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2005, 109, 24211-24214 Published on Web 12/06/2005
Interface-Specific χ(4) Coherent Raman Spectroscopy in the Frequency Domain Shoichi Yamaguchi and Tahei Tahara* Molecular Spectroscopy Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ReceiVed: July 29, 2005; In Final Form: NoVember 16, 2005
We demonstrate interface-specific fourth-order (χ(4)) coherent Raman spectroscopy in the frequency domain for the first time. Because the χ(4) Raman spectroscopy uses only visible (vis) or near-IR light, it is expected to be a potential alternative to the widely utilized IR-vis sum frequency generation spectroscopy that cannot be applied to interfaces buried in thick IR absorbers such as water. We present the vibrational |χ(4)|2 spectrum of rhodamine 800 at the air/water interface in a wide spectral range 100-3600 cm-1. Comparison of the |χ(4)|2 spectrum with the |χ(3)|2 spectrum leads us to conclude that the present χ(4) spectroscopy successfully probes the interface distinguished clearly from the bulk.
Introduction
SCHEME 1: Structure of Rhodamine 800 (R800).
Even-order nonlinear spectroscopy is a powerful tool to study molecular structures at interfaces.1-5 The IR-visible (vis) sum frequency generation (SFG) spectroscopy, which takes advantage of the interface specificity of χ(2) (the nth-order nonlinear optical susceptibility is written as χ(n)), is the most important method to acquire interfacial vibrational spectra that are essential for the assignments of molecular species and the determination of functional-group orientation at interfaces. IR-vis SFG has been applied to gas/liquid and gas (or vacuum)/solid interfaces more often and more successfully than to liquid/liquid and liquid/solid interfaces,3-5 because IR light is strongly absorbed by most of liquid and solid materials. Recently, interface-specific vibrational χ(4) spectroscopy was newly reported by several groups.6-12 Because the χ(4) spectroscopy does not employ IR but uses only vis or near-infrared (NIR) light, it can be applied to interfaces buried in thick IR absorbers that are transparent in the vis or NIR region. In this sense, the χ(4) spectroscopy is expected to be a potential alternative to IR-vis SFG. All the χ(4) spectroscopic measurements reported so far were performed in the time domain.6-16 Although the time-domain spectroscopy can provide frequency-domain vibrational spectra by the Fourier transformation, its spectral range is usually below 1000 cm-1 because of the limited bandwidth of the available ultrashort laser pulses. In the present study, we first demonstrate the multiplex vibrational χ(4) spectroscopy in the frequency domain, which gives spectral data of a very wide range from 100 cm-1 up to 3600 cm-1. We present the |χ(4)|2 spectrum of a surface-active dye rhodamine 800 (R800, Scheme 1) at the air/water interface and compare it with the coherent anti-Stokes Raman scattering (CARS) spectrum of R800 in water measured with the reflection configuration. The comparison shows that the present χ(4) spectroscopy successfully probes the interface distinguished clearly from the bulk. 10.1021/jp0542064 CCC: $30.25
Experimental Section The energy diagram of the vibrational χ(4) spectroscopy in the frequency domain is shown in Figure 1a. As indicated by the left two arrows, the difference frequency of the ω1 and ω2 pulses is in resonance with a vibrational level in the electronic ground state. With the subsequent two ω1 pulses, the fourthorder nonlinear polarization of ω1 - ω2 + ω1 + ω1 ) 3ω1 ω2 is generated. All the transitions are electronically resonant in the case of the present sample. Because the χ(4) signal is interface-specific and is resonance-enhanced vibrationally, we can regard the homodyne-detected |χ(4)|2 spectrum as a vibrational spectrum of interfacial molecules when it is plotted as a function of ω1 - ω2. If the last two ω1 pulses are replaced with one as in Figure 1b, the diagram represents the third-order polarization of 2ω1 - ω2 that gives the vibrational |χ(3)|2 (CARS) spectrum of bulk molecules.1,17 Figure 2 shows the χ(4) experimental setup that was developed on the basis of the electronic χ(2) multiplex SFG setup recently reported by us.18 A femtosecond Ti:sapphire regenerative amplifier (Spitfire, Spectra Physics) was used as a laser light source. A part of the amplifier output centered at 795 nm was focused into water, and the white light continuum generated was used as the ω2 pulse. Wavelength components shorter than 800 nm in the white light continuum were eliminated by a color glass filter. The spectrum of the ω2 pulse extended to 1.2 µm. The other part of the amplifier output was passed through an NIR interference filter (F1.5-794.7-4-1.00, CVI) for narrowing the bandwidth to about 20 cm-1, and it was used as the ω1 pulse. The ω1 and ω2 pulses were noncollinearly focused onto the same spot of about 0.1 mm in diameter at the air/ © 2005 American Chemical Society
24212 J. Phys. Chem. B, Vol. 109, No. 51, 2005
Letters
Figure 1. Energy diagrams of the (a) χ(4) and (b) χ(3) vibrational spectroscopies.
