Fourier Transform Infrared Spectroscopy Using Polarization

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J. Phys. Chem. B 2003, 107, 7812-7819

Fourier Transform Infrared Spectroscopy Using Polarization Modulation and Polarization Selective Techniques for Internal and External Reflection Geometries: Investigation of Self-Assembled Octadecylmercaptan on a Thin Gold Film E. Hutter,† K. A. Assiongbon,‡ J. H. Fendler,*,† and D. Roy*,‡ Center for AdVanced Materials Processing and Department of Physics, Clarkson UniVersity, Potsdam, New York 13699 ReceiVed: April 7, 2003

By combining the polarization modulation (PM) technique with Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS), one can substantially improve the detection sensitivity and data collection efficiency. Using PM, the step of “background subtraction” can be eliminated for quantifying adsorbed species or covalently attached monolayers, and the IR spectra can be taken in situ in aqueous solutions. We demonstrate here that such a combination of PM and FT-IRRAS is possible, not only for external reflection measurements, but also for the attenuated total internal reflection (ATR) arrangement. The unique advantage of this second combination is that it couples the surface sensitivity of ATR with the experimental convenience and rapid data collection capability of polarization modulated FT-IRRAS. In the present work, we combine numerical calculations with experiments to study octadecylmercaptan (ODM) monolayers, self-assembled onto a gold-coated calcium fluoride prism. Results, including IR spectra (2800-3000 cm-1) of ODM on Au are presented for PM as well as for parallel (p) and perpendicular (s) polarization selective detection schemes and for both external reflection and ATR experimental geometries.

1. Introduction Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) is a powerful in situ technique for studying a wide variety of biological and chemical systems.1,2 Traditionally, both external and internal reflection geometries have been used for FT-IRRAS, and the relative merits of the two methods have been reviewed by several authors.1-6 During the past decade, however, the internal reflection method has become the main focus of a large number of FT-IRRAS studies involving selfassembled monolayers (SAMs) and biological membranes on metal surfaces.2 Among the latter studies, the parallel (p) and perpendicular (s) polarization selective (PS) attenuated total reflection (ATR) technique has gained considerable popularity.2,7-11 Because ATR uses evanescent optical fields (that are localized near the surface), this technique is intrinsically surface sensitive. The main advantage of using polarization selective FT-IRRAS (abbreviated here as PS-IRRAS) with ATR is that in this approach, one can utilize the capabilities (surface sensitivity and the ability of determining molecular orientation) of both techniques in a single framework. Nevertheless, ATR based PS-IRRAS studies of interfacial layers are typically designed to measure the dichroic ratio, and this in turn requires separate measurements of reflection-absorption spectra using both s- and p-polarized lights. Depending on the experimental system, absorption spectra collected in this approach can be affected by time-dependent fluctuations of sample stability, as well as by those of the instrumental detection sensitivity.12-15 An effective experimental method of resolving these problems, * To whom correspondence should be addressed. Fendler. E-mail: [email protected], Phone: (315) 268 7113. Fax: (315) 268-4416. Roy. E-mail: [email protected]. Phone: (315) 268-6676. Fax: (315) 2686610. † Center for Advanced Materials Processing, Box 5814. ‡ Department of Physics, Box 5820.

namely, the polarization modulation FT-IRRAS (commonly abbreviated as PM-IRRAS), has also emerged during the recent years.12-37 The earlier PM-IRRAS experiments focused on molecular adsorbates on metal substrates.6,38-41 A considerable fraction of the more recently reported applications of PM-IRRAS also deals with metal surfaces.16-31 To our knowledge, however, these PM-IRRAS studies have been applied to external reflection geometries. The question regarding the applicability of PMIRRAS to ATR has remained essentially unexplored. The primary goal of the present work is to address the relevant theoretical and experimental aspects of integrating the ATR method with PM-IRRAS. PM-IRRAS utilizes the observation that on a metal substrate, only molecular dipoles having a surface-normal component can be observed in an external reflection IR spectrum.15,42,43 This effect is a consequence of the so-called “surface selection rule” for IR absorption by molecular layers on metals and arises from the high reflectivity and considerably large extinction coefficients (>10, electrical conductivity > 107 Ω-1 m-1) of metals at IR wavelengths. Under these conditions, and near grazing incidence, the (surface-normal) z-component of the effective p-polarized field (Ez) becomes large compared to both the x-component of the p-polarized field (Ex) and the s-polarized field (Ey). Thus the surface-normal dipole component of an adsorbed molecule interacts efficiently with the z-component of a p-polarized field. The relatively weaker fields Ex and Ey can only have a weak effect on any nonzero component of the dipole moment parallel to the surface. As a result, one detects a significantly larger optical absorption with a p-polarized incident beam than in the case of an s-polarized beam.4,5,42-44 Because of this surface selection rule for IR absorption, the reflectivity Rs of a bare metal substrate measured with spolarized light does not change significantly upon the adsorption

10.1021/jp034910p CCC: $25.00 © 2003 American Chemical Society Published on Web 07/04/2003

