A Spectroscopic Ellipsometry, Surface Plasmon Resonance, and X-ray

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A Spectroscopic Ellipsometry, Surface Plasmon Resonance, and X-ray Photoelectron Spectroscopy Study of Hg Adsorption on Gold Surfaces Todd Morris and Greg Szulczewski* The University of Alabama, The Department of Chemistry, Tuscaloosa, Alabama 35487 Received October 8, 2001. In Final Form: December 19, 2001 We report the results of a spectroscopic ellipsometry (SE) and surface plasmon resonance (SPR) study of mercury vapor adsorption on polycrystalline and (111) gold films. We found that, when Hg adsorbs onto gold films, the change in the ellipsometric parameter delta (∆) is proportional to the thickness of the adsorbed Hg layer (i.e., Drude approximation). On polycrystalline Au surfaces, ∆ increases linearly during the first 15-30 min of Hg exposure, remains constant between 30 and 45 min, and slowly increases beyond 50 min. In contrast, on Au(111) surfaces, ∆ increases linearly with Hg exposures up to 30 min but remains constant thereafter. In situ SPR reflectivity curves were measured on ∼50-nm polycrystalline Au films exposed to Hg vapor (∼15 ppm in air). The SPR minimum angle shifts linearly during the first 30 min of Hg exposure. Simulations of the SPR curves predict that the shift in the SPR minimum angle is directly proportional to the mercury coverage. X-ray photoelectron spectroscopy (XPS) revealed that the saturation surface coverage of Hg on Au(111) is 30% lower than that on polycrystalline gold films. The data presented here show that Hg adsorption on atomically flat gold is limited to the surface, whereas Hg diffuses into the bulk on polycrystalline films.

Introduction collection1

and Gold films play an important role in the detection2 of elemental mercury because of the strong affinity of these metals for one another. High-surfacearea gold “traps” (usually gold-coated sand) are used to preconcentrate Hg vapor for analysis by atomic absorption/ fluorescence spectrometry.2 In these analytical methods, Hg is sequestered from the vapor phase by adsorption onto gold. The gold film is heated to desorb the Hg, and the resulting vapor is admitted into an optical cell where either the absorption or emission is recorded. In addition to its important role as a Hg collector, Au can also function as a transducer. Several methods have been developed that exploit changes in a physical property of a gold film to detect Hg adsorption. For instance, resistivity changes in gold films can be correlated with Hg adsorption.3-5 McNerrey et al. were the first to demonstrate that small resistance changes in Au films (∼10-40 nm thick) can be used to detect Hg adsorption. They showed that quantities as small as ∼0.1 ng of Hg in air could be detected.3 Numerous types of gravimetric sensors for Hg have been described over the past 25 years.6-11 In these devices, the mass change when Hg adsorbs onto a gold-coated piezoelectric substrate is recorded. To our knowledge, there is only one report in * Corresponding author. Address: The University of Alabama, Department of Chemistry, 6th Avenue Lloyd Hall, Tuscaloosa, AL 35487. E-mail: [email protected]. Phone: 205-348-0610. Fax: 205348-9104. (1) Schroeder, W. H.; Hamilton, M. C.; Stobart, S. R. Rev. Anal. Chem. 1985, 8, 179-209. (2) French, N. B.; Priebe, S. J.; Hass, W. J. Anal.Chem. 1999, 71, 470A-475A. (3) McNerney, J. J.; Buseck, P. R.; Hanson, R. C. Science 1972, 178, 611-612. (4) George, M. A. Electrical, Spectroscopic, and Morphological Investigations of Mercury Adsorption on Thin Films. Ph.D. Thesis, Arizona State University, Tempe, AZ, 1991. (5) George, M. A.; Glausinger, W. S. Thin Solid Films 1994, 245, 215-224. (6) Scheide, E. P.; Taylor, J. K. Environ. Sci. Technol. 1974, 13, 10971099.

