Fluka (purity 99.8%). The sample holder is made of Teflon to reduce any possible contamination. All experimental data have been repeated several times. Results and discussion We measured the SFG spectra of the CH group at all four polarization combinations for the air/methanol interface at the ambient temperature (k), and all spectra are consistent with those previously reported [8,0,].Fig. shows SFG spectra at two polarization combinations (ssp and sps). The spectra for the other two (pss and ppp) polarization combinations are similar to that of the sps, which is close to noise level. So they are not displayed here. The strong dependence of SFG spectra signal on polarization combination indicates that methanol molecules are highly ordered at the interfaces. In ssp spectra, thepeakat88cm belongs to CH group s symmetric stretching vibration mode, while the one at 940 cm is the Fermi resonance peaks, which is the Fermi resonance enhanced and shifted mode of the double frequency of the bending mode of the CH group. Above peak positions are clearly assigned in the literature [8,0,,].In Raman and infrared spectra, the peak at 90 cm usually Chinese Science Bulletin Vol. 48 No. 0 October 00 85

4 is assigned as Fermi resonance between the sum frequency of two bending vibrational modes and the CH group anti-symmetry stretching mode. However, in the SFG experiment, the anti-symmetry peak has not been clearly observed. While some people also considered the 90 cm peak as another Fermi resonance peak with the symmetry-stretching mode []. Therefore, more research about the assignment of this peak is needed in future studies. All SFG spectra bandwidth is within 7 cm,this is consistent with the characteristic width of condensed matter and interfacial vibrational spectra. Fig.. At 88 cm, dependence of SFG signal intensity on the detection polarization angle. Fig.. Air/methanol interfacial SFG vibrational spectra. From Fig., the intensity at 88 cm of sps polarization combination is far less than that of ssp. In literature, the intensity ratio is Issp eff, ssp L ( ω) L ( ω ) L ( ω )sinβ yy yy zz yyz > 00. Isps L ( ω) L ( ω ) L ( ω )sinβ eff, sps yy zz yy yzy When all refractive index and input angle parameters included this equation, we have / > 0 [0].As mentioned above, this result gives a big uncertainty to the orientation angle. In the null angle measurement, with all other conditions fixed, we set Ω 45, i.e. counterclockwise away from p at 45. At the fixed infrared frequency at 88 cm and input polarization angle Ω 0,thep direction, we measure the SFG signal intensity at different detection polarization angle Ω s. The result is shown in Fig., in which the SFG intensity shows a good periodicity with Ω s. Using eq. (7), we can fit the null angle Ω s 0. ±.. Now we can put all the related parameters into eq., and the parameters are: n (ω ).0, n (ω ).0, n (ω s ).0, n (ω )., n (ω )., n (ω s )., n (ω ).6, n (ω ).6, n (ω s ).6. Those values are consistent with those used in literature [5,6]. So we have the orientational parameter: D cosθ cos θ tan Ω tan Ωs With the value s. (8) Ω s 0. ±., we get D.9 B0.07. Assuming the methanol orientation distribution as a δ function as in literature, we get the methyl orientation angle θ.8 ±.4. This is the best value for the orientation angle of the CH group of methanol up to now. 4 Conclusion The null angle approach for obtaining accurate orientational parameter of molecular groups at the interface is novel in SFG interfacial studies. Since the methyl group has been the most widely studied molecular group in SFG-VS, accurate measurement of its orientation has important implications. We have shown that the null angle approach is simple, accurate, and unambiguous results can be achieved. It can also be generally applied to all in SFG studies. When there are overlapping spectra peaks, the situation will become a little bit complex, and spectra intensity fitting is needed to obtain accurate null angle values. In conclusion, its application may change the current situation in the SFG-VS studies of various interfaces, and push the advancement of SFG as a unique interfacial chemical analysis technique. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No ), the Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KJCXZ-Hz-05), and the National 97 Project (Grant No. G ). References. Shen, Y. R., Surface properties probed by second-harmonic and sum-frequency generation, Nature, 989, 7(9): Shen, Y. R., Optical second harmonic generation at interfaces, Annu. Rev. Phys. Chem., 989, 40: Chinese Science Bulletin Vol. 48 No. 0 October 00