FT-IR Using Polarization Modulation

J. Phys. Chem. B, Vol. 107, No. 31, 2003 7813 of incidence for both reflection geometries. The p- and spolarized directions of the electric field vector are indicated in the figure. For internal reflection, the z-axis is directed from phase 1 to phase 4, and this direction is reversed for external reflection. The origin, z ) 0, for internal reflection is at the 1-2 interface. For external reflection, the origin is placed at the 4-3 interface. With these definitions, and for both reflection geometries, the complex electric field Em of the probe light (of wavelength λ) in phase m is written as

Em ) E h 0+ h 0m exp[j(kmzz - ωt)] + E m exp[-j(kmzz + ωt)] (1)

Figure 1. Schematic diagram (not drawn to scale) of the four-phase stratified medium studied in the present work. For external and internal reflections, the IR beam enters the stack from medium 4 (air) and medium 1 (CaF2 prism), respectively. Media 2 and 3 represent a gold film and a SAM of ODM, respectively. For each reflection geometry, the positive z-direction is chosen along the vertical component of the direction of optical incidence.

of a sample molecular layer on the substrate. On the other hand, the p-polarized reflectivity Rp of the same metal drops noticeably when the molecular dipoles are adsorbed on the metal. Thus for molecular dipoles on a metal surface, the task of background subtraction (that is, correcting for optical absorption by the substrate and the ambient medium) is considerably simplified in PM-IRRAS by directly measuring the dichroic difference, (Rp - Rs).12-15 To incorporate this useful feature of PM-IRRAS in the ATR framework, it is first necessary to examine the role of the surface selection rule for IR absorption in the ATR geometry. We address this issue with numerical calculations of polarization-dependent mean square electric fields and reflectivities for a standard multilayer configuration. As a model system, we choose a four-phase device where a thin Au film, deposited on the base of a right-angled CaF2 prism, serves as the substrate for a SAM of octadecylmercaptan (ODM) in a surrounding medium of air. With calculated results, we demonstrate that the PM-IRRAS method should be comparably effective for both external reflection and ATR. We measure the reflection-absorption spectrum of ODM in the ∼2800-3000 cm-1 region using grazing angle external reflection geometry and three different polarization schemes: (i) fixed p-polarization, (ii) fixed s-polarization, and (iii) PM using a double modulation digital technique. The same three experiments are then repeated for the ATR configuration, and four sets of FT-IRRAS data (PS-IRRAS and PM-IRRAS for external reflection and ATR) are compared. 2. Theory The four-phase system considered in our present study is shown in Figure 1. Phases 1, 2, 3, and 4 represent a right-angled CaF2 prism, a thin film (∼11 nm) of Au deposited on the base of the prism, a SAM of ODM, and air, respectively. The dielectric function and the thickness of the mth medium are denoted as m and dm, respectively. For internal reflection, the probe light beam is incident from phase 1 at an angle φin, and to introduce ATR, we maintain φin > φc, where φc is the critical angle. For external reflection, light is incident from phase 4 at an angle φex (near grazing incidence). The experimental system is described with a Cartesian coordinate system with the x-direction chosen parallel to the phase boundaries. The xyplane for each interface is isotropic, and the xz plane is the plane

0( where j ) x-1; E h 0( m ) Em exp(jkxx); x and z represent spatial coordinates, ω is the angular frequency, and t represents time; E h 0( m represents the electric field amplitudes; kmx and kmz are the x and z components of the light wave vector in medium m, respectively; kmx is continuous across the boundaries; kmx ) k4x ) (2π/λ)x4 sin φex for external reflection and kmx ) k1x ) (2π/λ) x1 sin φin for internal reflection; kmz ) (2π/λ)ξm; and ξm is a dielectric parameter, defined as ξm ) (m - 4 sin2 φex)1/2 for external reflection and as ξm ) (m - 1 sin2 φin)1/2 for internal reflection. Here, m ) (mr + jmi); mr and mi are the real and imaginary parts of m, respectively; mr ) nm2 - κm2; mi ) 2nmκm; nm and κm are the real and imaginary parts of the complex refractive index nˆ m in layer m, respectively; nˆ m ) nm + jκm. In a given phase m, the electric field amplitudes in the positive z-direction (for incidence or transmission) are denoted as Emγ0+ (γ ) x, y, or z). The corresponding field amplitudes in the negative z-direction (for reflection) are denoted as Emγ0-. The reflectivities of the multilayer system in external reflection are obtained as44,45 Rp(ex) ) |E4x0-/E4x0+|2 and Rs(ex) ) |E4y0-/E4y0+|2 for p- and s-polarized lights, respectively. The corresponding reflectivities for internal reflection are written as Rp(in) ) |E1x0-/E1x0+|2 and Rs(in) ) |E1y0-/E1y0+|2, respectively. Detailed formulas for these reflectivity parameters have been derived by several authors2-5,44,45 and will be omitted here. The reflectivity formulas discussed by Hansen45 will be used for numerical calculations and data analysis in our present work. Let us now briefly discuss how the above-mentioned reflectivity parameters are detected by PM experiments. In the present work, we use the photoelastic double modulation technique, where for both external reflection and ATR, the digitally processed PM-IRRAS signal has the form12,35