which an optical technique has been used to detect Hg adsorption onto gold films. Butler et al. showed that changes in the reflectivity of visible light from Au-coated optical fibers (∼30-nm film deposited onto the cleaved end of a 50-µm-diameter multimode fiber) could be correlated to Hg adsorption.12 Butler et al. stated that the optical response was independent of the wavelength of light, but most of the measurements were conducted with 860-nm radiation. In this paper, we demonstrate that surface plasmon resonance (SPR) and spectroscopic ellipsometry (SE) are complementary techniques that are capable of monitoring Hg chemisorption onto Au films. By comparing the optical data to X-ray photoelectron spectroscopy (XPS) results on polycrystalline and Au(111) surfaces, we have been able to distinguish between Hg adsorption and diffusion into the bulk. To our knowledge, this is the first study to utilize SE and SPR to monitor Hg adsorption on Au films. Experimental Section Two types of gold films were used in this work. Gold films were prepared by vapor deposition in a high-vacuum chamber (base pressure ≈ 2 × 10-7 Torr).13 A quartz crystal microbalance (Inficon) with gold-coated 8-MHz AT-cut crystals was used to monitor the Au flux and total accumulated thickness. The microbalance was calibrated against X-ray scattering measurements and found to be accurate to within 5%. Gold films (∼200 (7) Mogilevski, A. N.; Mayorov, A. D.; Stroganova, N. S.; Starovski, I. P.; Galkina, L.; Spassov, L.; Mihailov, D.; Zaharieva, R. Sens. Actuators A 1991, 28, 35-39. (8) Thundat, T.; Wachter, E.; Sharp, A. S. L.; Warmack, R. J. Appl. Phys. Lett. 1995, 66, 1695-1697. (9) Schweyer, M. G.; Andle, J. C.; McAllister, D. J.; Vetelina, J. F. Sens. Actuators B 1996, 35-36, 170-175. (10) Caron, J. J.; Haskell, R. B.; Benoit, P.; Vetelino, J. F. IEEE Trans. Ultrason., Ferroelectr. Freq. Control 1998, 45, 1393-1397. (11) Ruys, D. P.; Andrade, J. F.; Guimares, O. M. Anal. Chim. Acta 2000, 404, 95-100. (12) Butler, M. A.; Ricco, A. J.; Baughman, R. J. J. Appl. Phys. 1990, 67, 4320-4326. (13) Szulczewski, G. J.; Selby, T. D.; Kim. K.-Y., Hassenzahl, J.; Blackstock, S. B. J. Vac. Sci. Technol. A 2000, 18, 1875-1880.

10.1021/la011525n CCC: $22.00 © 2002 American Chemical Society Published on Web 02/14/2002