Figure 2. Experimental setup of the homodyne vibrational χ(4) spectroscopy. Labels L and M denote a lens and a mirror, respectively. Some optical elements shown in the side (or top) view are not shown in the other view for clarity.
water interface of a sample solution at an incident angle 45° from normal. The angle between the ω1 and ω2 beams was set at about 10°. The linear polarization of the ω1 and ω2 beams was in the plane of reflection (i.e., p-polarization). The pulse energy for ω1 was 2 µJ, and that for ω2 was 6 µJ. Time delay between the ω1 and ω2 pulses was controlled by a translation stage. When the ω1 and ω2 pulses were temporally overlapped, the χ(4) signal (3ω1 - ω2) was generated at the interface. The whole spectral components of the ω2 pulse (∼800-1200 nm) had sufficient temporal overlap with the ω1 pulse at one time delay point. It was confirmed by the electronic SFG (ω1 + ω2) spectrum of a quartz plate shown in Figure 3a, which was measured as described in our previous paper.18 An iris behind the sample blocked the ω1 and ω2 pulses but passed the whole 3ω1 - ω2 spectral components that diverged because of the phase matching condition. Although the ω1 and ω2 beams were noncollinear, the phase matching condition was approximately satisfied owing to the small crossing angle between these beams. The χ(4) signal light was collected and collimated by a planoconvex fused silica lens with UV antireflection coating, and it was focused by another identical lens onto the entrance slit of a single polychromator (HR-320, Jobin Yvon) that was equipped with a grating of 600 grooves/mm and 300 nm blaze wavelength. The center wavelength of the polychromator was set at 376 nm. The spectrally dispersed signal light was detected by a liquidnitrogen-cooled CCD (Spec-10:2KBUV, Princeton Instruments) that had 2048 × 512 pixels. Unwanted scattering light was rejected by a dielectric filter and color glass filters in front of the polychromator. Trivial background signals including CCD noises were measured at a time delay when the ω1 and ω2 pulses were not temporally overlapped (typically, at 20 ps). The
Figure 3. (a) Electronic SFG spectrum of a quartz plate. (b) Vibrational |χ(4)|2 spectrum of R800 at the air/water interface. (c) CARS (vibrational |χ(3)|2) spectrum of R800 in water measured with the reflection configuration. (d) Spontaneous Raman spectrum of R800 powder. The insets sketch the regions probed. Small dots stand for solvent molecules, and large open ovals for solute molecules.