I(d3)PM ≈

R0[Rp(d3) - Rs(d3)]

(2)

[Rp(d3) + Rs(d3)]

Here R0 is a constant for a fixed experimental setup, R0 ) [J2(φ0)G0], G0 is a constant factor that accounts for the different gains and filtering characteristics of the two channels of the double modulation setup, J2(φ0) is the Bessel function of order 2, φ0 is the maximum phase shift introduced in the incident light by the modulator, and φ0 depends on λ and the maximum applied voltage at the modulator. In the presence of the sample molecule in the third phase, d3_* 0. Equation 2 can be rearranged as

[

I(d3)PM ) R0

Rp(d3) Rs(d3)

][

-1 1+

]

Rp(d3) Rs(d3)

-1

(3)

The quantity expressed in eq 3 is related to the optical absorption A in the sample layer 3. The explicit form of this relation depends on the experimental system and, to some extent, on

7814 J. Phys. Chem. B, Vol. 107, No. 31, 2003

Hutter et al.

the reflection geometry. However, for a thin sample layer and in the presence of the earlier mentioned surface selection rule, the formula for the PM-IRRAS signal is considerably simplified. Let us first consider the case of external reflection, where the active role of the surface selection rule has been confirmed for numerous systems.2 This selection rule implies that12

Rs(d3) ≈ Rs(0)

(4)

Where Rs(0) is the s-polarized reflectivity of the adsorbate-free metal. Furthermore, because absorption in layer 3 is larger for p-polarization than for s-polarization, Rp(d3) should be small compared to Rs(d3) according to the aforementioned surface selection rule. Using the latter condition, we assume that only first-order terms in [Rp(d3)/Rs(d3)] are relevant in PM-IRRAS, that is

[ ]

2

Rp(d3)

,1

Rs(d3)

(5)

By expanding the second square bracketed term on the righthand side of eq 3 in a binomial series and using eq 5, we have

[

]

Rp(d3)

I(d3)PM ) -R0 1 -

Rs(d3)

[

≈ R0 + 2R0

Rp(d3) Rs(d3)

2

-1

]

(6)

Using the standard definition of optical absorbance (for phase 3),6,12

[

A ≈ Ap ) 1 -

]

Rp(d3) Rp(0)

and using eq 4 in eq 6, we obtain

[

I(d3)PM ) R0

2(1 - A)Rp(d3) Rs(0)

(7)

]

-1

(8)

(9)

Setting d3 ) 0 in eq 6 gives I(0)PM ) R0[{2Rp(0)/Rs(0)} - 1]. By combining this last result with eqs 8 and 9, we have

hI PM ) 1 + R1A

normalized PM-IRRAS data, we define the normalized PSIRRAS signal hIPS as

hI PS )

It is customary to normalize the PM signal with respect to a background signal I(0)PM, collected in the absence of the sample SAM in the multilayer system.12,13,35 The normalized PMIRRAS signal IPM has the form

hI PM ) I(d3)PM/I(0)PM

Figure 2. Numerically calculated spatial variations (along the zdirection) of normalized mean-square electric fields in the medium of optical incidence. Panels A and B correspond to external reflection (φex ) 80.0 deg) and ATR (φin ) 71.7 deg), respectively. The parameters are described in the text. In both panels, plots a (〈Emx2〉) and c (〈Emz2〉) correspond to p-polarized light, and plot b (〈Emy2〉) corresponds to s-polarized light (m ) 1 or 4). All fields are normalized with respect to the incident mean square electric field (〈E+4x2〉 in A, and 〈E+1x2〉 in B). The coordinate z ) 0 corresponds to the location of the 4-3 interface for external reflection and that of the 1-2 interface for internal reflection.

(10)

where R1 ) [2Rp(0)/{Rs(0) - 2Rp(0)}]; R1 is a constant that depends on the background (phases 1, 2, and 4) contribution to the PM-IRRAS signal but is independent of the sample SAM properties. Let us now examine how the normalized PM-IRRAS signal expressed in eq 10 compares with the quantities measured in PS-IRRAS. For PS-IRRAS, the reflectivities Rp(d3) (or Rs(d3)) in the presence of the SAM and Rp(0) (or Rs(0)) in the absence of the SAM are measured separately by using a fixed state (p or s) of polarization. The background-corrected PS-IRRAS signals are written as I(d3)PSp ) [Rp(d3) - Rp(0)] and I(d3)PSs ) [Rs(d3) - Rs(0)] for p- and s-polarizations, respectively. To express the PS-IRRAS data in a form comparable to that of the

)

I(d3)pPS - I(d3)sPS I(d3)pPS + I(d3)sPS [Rp(d3) - Rp(0)] - [Rs(d3) - Rs(0)] [Rp(d3) + Rs(d3)] - [Rp(0) + Rs(0)]