Study of Hg Adsorption on Gold Surfaces nm) for XPS and SE were deposited onto Cr-primed Si(100) wafers (Silicon Quest International). For SPR measurements, ∼50-nm Au films were deposited onto BK-7 glass (Schott). To improve adhesion between the glass and Au, a monolayer of a fourthgeneration starburst poly(amidoamine) dendrimer was first adsorbed onto the glass.14,15 Au(111) grown on mica was purchased from Molecular Imaging. Both the polycrystalline and Au(111) samples were characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). A typical polycrystalline sample contained islands that were ∼50 nm in diameter and ∼2 nm in height. The Au(111) samples were characterized by large (∼200 nm) atomically flat terraces separated by single-atom steps. XPS spectra were acquired using a Kratos Axis 165 spectrometer in a chamber with a base pressure of ∼3 × 10-10 Torr. The pressure during analysis was less than 1 × 10-9 Torr. Monochromatic Al KR radiation at 1486.7 eV was used as the excitation source in all quantitation experiments. Angle-resolved XPS utilized a conventional Al “flood” X-ray source. All spectra were acquired with an 80-eV pass energy. We did not observe any significant loss of mercury from the samples. We used the Shirley background subtraction technique16 and atomic sensitivity factors from Kratos to determine the relative Au/Hg atomic ratio from the measured Au(4f) and Hg(4f) peak areas. Ellipsometry measurements were made using a variable-angle spectroscopic ellipsometer (J. A. Woollam Co., Inc.) at 65°, 70°, and 75° angles of incidence and a wavelength range of 300-1000 nm. The parameters psi, Ψ, and delta, ∆, were measured and converted to either the complex dielectric function ( ) 1 + i2) or the complex index of refraction (N ) n + ik) using the Woollam software. The measured uncertainties (i.e., standard deviations) in Ψ and ∆ were (0.05° and (0.12°, respectively. At 632 nm, we found that our polycrystalline gold films had 1 ≈ -9.5 and 2 ≈ 1.1. SPR reflectivity curves were measured in the Kretchmann configuration17 with a home-built apparatus. Our design is similar to the one described by Ehler and Noe.18 The output of a 15-mW polarized HeNe laser (632 nm) was attenuated to ∼1 mW with a pair of neutral density filters. The beam was mechanically chopped (Stanford Research Systems, model 540) at 4 kHz and passed through a 500:1 polarizer to generate p-polarized radiation (with respect to the plane of incidence). The Au/glass slides were placed against one side of a 45° prism (BK-7 glass) with a drop of index-matching fluid (n ) 1.515) to ensure optical coupling between the two substrates. The other leg of the prism was coated with a 200-nm silver film. The 632-nm radiation exited the prism by reflection off the Ag film and was detected with a Si photodiode (Thorlabs, model DET 100). The output of the photodiode was sent to a lock-in amplifier (Stanford Research Systems, model 830), where it was referenced against the chopper frequency, and the DC voltage was recorded by a personal computer. The prism was mounted to a computer-controlled high-resolution rotation stage (Newport, model URM100C). The resolution of the stage was 0.001°. In a typical SPR experiment, the step size was 0.01°. The gold samples were exposed to mercury in two ways. Samples for XPS and ellipsometry were placed in a small capped glass jar (volume ≈ 300 mL) containing ∼5 g of liquid mercury. Because of its volatility, Hg soon reached a saturated concentration of ∼13-17 ng/mL at 21-23 °C in the jar (or ∼15 ppm).4 The “mercury exposure time” was the time in the sealed jar. For the SPR measurements, Hg-saturated air was obtained in a similar manner. About 1 g of Hg was held in a 10-mL test tube. The test tube was placed inside a sealed flask, and the Hg-saturated air was pumped into a Teflon cell (∼1 mL volume) that was held firmly against the gold film with a Kalrez O-ring. The measured flow rate of the mercury-saturated air was ∼3-4 mL/min. Any excess mercury was bubbled through a scrubbing solution (HNO3/ KMnO4) to oxidize the excess mercury. The scrubbing solution (14) Baker, L. A.; Zamborini, F. P.; Sun, L.; Crooks, R. M. Anal. Chem. 1999, 71, 4403-4406. (15) Rar, A.; Zhou, J. N.; Liu, W. J.; Barnard, J. A.; Bennett, A.; Street, S. C. Appl. Surf. Sci. 2001, 175/176, 134-139. (16) Shirley, D. A. Phys. Rev. B 1972, 5, 4709-4714. (17) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569-638. (18) Ehler, T. T.; Noe, L. J. Langmuir 1995, 11, 4177-4179.

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Figure 1. X-ray photoelectron spectra of polycrystalline Au films exposed to ∼15 ppm Hg in air for (a) 0, (b) 5, (c) 10, (d) 60, and (e) 180 min. also provided a convenient means of monitoring the Hg flow. When elemental mercury is oxidized to Hg2+, the MnO4- ion is reduced to MnO2. The result is a decrease in the absorbance (measured with a UV-vis spectrometer) of the MnO4- ion at ∼550 nm. In a set of measurements with the gold film removed from the flow cell, we found that the flow rate of Hg-saturated air was constant throughout the duration of the experiments.

Results We used XPS to determine the absolute surface coverage of Hg on the Au films. In Figure 1, we show the Hg(4f) and Au(4f) regions as a function of mercury exposure time on polycrystalline gold. It is evident that the surface coverage of mercury increases rapidly and saturates near 30 min (see inset in Figure 1). Quantitative analysis places the absolute Hg surface coverage at ∼9% of a monolayer or ∼1 × 1014 Hg atoms/cm2. These data suggest that the sticking coefficient is independent of coverage until most available adsorption sites are occupied. On a Au(111) surface, the absolute saturation coverage was measured to be ∼0.7 × 1014 Hg atoms/cm2. This difference in measured coverage was observed for many samples and is outside the uncertainty of the measurements. Figure 2 shows plots of ∆ versus wavelength for Au(111) and polycrystalline Au surfaces before and after exposure to ∼15 ppm of Hg for 30 min. Across the investigated wavelength range, three trends are apparent. First, the ∆ value is sensitive to the morphology of the clean gold surface. Specifically, ∆ for the polycrystalline gold film is ∼5-10° lower than that for the Au(111) sample. Second, the change in ∆ after Hg exposure is larger for the polycrystalline gold than for Au(111). These results are consistent with the XPS measurements, which re-