5 . Corn, R. M., Higgins, D. A., Optical second harmonic generation as a probe of surface chemistry, Chem. Rev., 994, 94: Eisenthal, K. B., Liquid interface probed by second-harmonic and sum-frequency spectroscopy, Chem. Rev., 996, 96: Miranda, P. B., Shen, Y. R., Liquid interfaces: A studty by sum-frequency vibrational spectroscopy, J. Phys. Chem. B, 999, 0: Buck, M., Himmelhaus, M., Vibrational spectroscopy of interface by infrared-visible sum frequency generation, J. Vac. Sci. Tcehnol. A, 00, 9: Richmond, G. L., Molecular bonding and interactions at aqueous surfaces as probed by vibrational sum frequency spectroscopy, Chem. Rev., 00, 0: Superfine, R., Huang, J. Y., Shen, Y. R., Nonlinear optical studies of the pure liquid/vapor interface: vibrational spectra and polar ordering, Phys. Rev. Letters, 99, 66(8): Du, Q., Superfine, R., Shen, Y. R. et al., Vibrational spectroscopy of water at the vapor/water interface, Phys. Rev. Letters, 99, 70: Wolfrum, K., Graener, H., Laubereau, A., Sum-frequency vibrational spectroscopy at the liquid-air interface of methanol water solution, Chem. Phys. Letters, 99, (,): Stanners, C. D., Du, Q., Chin, R. P. et al., Polar ordering at the liquid-vapor interface of n-alcohol (C-C8), Chem. Phys. Letters, 995, : Zhang, D., Gutow, J. H., Eisenthal, K. B., Structure phase of small molecules at air/water interfaces, J. Chem. Soc. Faraday Trans., 996, 9(4): Yeh, Y. L., Zhang, C., Shen, Y. R. et al., Structure of the acetone liquid/vapor Interface, J. Chem. Phys., 00, 4: Shultz, M. J., Baldlli, S., Schnitzer, C. et al., Aqueous solution/air interfaces probed with sum frequency generation spectroscopy, J. Phys. Chem., 00, 06(): Zhuang, X., Miranda, P. B., Kim, D. et al., Mapping molecular orientation and conformation at interfaces by surface nonlinear optics, Phys. Rev. B, 999, 59: Wei, X., Hong, S. C., Zhuang, X. W. et al., Nonlinear optical studies of liquid crystal alignment on a rubbed polyvinyl alcohol surface, Phys. Rev. E, 000, 6(4): Wang, J., Chen, C. Y., Buck, S. M. et al., Molecular chemical structure on poly (methyl methacrylate)(pmma) surface studied by sum frequency generation (SFG) vibrational spectroscopy, J. Phys. Chem. B, 00, 05: Chen, Z., Shen, Y. R., Somorjai, G. A., Students of polymers structures by sum frequency generation vibrational spectroscopy, Annu. Rev. Phys. Chem., 00, 5: Kim, G., Gurau, M., Kim, J. Y. et al., Investigation of lysozyme adsorption at the air/water and quartz/water interfaces by vibrational sum frequency spectroscopy, Langmuir, 00, 8: Lahann, J., Mitragotri, S., Tran, T. N. et al., A reversibly switching surface, Science, 00, 99(5605): Kim, J., Somorjai, G. A., Molecular packing of lysozyme, fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy, J. Am. Chem. Soc., 00, 5(0): Shen, Y. R., The principles of nonlinear Optics, New York: Wiley, Heinz, T. F., Nonlinear Optical Effects at Surfaces and Interfaces, in Nonlinear Surface Electromagnetic Phenomena (eds. Ponath, H., Stegman, G.), New York: Elsevier, 99, Hirose, C., Akamatsu, N., Domen, K., Formulas for the analysis of surface sum-frequency generation spectrum by CH stretching modes of methyl and methylene groups, J. Chem. Phys., 99, 96: Simonelli, D., Shultz, M. J., Sum frequency generation analysis of molecular ammonia on the surface of concentrated solutions, J. Chem. Phys., 000, (5): Lobau, J., Wolfrum, K., Sum-frequency spectroscopy in total internal reflection geometry: signal enhancement and access to molecular properties, J. Opt. Soc. Am., 997, 4(0): Heinz, T. F., Tom, H. W. K., Shen, Y. R., Determination of molecular orientation of monolayer adsorbates by optical second-harmonic generation, Phys. Rev. A, 98, 8(): Zhang, T. G., Zhang, C. H., Wong, G. K., Determination of molecular orientation in molecular monolayers by second-harmonic generation, J. Opt. Soc. Am. B, 990, 7(6): Hicks, J. M., Kemnitz, K., Eisenthal, K. B., Studies of liquid surfaces by second harmonic generation, J. Phys. Chem., 986, 90: Zhang, D., Gutow, J., Eisenthal, K. B., Vibrational spectra, orientations, and phase transitions in long-chain amphiphiles at the water interface: probing the head and tail groups by sum frequency generation, J. Phys. Chem., 994, 98: Shen, Y. R., Surface nonlinear optics: a historical perspective, IEEE J. on Selected Topics in Quantum Electronics, 000, 6(6): Ye, S., Nihonyanagi, S., Uosaki, K., Sum frequency generation (SFG) study of the ph-dependent water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) monolayer, Phys. Chem. Chem. Phys., 00, Herzberg, G., Molecular spectra and molecular structure, Infrared and Raman Spectra of Polyatomic Molecules, Inc. Princeton, New Jersey, Toronto New York, London: D. Van Nostrana Company, 945, Serrallach, A., Meyer, R., Gunthard, H. H., Methanol and deuterated species: infrared data, valence force field, Rotamers, and conformation, J. Molecular Spectroscopy, 974, 5: Hester, R. E., Plane, R. A., Raman spectra of methanol solutions-i saturated solutions of some electrolytes, Spectrochimica Acta, 967, A: Bass, M., Brewer, L., Disalvo, F. J., CRC Handbook of Chemistry and Physics 7rd, 99 99, 8 4. (Received April 6, 00; accepted June 8, 00) Chinese Science Bulletin Vol. 48 No. 0 October 00 87