(11)

Dividing both the numerator and the denominator on the righthand side of eq 11 by Rp(0) and using the definition of absorbance from eq 7, one can express eq 11 as follows:

hI PS )

A - β1 β0 - β 2

(12)

where β0 ) 1 + [Rs(0)/Rp(0)], β1 ) [Rs(0) - Rs(d3)]/Rp(0), and β2 ) [Rp(d3) + Rs(d3)]/Rp(0). For systems involving strong optical absorption, the right-hand side of eq 12 can be further simplified. For such systems, it is reasonable to assume that the inequality in eq 5 can be extended to first-order values of Rp(d3) and Rs(d3), so that Rp(d3) , Rs(d3). With this latter assumption, and using eq 4, we have β2 ≈ [Rs(0)/Rp(0)] ) (β0 - 1); then eq 11 takes the simple form

hI PS ≈ [A - β1] ≈ A

(13)

The last identity in eq 13 is based on the assumption that [Rp(0) -Rp(d3)] . [Rs(0) - Rs(d3)], that is A . β1. As shown later in this paper (in Figure 5), this particular condition (IhPS ∼ A) applies to values of φin that are greater than, but close to, φc. Both the PM-IRRAS signal normalized according to eq 10, and the PS-IRRAS signal normalized according to eq 13 are

FT-IR Using Polarization Modulation

Figure 3. Numerically calculated spatial variations (along the zdirection) of normalized mean square electric fields in the different phases of the multiphase system considered in Figure 1. Panels A and B correspond to the external and internal reflection cases, respectively. The parameters of calculations, as well as the normalization factors, are the same as those in Figure 2. Only a small section of the incident medium (phase 4 in A and phase 1 in B) is shown for each reflection geometry (the detailed field variations in these phases have been already presented in Figure 2). The vertical lines indicate the phase-boundaries, and z ) 0 is located at the first interface of optical incidence (4-3 boundary in A and 1-2 boundary in B). In both panels, plots a (〈Emx2〉) and c (〈Emz2〉) correspond to p-polarized light, and plot b (〈Emy2〉) corresponds to s-polarized light (m ) 2-4).

J. Phys. Chem. B, Vol. 107, No. 31, 2003 7815

Figure 5. Numerically calculated reflectivity (Rp in plot a and Rs in plot b) for the four-phase system of Figure 1 under the conditions of external reflection (panel A) and ATR (panel B). The system is characterized in terms of the parameters used in Figures 2-4. In panel B, only the angular range relevant for ATR (φin > φc ) 44.968°) is considered.

light. Nevertheless, the absorbance measured by PM-IRRAS involves both p- and s-polarized lights. Therefore, for a proper comparison of the PM and PS techniques, it is appropriate to include both polarization states in the analyses of both these techniques. 3. Numerical Calculations

Figure 4. Calculated angular dependencies of normalized mean-square electric fields in the Au substrate phase 2 (plots a, c, and e) and SAM phase 3 (plots b, d, and f). The cases of external reflection and ATR are shown in panels A and B, respectively. The parameters used for calculations are the same as those in Figures 2 and 3. In each panel, plots a-f represent 〈E2x2〉, 〈E3x2〉, 〈E2y2〉, 〈E3y2〉, 〈E2z2〉, and 〈E3z2〉, respectively. Plots a and b are coincident for both reflection geometries.

proportional to the absorbance A of the sample SAM in phase 3. We note here that the sample absorbance, as defined in eq 7, can be directly measured by PS-IRRAS, only using p-polarized

Spatial Variations of Mean Square Electric Fields. If the surface selection rule for IR absorption is active in a certain optical arrangement of the multilayer system, it should be possible to detect signature features of this selection rule in the z-dependent electric field components within different layers of the system. Here, we numerically calculate these fields for the four-phase system of Figure 1. We adapt the formulas discussed by Hansen3,45 and other authors,1,2,5,44 along with parameters that are appropriate for the actual experimental system of our present work. We use the following experimentally controlled or measured parameters: λ ) 3333 nm (3000 cm-1); d2 ) 10.98 nm (for the Au film, value reported later in this paper); d3 ) 1.87 nm (for the ODM layer on Au, value reported later in this paper). We also use the published optical constants: 1 (for CaF2) ) 2.002;46 2 ) -404.521 + j(75.79);47 4 ) 1, n3 ) 1.5;43 κ3 ) 0.1;43 3 ) (n2 - κ32) + j(2n3κ3) ) 2.24 + j(0.3). The critical angle, φc, for ATR in this system is calculated to be 44.968 deg. For the fields calculated at fixed angles, we use φex ) 80 deg at grazing incidence and φin ) 71.7 deg (our ATR experiments are typically performed in the 68-72 deg range). To calculate angular dependencies of the fields, we vary φex between 0 and 90 deg and vary φin over a range (φc < φin e 90 deg) that is relevant for ATR. Numerically obtained mean square electric fields in the first incidence phase of the four-phase system are shown in parts A (external reflection) and B (internal reflection) of Figure 2 as functions of the distance (z) along the surface normal. All these electric fields are normalized with respect to their respective incident mean-square fields. For each reflection geometry, plots