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Figure 2. Plot of ∆ for Au(111) and polycrystalline Au films before and after exposure to ∼15 ppm Hg vapor for 30 min. The solid lines and dashed lines represent Au(111) and polycrystalline gold, respectively. In both cases, the upper line is the result after Hg exposure.

vealed the polycrystalline Au film had ∼30% higher Hg surface coverage than Au(111). Third, the change in ∆ is greatest between 500 and 550 nm for both the polycrystalline and Au(111) samples. Consequently, we monitored the time-dependent change in ∆ with Hg exposure at 530 nm (see Figure 3). On both the polycrystalline and singlecrystal Au samples, the change in ∆ change was approximately linear for first ∼30 min. On the Au(111) sample, we observed no further change in ∆ after 30 min. In contrast, ∆ continued to increase on the polycrystalline sample after 60 min. Figure 4 shows SPR reflectivity curves before and after exposure to Hg for 60 min. The solid lines are the best-fit SPR curves using the optical constants (at 632 nm) listed in Table 1. The fits were determined by solving the Fresnel equations using the N-phase model of Hansen.19 We note that this procedure has been successfully employed to model experimental SPR reflectivity curves.20,21 However, it is well-known that, using single-wavelength SPR, it is not possible to uniquely determine both the optical constants and film thickness.22 Our approach was to determine the best optical constants and thickness of the gold film before Hg exposure. We always used the thickness measured by the QCM as an initial estimate of the gold film thickness and typical optical constants for thermally evaporated gold films.20 We allowed these parameters to vary in the fit until the best values were obtained (i.e., the smallest difference between the experimental and calculated values). For every sample, we always determined (19) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380-390. (20) Hanken, D. G.; Jordan, C. E.; Frey, B. L.; Corn, R. M. Electroanal. Chem. 1998, 20, 141-225. (21) Frutos, A. G.; Corn, R. M. Anal. Chem. 1998, 70, 449A- 455A. (22) Peterlinz, K. A.; Georgiadis, R. Opt. Commun. 1996, 130, 260266.

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Figure 3. Change in ∆ at 530 nm for polycrystalline and Au(111) samples after exposure to ∼15 ppm Hg. The open circles and closed circles represent the data on the Au(111) and polycrystalline surfaces, respectively.

the best values and kept those values fixed when we included Hg in the fit. We note that our Au films were stored in air and not cleaned, so an adventitious contamination layer was present on the surface.23 As a result, it was necessary to include a “contamination layer” in the model to fit the experimental SPR data for bare gold films. Interestingly, it was necessary to remove the contamination layer from the model after Hg exposure to achieve the best fit. On this latter point, there is evidence to suggest that Hg adsorption onto Au surfaces displaces the ubiquitous physisorbed contamination layer.4 In general, the optical constants that we measured for our gold films are consistent with published values.24 Figure 5 shows the absolute value of the change in the SPR minimum angle with Hg exposure. The change is linear during the first 30 min. Figure 6 shows simulations of SPR curves using the experimentally determined optical constants of our gold films and published optical constants of liquid mercury.25 As the Hg layer thickness is increased from 0.1 to 0.6 nm, the minimum angle decreases, and the reflectivity at the minimum angle increases. Both of these trends are observed in the experimental measurements. Although not as obvious as the two trends mention above, the width of the experimentally measured SPR curves also increases. In simulations (not shown), the full width of the SPR curve increases from ∼3° to ∼6° as the thickness of the Hg layer increases from 0.3 to 3 nm. Discussion The vast majority of studies that have examined the adsorption of Hg to gold have been on polycrystalline Au films. Despite the fact that several different analytical (23) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (24) Georgiadis, R.; Peterlinz, K. P.; Peterson, A. W. J. Am. Chem. Soc. 2000, 122, 3166-3173.

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Figure 5. Plot of the change in SPR minimum angle versus Hg exposure time.