bandwidth of the hyper-Rayleigh scattering light of the ω1 pulse approximately corresponded to the spectral resolution for the |χ(4)|2 spectrum, which was 54 cm-1. The typical measurement time to obtain a |χ(4)|2 spectrum was 1 h. For the χ(3) measurement, the χ(4) setup was used as it was, except the following two points: First, the center wavelength of the polychromator was changed to 690 nm. Second, a different dielectric filter and color glass filters were used to reject
Letters unwanted scattering light. The propagation direction of the χ(3) signal light (2ω1 - ω2) was slightly different from that of the χ(4) signal (3ω1 - ω2), but the difference was compensated by the plano-convex fused silica lenses that were large enough. This χ(3) measurement can be regarded as multiplex CARS in the reflection configuration. The bandwidth of the Rayleigh scattering light of the ω1 pulse approximately corresponded to the spectral resolution for the |χ(3)|2 spectrum, which was 25 cm-1. The typical measurement time to obtain a |χ(3)|2 spectrum was 8 min. A cylindrical quartz cell was used to contain the sample solution that was stirred in order to avoid any heating or other photoinduced processes. R800 was purchased from Exciton as LD 800 (perchlorate) and dissolved in HPLC-grade distilled water (Wako) without further purification. The bulk concentration of R800 was saturated (8.3 × 10-5 mol dm-3). The spontaneous resonance Raman spectrum of R800 powder was measured using a commercial Raman spectrometer (Labram, Dilor ISA) with a 514.5 nm excitation laser. The spectral resolution for the spontaneous Raman spectrum was 20 cm-1. All the experiments were performed at room temperature, 299 K. Results and Discussion Figure 3b shows the vibrational |χ(4)|2 spectrum of R800 at the air/water interface, and Figure 3c depicts the |χ(3)|2 (CARS) spectrum of R800 in water measured with the reflection configuration. The trivial background signals were already subtracted, and the intensities were normalized by the CCD exposure time. For comparison, the spontaneous resonance Raman spectrum of R800 powder is also shown in Figure 3d. Although the bands in the spontaneous Raman spectrum are much narrower than those in the |χ(4)|2 and |χ(3)|2 spectra because of the difference in the spectral resolution and the sample forms (i.e., solution and powder), the band positions and the spectral patterns of the |χ(4)|2 and |χ(3)|2 spectra are essentially in good agreement with those of the spontaneous Raman spectrum. It means that the |χ(4)|2 and |χ(3)|2 spectra are vibrational spectra of R800 as expected. We can readily see that these three spectra show the vibrational bands at the same wavenumbers within the present spectral resolution: 1200, 1350, 1500, 1650, and 2220 cm-1. The bands around 3000 cm-1 are recognized in the spontaneous Raman and |χ(3)|2 spectra, but not in the |χ(4)|2 spectrum. The |χ(3)|2 spectrum is a typical electronic-resonant CARS spectrum. Especially, the 2220 cm-1 band shows a derivative line shape that is characteristic of frequency-domain homodyne spectroscopies. The bands at 1650, 2220, and 3000 cm-1 are tentatively assigned to the CdC (or CdN), CtN, and CH stretching modes of R800, respectively. The same vibrational assignments can be applied to the |χ(4)|2 spectrum, except the CH stretching. The asymmetric line shapes in the |χ(4)|2 spectrum are considered to be caused by “interferences” between the congested vibrational bands. We cannot observe a vibrational band assignable to water in the |χ(4)|2 and |χ(3)|2 spectra, because water is electronically nonresonant under the present experimental condition. The most remarkable difference between the |χ(4)|2 and |χ(3)|2 spectra is the absence of the nonresonant background in the |χ(4)|2 spectrum. A CARS spectrum of a liquid sample always has a finite contribution of the nonresonant background as seen in the present |χ(3)|2 spectrum. Generally, the nonresonant background is predominantly ascribed to the solvent,19,20 because the concentration of the solvent is much larger than that of the solute. The absence of the nonresonant background in the |χ(4)|2