7816 J. Phys. Chem. B, Vol. 107, No. 31, 2003 a (x-component) and c (z-component) correspond to the ppolarized field and plot b (y-component) corresponds to the s-polarized field. The expected sinusoidal behaviors of electric fields in the incident media are observed here. The relative magnitudes and phases of the different field components are also observed. Normalized mean square electric fields in the intermediate and final phases are shown in parts A (external reflection) and B (internal reflection) of Figure 3. A small section of the first incidence phase is also included to provide a reference point for comparison among the different phases. In each panel, plots a (x-component) and c (z-component) correspond to p-polarized fields and plot b (y-component) corresponds to s-polarized fields. Let us first examine the graphs in panel A for external reflection. As expected in terms of electromagnetic boundary conditions, the x- and y-directional fields are continuous and the zdirectional field is discontinuous at the boundaries. Furthermore, 〈Emz2〉 (plot c) in each phase is considerably larger than the corresponding values of 〈Emx2〉 (plot a, amplified by a factor of 100) and 〈Emy2〉 (plot b, amplified by a factor of 100). It is this predominance of the electric field in the direction of surface normal that leads to the earlier mentioned surface selection rule. When p-polarized light is used, molecular dipoles in phase 3 with components along the surface-normal interact with the relatively large field Ez. In comparison with this case, the other field component, Ex, in p-polarization is much smaller, and even if it interacts with any nonvanishing dipole moments parallel to the surface, the resulting optical absorption would be smaller. A similar situation is found for the surface-parallel field Ey in s-polarization. As a result, only dipoles with components normal to the surface in the SAM phase would show measurable absorption peaks in p-polarized external reflection IRRAS. The corresponding spectrum with s-polarization would be dominated by the background response (media 2 and 4), with a relatively small contribution from the SAM phase. This polarizationspecific response of the adsorbed molecule provides the phenomenological basis for eqs 4 and 5. In PM measurements involving metal substrates, this also provides a relatively simple method of obtaining background-corrected spectra in the form of the quantity [Rp(d3) - Rs(d3)] and eliminates the need of additional data collection for the background spectrum. We note in this context, that the normalized form of the PM signal in eq 9 allows for a relatively straightforward method of excluding the instrumental constant R0 from data analysis.12 However, many PM experiments, especially those designed for simple IR absorption measurements, can be performed without including this normalization step. Let us now consider the case for internal reflection in Figure 3B. The expected discontinuity in the z-directional field is observed at the phase boundaries. In addition, as for external reflection, here again we find that in all the phases considered, 〈E3z2〉 of a p-polarized light (plot c) is noticeably larger than both 〈E3x2〉 (plot a, in p-polarization) and 〈E3y2〉 (plot b, in s-polarization). In fact, by comparing each graph in Figure 3B with its counterpart in Figure 3A, we find that the overall optical behaviors of the multiphase system are comparable in the two reflection geometries. In view of the foregoing discussion, these observations suggest the surface selection rule for IRRAS plays an active role, not only in external reflection, but also in ATR from the multiphase system. Angular dependencies of normalized mean square electric fields in the intermediate layers (2 and 3) of the four-layer system for external reflection are examined in Figure 4A. Each field 〈Emγ2〉 (m ) 2, 3; γ ) x, y, z) in phase m is calculated