Figure 4. Experimental and simulated surface plasmon resonance reflectivity curves before and after Hg exposure. The circles and squares represent the data for the clean and Hgcovered samples, respectively. The inset shows the differential reflectivity. Table 1. Optical Constants and Film Thicknesses Used in SPR Calculations in Figure 4 layer BK-7 glass Au contamination Hg air

thickness (nm) 49.3 0.5 0.2

1

2

2.2952 -11.04 2.10 -23.46 1.001

0 1.51 0 20.78 0

methods have been used to determine Hg surface coverage, there is general agreement that the saturation coverage is ∼1 × 1014 Hg atoms/cm2.1,4,26,27 The theoretical number of surface atoms on the Au(111) face is 1.5 × 1015 Au atoms/ cm2. Our XPS measurements on polycrystalline films are also in agreement with the previous measurements. However, we find that, under identical Hg exposures the absolute Hg surface coverage on Au(111) is ∼30% less than that on the polycrystalline sample. In addition, angleresolved XPS measurements (data not shown) indicate that a ∼0.3-nm Hg layer forms on Au(111) and a ∼1.0-nm Hg layer forms on a polycrystalline Au film after 30 min of Hg exposure. (25) Palik, E. D. Handbook of Optical Constants of Solids II; Academic Press: New York, 1991. (26) Dumarey, R.; Dams, R, Hoste, J. Anal. Chem. 1985, 57, 26382643. (27) Battistoni, C.; Bemporad, E.; Galdikas, A.; Kaciulis, S.; Mattogno, G.; Mickevicius, S.; Olevano, V. Appl. Surf. Sci. 1996, 103, 107-111.

Figure 6. Effect of increasing Hg layer thickness on simulated SPR reflectivity curves. The inset shows that the minimum angle decreases linearly with increasing Hg layer thickness.

We measured the Au(4f7/2) and Hg(4f7/2) binding energies to be 83.9 and 99.8 eV, respectively. Brundle and Roberts28 dosed Hg vapor onto Au under ultrahigh-vacuum condi-

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tions and measured the Au(4f7/2) and Hg(4f7/2) binding energies to be 83.7 and 99.6 eV, respectively. As a result, the small differences in the measurements can be attributed to a difference in the surface conditions or simply in the spectrometer calibration. For comparison, we note that Svensson et al. measured the Hg(4f7/2) binding energy of solid Hg29 to be 99.9 eV, so there is only a small perturbation in the binding energy when Hg atoms adsorb onto Au. After a 30 min exposure, we find that the Hg(4f) peak area saturates and the Au(4f) peak area decreases. This observation suggests that mercury diffuses into the bulk of the polycrystalline sample and beyond the sampling depth of XPS. A comparison of the measured mean free paths of the Hg(4f) and Au(4f) photoelectrons supports this conclusion. The mean free path of Hg(4f) photoelectrons emerging from Au films has been measured to be about 0.9 nm.28 The mean free path of the Au(4f) photoelectrons has been measured to be ∼4 nm.30 The effective sampling depth is about three times the mean free path or ∼3 nm for Hg and ∼12 nm for Au. As a result, mercury atoms that diffuse ∼3-10 nm below the interface will not lead to an increase in the Hg(4f) intensity but will attenuate the Au(4f) intensity. Because XPS is a very surface-sensitive technique, we elected to use ellipsometry because light penetrates further below a metal surface. For example, at 632 nm, the penetration depth is estimated to be ∼25 nm.31 Gold films have been characterized by ellipsometry for decades. It is well-known that surface morphology and roughness will strongly affect the measurement of Ψ and ∆. Perhaps Aspens and co-workers performed the most thorough characterization of Au films by spectroscopic ellipsometry.32 They prepared Au films under a variety of growth conditions and examined the Au microstructure by transmission electron microscopy. They demonstrated that voids in polycrystalline samples strongly affect the determination of the imaginary component of the dielectric function above 2.5 eV (∼500 nm). The result is an increase in ∆ for smoother films. Our gold samples exhibit the characteristic features reported by Aspens, namely, ∆ is larger for a pristine surface (see Figure 2). When we exposed Au(111) and polycrystalline gold samples to identical Hg concentrations, we always observed that the change in ∆ was greater for the polycrystalline sample. It is well-known that ∆ is very sensitive to the phase between the incident and reflected s- and p-polarzied waves.33 Many adsorbate systems have been found to satisfy the Drude approximation, ∆ - ∆o ≈ d, where d is the adsorbed layer thickness, ∆o is measured on the clean substrate, and ∆ is measured with adsorbed layer on the substrate. We used our XPS data to determine whether the Hg/Au adsorbate/surface system conforms to the Drude approximation. Inspection of Figure 3 reveals that ∆ rapidly increases during the first 20 min of Hg exposure and plateaus near 30 min. The same trend is seen in the XPS measurements (see inset in Figure 1). The Hg surface coverage rapidly increases but plateaus after a 30-min Hg exposure. Consequently, we suggest that the change (28) Brundle. C. R.; Roberts. M. W. Chem. Phys. Letts. 1973, 18, 380-381. (29) Svenson, S.; Martensson, N.; Basilier, E.; Malquist, P. A.; Gelius, U.; Siegbahn, K. J. Electron. Spectrosc. Relat. Phenom. 1976, 9, 51-65. (30) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 16701673. (31) Tompkins, H. G.; McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry; Wiley: New York, 1999. (32) Aspens, D. E.; Kinsborn, E.; Bacon, D. D. Phys. Rev. B 1980, 21, 3290-3299. (33) Tompkins, H. G. A User’s Guide to Ellipsometry; Academic Press: New York, 1993.