J. Phys. Chem. B, Vol. 109, No. 51, 2005 24213 spectrum indicates that the concentration ratio of water to R800 at the interface is far lower than that in the bulk. This is consistent with the surface activity of R800 that brings about a positive surface excess of R800 and a relative decrease of the interfacial water concentration. The insets of Figure 3 schematically show the interfacial regions of the sample solution probed by the χ(4) and χ(3) measurements. The absence of the nonresonant background confirms that the air/water interface is successfully probed in the χ(4) spectroscopy. From a theoretical study,21 we can consider that the interface has only a few molecular diameter thickness. In contrast, the χ(3) spectroscopy, i.e., CARS in the reflection configuration,22,23 is supposed to probe the “interfacial” region of wavelength-order thickness. Therefore, the region probed by the χ(3) spectroscopy should be essentially regarded as bulk. We roughly estimate the concentration ratio of water to R800 ([H2O]/[R800]) in the regions probed by the χ(4) and χ(3) measurements to rationalize the above argument. In the χ(3)probed region, [H2O]/[R800] is 6.7 × 105, which is directly calculated from the concentration in the bulk. On the other hand, at the χ(4)-probed interface, [H2O]/[R800] is estimated to be 3 × (∼101-102) by adopting a typical interfacial population density for a surface-active dye (∼1-0.1 molecules/nm2)24 and taking the thickness of the interface to be 1 nm. Therefore, the ratio of [H2O]/[R800] at the interface to that in the bulk is 5 × (∼10-5-10-4), which is small enough to explain the absence of the nonresonant background due to water in the |χ(4)|2 spectrum. Conversely, we will be able to estimate the interface thickness experimentally, using the interfacial population density of the solute molecules, if we can improve the signal-to-noise ratio of the |χ(4)|2 spectrum well enough to identify a small but finite contribution of the nonresonant background due to water. As already mentioned, the small signal due to the CH stretching can be seen in the |χ(3)|2 spectrum around 3000 cm-1, but not in the |χ(4)|2 spectrum. We think that there are two reasons for it: One is that the contribution of a small vibrational band can be enhanced by the nonresonant background in the |χ(3)|2 spectrum, because the small vibrational contribution is multiplied by the finite nonresonant background term in the absolute square operation in |χ(3)|2. On the other hand, the absence of the nonresonant background in the |χ(4)|2 spectrum makes the relative contribution of the small band even smaller. The other reason is that the resonance enhancement factor for the CH stretching region in the |χ(4)|2 spectrum is probably smaller than that for the fingerprint region. The phase matching for the χ(3) spectroscopy may be also noteworthy. The χ(3)-probed bulk region is expected to be about 3 orders of magnitude thicker than the χ(4)-probed interface. Nevertheless, it is much thinner than a typical sample used in a CARS measurement with the transmission configuration. This allows phase matching for the very wide spectral range in the present CARS detection.25 Last, we discuss a possibility of cascade processes that may contribute to the signal. Figure 4a shows a cascade process where χ(3) and χ(2) processes sequentially take place: The CARS signal light of 2ω1 - ω2 is generated in the χ(3) process, and the sum frequency mixing of the CARS light and the ω1 pulse gives the cascade signal of 3ω1 - ω2 by the χ(2) process. This CARS-SFG cascade signal is undesired, because it does not provide an interface-specific vibrational spectrum. However, this cascade signal does not contribute to the present |χ(4)|2 spectrum, because the CARS light is too weak to generate the 3ω1 - ω2 signal observed. Estimating from the |χ(3)|2 spectrum in Figure 3c and an electronic |χ(2)|2 spectrum measured separately, the
24214 J. Phys. Chem. B, Vol. 109, No. 51, 2005
Letters (B) (no. 15750023) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. References and Notes
Figure 4. Energy diagrams of the cascade processes. The labels of the energy levels have the same meaning as in Figure 1.
intensity of the cascade signal is at most 10-9 counts/s, which is 10-8 times smaller than the signal intensity observed in the χ(4) experiments. Although we can think of another cascade process shown in Figure 4b, the intensity of this signal is also proportional to |χ(2)|2|χ(3)|2 and must be on the same order as that of the CARS-SFG cascade signal. Consequently, we can conclude that the |χ(4)|2 spectrum obtained is free from these unwanted cascade processes. In summary, we demonstrated the vibrational χ(4) spectroscopy in the frequency domain for the first time. With this spectroscopy, we clearly observed the fingerprint vibrational spectrum of R800 at the air/water interface for the wide spectral region from 100 cm-1 to 3600 cm-1. The data indicated that the interface of a few molecular diameter thickness was successfully probed by the frequency-domain χ(4) spectroscopy. Acknowledgment. The authors gratefully acknowledge helpful comments by two anonymous reviewers. Portions of this work were supported by a Grant-in-Aid for Young Scientists
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