Hutter et al. within but at the end of that phase. Thus for external reflection in Figure 4A (z ) 0 at the 3-4 boundary), 〈E2γ2(z)〉 and 〈E3γ2(z)〉 are calculated at z ) d3 + d2 and at z ) d3, respectively. Similarly, for ATR in Figure 4B (z ) 0 at the 1-2 boundary), 〈E2γ2(z) 〉 and 〈E3γ2(z) 〉 are calculated at z ) d2 and z ) d2 + d3, respectively. In Figure 4A, 〈E2z2〉 (plot e) and 〈E3z2〉 (plot f) are found to be the largest among the six field components compared. As previously noted by other authors,1,3-6 the maximum in both 〈E2z2〉 and 〈E3z2〉 occurs near grazing incidence. The values of 〈E2x2〉 (plot a) and 〈E3x2〉 (plot b) are close in the entire practical range of φex. Angular dependencies of 〈E2y2〉 (plot c) and 〈E3y2〉 (plot d) are different, but their relative magnitudes are similar. Angular dependencies of the normalized mean square electric fields in layers 2 and 3 for ATR are shown in Figure 4A. The situation observed here is similar in many respects to that found in Figure 4B. 〈E3z2〉 (plot f) in Figure 4B represents the strongest of the six field components considered. In addition, 〈E3z2〉 is largest at angles that are slightly higher than the critical angle. At higher incident angles, 〈E3z2〉 as well as the other field components become essentially independent of variations in φin, but 〈E3z2〉 still exhibits its predominance. The values of 〈E2x2〉 (plot a) and 〈E3x2〉 (plot b) are very close throughout the angular range of Figure 4B, and their differences are not detected at the resolution of this figure. 〈E2y2〉 (plot c) is larger than 〈E3y2〉 (plot d), but both decrease slowly and almost linearly as φin is increased. In view of the discussion presented in the context of Figure 3, the predominance of 〈E3z2〉 in Figure 4 can be characterized again as a signature feature of the surface selection rule in both reflection geometries considered here. Reflectivity. Angle-dependent IR reflectivities of the fourphase system, as calculated for external reflection and ATR configurations, are shown in parts A and B of Figure 5, respectively. In each panel, plots a and b represent Rp(d3) and Rs(d3), respectively. In Figures 2-4, we have seen how the surface selection rule for IR absorption acts under the conditions of both reflection geometries examined here. The smaller values of Rp(d3) than those of Rs(d3) at all angles considered in both panels of Figure 5 are consistent with the results of Figures 2-4. The observed angular changes of Rp(d3) and Rs(d3) originate from collective effects of the angle-dependent electric fields (shown in Figure 4) in intermediate phases of the experimental system. It should be noted, however, that the difference between Rs and Rp in most parts of the available range of incidence angles for ATR (Figure 5B) is less drastic than that observed near grazing incidence for external reflection (in Figure 5A). Therefore, for the former case, the approximation of eq 5 may not be as good as for the latter case. The results of Figure 5B indicate the presence of a relatively small range of φin (φin > φc), very close to φc, where the description of eq 5 is most prominent. The calculated results presented in Figures 2-5 are consistent with previously published results for similar systems.2-6,43,45 At the same time, the present calculations indicate that the surface selection rule of IRRAS for metal substrates should be operative in both external reflection and ATR studies of the ODM-SAM. This justifies the use of eqs 2, 5, 8, and 9 to establish the theoretical basis for combining the ATR method with PM-IRRAS. 4. Experimental Section Materials. Gold shots (6.35 mm and down, semispherical Premion, 99.999%; Alfa Aesar), SF10 glass slides (3 ft × 1 in.; SCHOTTGlass, Inc.), SF10 glass prisms (International

FT-IR Using Polarization Modulation

J. Phys. Chem. B, Vol. 107, No. 31, 2003 7817

TABLE 1: Comparison of Peak Characteristics of IR Spectra in Figures 6 and 7 absorption peak assignment and characteristicsa 1 (CH2 sym)

2 (CH3 sym)

3 (CH2 asym)

4 (CH3 asym)

reflection geometry

polarization control and FTIRRAS data

position, cm-1

area, au

position, cm-1

area, au

position, cm-1

area, au

position, cm-1

area, au

external reflection ATR external reflection ATR

fixed polariza tion, hIPS (eq 7) fixed polarization, hIPS (eq 7) polarization modulation, hIPS (eq 3) polarization modulation, hIPS (eq 3)

2849 2849 2848 2849

0.017 0.017 0.016 0.045

2877 2874 2880 2876

0.002 0.002 0.006 0.011

2917 2917 2916 2917

0.06 0.065 0.111 0.17

2963 2963 2965 2963

0.012 0.011 0.009 0.026

a

1-4 indicate peak numbers in Figures 6 and 7; a.u.) arbitrary units; sym ) symmetric; asym ) asymmetric.

Scientific Inc.), CaF2 prisms (Optimax), refractive index matching fluid (R. P. Cargille Laboratories), and octadecylmercaptan (ODM; Aldrich) were used as received. Ultrapure water was obtained from a Millipore Milli-Q column system provided with a Milli-pak filter of 0.22-µm pore size at the outlet. Fabrication of Gold Films and Self-Assembly of ODM. The gold film was evaporated on the CaF2 prism (and for some measurements, on an SF10 slide) by using an Edwards AUTO 306 compact vacuum coater, operated at 10-6 Torr. The thickness and deposition rate of the films were monitored by a built-in quartz crystal microbalance (QCMB), oscillating 6 MHz. The deposited mass and the film thickness were calculated from the change of oscillation frequency, displayed on a frequency counter. The thickness of the film was additionally checked by surface plasmon resonance (SPR) spectroscopy.48 The rate of deposition was maintained at a relatively fast rate of 0.2 nm/s to ensure that the resulting Au film was continuous (deposition rates that are favorable for Au island formation are about 100 times slower49). The Au film also exhibited the characteristic color of a continuous layer. Self-assembly of ODM was carried out by immersing the piranha-cleaned gold film into 1 mM ethanolic solution of the ODM for an hour. Subsequently, the film was washed with ethanol and methanol. FT-IRRAS. All IR spectra were recorded with a Digilab FTS 7000 spectrometer, using a liquid nitrogen cooled narrow band HgCdTe detector. A gold grid polarizer was used to obtain either s- or p-polarized radiation. The width (2 cm-1), resolution (8 cm-1), and sensitivity (1) of the aperture were kept fixed in all experiments. External reflection of the IR beam from the sample was measured at grazing angle (80 deg). For internal reflection, the gold-coated prism was turned around in such a way that the beam passed through the prism first and was reflected (internally) at the gold-coated base of prism and air interface at an angle around 70 deg. ATR measurements performed with φin between 67.5 and 72.0 deg yielded very similar results (as expected according to the calculated results of Figure 4B). The experiments with polarized but unmodulated light were done in a rapid scan mode, at 20 kHz, using an undersampling ratio (UDR) of 2, and co-adding 104 scans for each spectra. The bare gold film was scanned as a background. In the PM experiments, the IR light was phase-modulated at 400 Hz. The undersampling ratio was 4 and a UDR 4 filter was used. The polarized radiation was modulated by a HINDS ZnSe photoelastic modulator operating at 37 kHz and at 0.5 λ (strain axis 45° to the polarizer) before hitting the sample. Each spectrum represents a single scan with a spectral range of 400-4000 cm-1. A digital signal processing protocol (DSP-3, “Win-IR Pro Version 3.1” from Digilab), incorporated into the BioRad spectrometer software, was used to obtain polarization modulated spectra. As previously discussed by Drapcho et al.,50 this method does not require the use of a lock-in amplifier.