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in ∆ at 530 nm is a measure of the Hg layer thickness (or surface coverage). For Hg exposures between 30 and 60 min, ∆ is constant (see Figure 3), but it slowly increases beyond 60 min. We attribute this behavior to signal the onset of Hg diffusion into the bulk. This is supported by the fact that ∆ remains constant on Au(111) for exposures beyond 30 min. The transition from Hg adsorption to diffusion can be also be detected by SPR. In Figure 5, we observe that the SPR minimum angle decreases linearly within the first 30 min of Hg exposure. A similar observation has been made for Ag deposited onto Au films:34 the minimum SPR angle decreased as the thickness of the silver film increases. This is because Ag has a more negative value of the real component of the dielectric function, 1, than Au (about -18 versus -11). Therefore, we can expect the SPR minimum angle to decrease when Hg adsorbs onto Au because 1 for Hg is more negative than 1 for Au as well (see Table 1). The simulations in Figure 6 show that the decrease in the SPR minimum angle is directly proportional to the thickness of the adsorbed Hg layer (0.12°/nm). We find that the SPR minimum angle begins to change nonlinearly for Hg exposures beyond 30 min. Once again, we attribute this spectroscopic change to signal Hg diffusion into the bulk on a polycrystalline sample. Very few studies of Hg adsorption on single-crystal gold surfaces have been performed.4,35,36 Recent scanning tunneling microscopy results by Levlin et al. show that mercury adsorption onto a gold surface depends on its morphology.36 Levlin et al. exposed Hg vapor in a nitrogen atmosphere to a Au(111) surface held at various temperatures and Hg concentrations. Levlin et al. were able to correlate the Hg surface coverage and scanning tunneling microscopy images to suggest that Hg does not diffuse into the bulk on atomically flat gold. They proposed that Hg adsorption occurs via a place-exchange mechanism. Our ellipsometry experiments on Au(111) and polycrystalline gold surfaces support the conclusions of Levlin et al. We are currently trying to prepare smoother Au films for further SPR studies. Conclusions We have demonstrated that spectroscopic ellipsometry and surface plasmon resonance spectroscopy can be utilized to monitor Hg adsorption onto Au films. Specifically, we have shown that the decrease in the SPR minimum angle and the increase in the ellipsometric parameter ∆ are proportional to Hg coverage. We have shown that the morphology of a gold film governs the extent of mercury diffusion into the bulk. On pristine Au(111) films, Hg adsorption is limited to the surface. In contrast, on polycrystalline Au films, mercury readily diffuses into the bulk as a result of the granular structure of the film. Acknowledgment. G.S. thanks the School of Mines and Energy Development at The University of Alabama for funding this work. We also acknowledge the National Science Foundation for use of shared instrumentation through Materials Research Science and Engineering Center Grant DMR-98-09423. T.M. thanks the Graduate Council at the University of Alabama for providing full support in the form of a graduate fellowship. LA011525N (34) Schro¨der, U. Surf. Sci. 1981, 102, 118-130. (35) Levin, M.; Niemi, H. E.-M.; Hautojarvu, P.; Ikavalko, E.; Laitinen, T. Fresenius’ J. Anal. Chem. 1996, 355, 2-9. (36) Levlin, M.; Ikavalko, E.; Laitinen, T. Fresenius’ J. Anal. Chem. 1999, 365, 577-586.