Figure 6. Experimentally recorded (and normalized according to eq 7) polarization selective IRRAS data for the four-phase system of Figure 1. Plots a and b correspond to the external reflection and ATR, respectively. The four peaks (labeled 1-4) represent the primary characteristic IR absorption bands of the ODM SAM in phase 3 of the multilayer system. Peak assignments and areas are summarized in Table 1.

5. Results and Discussion Thicknesses of the Au Substrate and the ODM Film. These thicknesses were determined with a two-solvent (ethanol and methanol) SPR technique,48 using incident probe lights at 632.8 and 670.0 nm. The thickness of the Au film was measured by QCMB and was further confirmed by SPR to be 10.98 nm. Due to this relatively thin layer of Au used here, the angle-resolved SPR data showed no distinct dip at the resonance angle. Therefore, the thickness of the ODM monolayer was determined in a separate set of experiments, using a gold film of known thickness (48.9 nm) on an SF10 glass slide/prism combination. The thickness of the ODM monolayer was determined to be 1.87 nm. Results of PS-IRRAS. The measured PS spectra of the CaF2/Au/ODM/air system, normalized in the form of eq 7, are presented in Figure 6. According to eq 10, the quantity plotted in this figure is proportional to the absorbance of the ODM SAM. Plots a and b correspond to the cases of external reflection and ATR, respectively. Four major peaks (labeled 1-4) are observed in the recorded PS-IRRAS data and are assigned in Table 1. The peak positions are consistent with those observed in previous PS-IRRAS studies of ODM SAMs on Au.51 We also note that the frequencies of the symmetric and asymmetric CH2 vibrations provide a measure of the degree of order in the adsorbed SAM.52 These frequencies for ODM, observed in Figure 6 (first two rows of Table 1) are also consistent with previous studies of similar SAMS.53 Both the symmetric and asymmetric CH2 vibrational modes in our experiments appear closer to their corresponding values for a crystalline sample than for a liquid sample,51,54 indicating a well-ordered SAM in the present case. In addition, the observed peak frequencies for external reflection in Figure 6 are equal or close to their respective values in ATR. This is an expected result because the same sample is used for the two reflection geometries. The slight differences of absorption frequencies in plots a and b of Figure 6 can be assigned to the differences in resolutions for the two setups used with the two reflection geometries.12,55

7818 J. Phys. Chem. B, Vol. 107, No. 31, 2003

Hutter et al. signature of the background spectra. This is indicated in the peak area ratios measured from Figure 7, as listed in Table 1. Nevertheless, all the relevant features of the PS spectra for both reflection geometries used are observed in the PM spectra. The calculated results in Figures 3-5 indicate that both external reflection and ATR geometries can be used for PM-IRRAS. The results of Figures 6 and 7 provide experimental evidence for this theoretical prediction.

Figure 7. Experimentally recorded (and normalized according to eq 3) polarization modulated IRRAS data for the four-phase system of Figure 1. Plots a and b correspond to the external reflection and ATR, respectively. The four peaks (labeled 1-4) in this figure correspond to the four characteristic IR absorption bands of ODM that are also observed in Figure 6. Peak assignments and areas are summarized in Table 1.

The integrated intensities (peak areas) of the four absorption bands observed in Figure 6 are listed in the first two rows of Table 1. For each peak, the integrated areas in the two reflection geometries are comparable. However, the peak shapes are noticeably different in the two cases. These differences in the peak shapes can be explained in terms of previously published theoretical considerations.55 Analyses of IR absorption peaks usually focus on the wavelength-dependent extinction coefficient (κ3 in the present case) of the absorbing sample. Often, however, IRRAS data are associated with a measurable wavelength dependence of the real part of the sample’s refractive index (n3 in our experiments). This latter effect can manifest itself in the Fresnel coefficients of the reflectivity equations and, in this way, can affect the size and shape of the IR absorption peaks. This phenomenon has been discussed in detail by Harrick4 and by Allara et al.55 The phase through which the light enters the SAM and the phase through which the light exits the SAM are switched as we change between external and internal reflection geometries. Therefore, the Fresnel coefficients are different for the external and internal reflection measurements performed in the present work. Consequently, the absorption peak shapes for the two reflection geometries are also somewhat different. Some differences in the absorption band shapes for external and internal reflections can also arise from different shapes of the background spectra, Rp(0) and Rs(0), collected in the two different reflection geometries. This can affect the absorption peak areas in the two cases. In fact, as indicated by the peak areas listed in Table 1, this background effect seems to manifest itself in a detectable way in the data of Figure 6. Allara et al. have discussed other possible reasons for distortion of IR absorption bands,55 which also can act with different strengths in external and internal reflections. Apart from these expected differences, the overall features of the external and internal reflection polarization selective IR spectra in Figure 6 are comparable. Results of PM-IRRAS. The measured PM-IRRAS data for the four-phase system, normalized in the form of eq 9, are presented in Figure 7. Plots a and b correspond to external reflection and ATR, respectively. The four expected absorption peaks (labeled 1-4) are observed in Figure 7. The peak frequencies (listed in the last two rows of Table 1) measured here do not exhibit any noticeable differences with respect to those measured by PS-IRRAS in Figure 6. The shapes of the PM-IRRAS peaks in Figure 7 show some differences between the cases of external reflection and ATR. These differences can be attributed to the different Fresnel factors for the two reflection configurations, as we have already discussed in the context of Figure 6. Like the case of PS-IRRAS in Figure 7, the PM-IRRAS spectra in Figure 7 also seem to contain a detectable

6. Conclusions In this paper, we have demonstrated that a combination of PM and FTIRRAS reflection-absorption measurements is possible for both external reflection and ATR configurations. Using numerical calculations, we have examined the IR reflection characteristics of a four-phase system involving a SAM of ODM on Au. The calculated results illustrate the theoretical basis for PM-IRRAS measurements using the Kretschmann geometry for ATR. Experimentally, we have performed a systematic comparison of a series of IR reflection absorption spectra of the above-mentioned four-phase system, collected under the conditions of PS and PM measurements using both external reflection and ATR configurations. The measured (and appropriately normalized) IR spectra for each reflection geometry are similar for the PS and PM experiments. The four characteristic IR absorption peaks of ODM appear in all these spectra and exhibit comparable characteristics in the different cases. The theoretical and experimental methods used in the present work can be extended to investigate other similar multilayer systems. In this way, by combining the unique advantages of PM-IRRAS and ATR in a single experimental framework, it should be possible to examine various previously unexplored details of different types of SAMs in different environments. Acknowledgment. We thank the US Department of Energy for supporting this work. E. Hutter thanks the National Science Foundation for financial support (Grant No. INT-0206923). References and Notes (1) Dluhy, R. A.; Stephens, S. M.; Widayati, S.; Williams, A. D. Spectrochim. Acta 1995, 51, 1413. (2) Axelsen, P. H.; Citra, M. J. Prog. Biophys. Mol. Biol. 1996, 66, 227. (3) Hansen, W. N. In AdV. Electrochem. Electrochem. Eng., Muller, R. H., Ed.; Wiley-Interscience: New York, 1973; Vol. 9, p 1. (4) Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corp.: Ossining, NY, 1979. (5) Mielczarski, J. A.; Yoon, R. H. J. Phys. Chem. 1989, 93, 2034. (6) Golden, W. G.; Saperstein, D. D.; Severson, M. W.; Overend, J. J. Phys. Chem. 1984, 88, 574. (7) Iwaki, M.; Andrianambinintsoa, S.; Rich, P.; Breton, J. Spectrochim. Acta 2002, 58, 1523. (8) Bu¨rgi, T. Phys. Chem. Chem. Phys. 2001, 3, 2124. (9) Futamata, M. Surf. Sci. 1999, 427-428, 179. (10) Singh, P. K.; Adler, J. J.; Rabinovich, Y. I.; Moudgil, B. M. Langmuir 2001, 17, 468. (11) Ishida, K. P.; Griffiths, P. R. Anal. Chem. 1994, 66, 522. (12) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectros. 1991, 45, 380. (13) Buffeteau, T.; Desbat, B.; Besbes, S.; Nafati, M.; Bokobza, L. Polymer 1994, 35, 2538. (14) Green, M. J.; Barner, B. J.; Corn, R. M. ReV. Sci. Instrum. 1991, 62, 1426. (15) Barner, B. J.; Green, M. J.; Sa´ez, E. I.; Corn, R. M. Anal. Chem. 1991, 63, 56. (16) Ozensoy, E.; Hess, C.; Goodman, D. W. J. Am. Chem. Soc. 2002, 124, 8524. (17) Shon, Y.-S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278.

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