Fe(CO)5 Thin Films Adsorbed on Au(111) and on Self-Assembled

Fe(CO)5 Thin Films Adsorbed on Au(111) and on Self-Assembled Organic Monolayers: II. Thermal Transformations. Christelle Hauchard, Christian Pépin, a...
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Fe(CO)5 Thin Films Adsorbed on Au(111) and on Self-Assembled Organic Monolayers: I. Structure Christelle Hauchard, Christian Pe´pin, and Paul Rowntree* De´ partement de Chimie, Universite´ de Sherbrooke, Sherbrooke, Centre for the Self-Assembled Chemical Systems, Que´ bec J1K 2R1, Canada Received March 4, 2005. In Final Form: June 23, 2005 The adsorption of Fe(CO)5 onto Au(111)/mica and C4, C8, C12, and C16 SAMs/Au(111)/mica surfaces has been studied using infrared spectroscopy to elucidate the coverage-dependent structures of these films and the intermolecular couplings that determine the form of the spectra. For all substrates, the first layer is composed of molecules physisorbed with one axial and two equatorial carbonyl groups directed toward the substrate; subsequent layers are preferentially oriented with the C3 molecular axis aligned perpendicular to the substrate (i.e., one axial carbonyl group directed toward the substrate). The axial vibrational band systematically shifts to higher frequencies with increasing surface coverage because of the effects of intermolecular coupling of the quasiparallel transition dipole moments. The strong effects of dipolar coupling are also witnessed by the trends of the band positions when the distance to the image plane is systematically varied using highly organized self-assembled organic substrates; no band shifts are observed when dilute Fe(CO)5 is embedded in Xe matrixes under identical experimental conditions. The as-deposited films are structurally stable below 125 K on Au(111)/mica surfaces and below 100 K on the organic self-assembled monolayers. The instability of the films above these temperatures demonstrates that the as-adsorbed films do not form thermodynamically well-defined phases but are structurally metastable. The results presented herein and in the companion paper provide a consistent framework to interpret the spectroscopy of these systems that resolves outstanding issues concerning these films and provides a structural model that explains the dynamic properties of these films during exposure to low-energy electron beams.

Introduction Organometallic adsorbates are the subject of widespread interest in surface science, because of their molecular complexity, ease of preparation and deposition, and accessibility to theoretical methods as tractable models to study extended metallic systems. In addition, the relative ease with which organometallic species can undergo decomposition to produce metallic deposits on surfaces via thermal processing,1-5 photolysis,6-9 and interaction with electron beams2,10,11 makes them versatile precursors for surface modifications. There is widespread interest in understanding (i) the structure of thin films composed of organometallic species, (ii) how these structures are influenced by the deposition substrate, and (iii) how the film structure can influence the decomposition behavior of the components. Unfortunately, many of the experimental techniques that are used in modern surface science (e.g., LEED, XPS, UPS, and TPD) can fragment these fragile molecular species, and nondestructive characterization methods are advantageous. This report explores the adsorption of iron pentacarbonyl [Fe(CO)5, herein referred to as IPC] on various substrates, to elucidate the structure of the as-deposited * To whom correspondence should be addressed. E-mail: [email protected]. (1) Ren, D.; Sung, D.; Gellman, A. J. Tribol. Lett. 2001, 10, 179. (2) Foord, J. S.; Jackman, R. B. Chem. Phys. Lett. 1984, 112, 190. (3) Rocklein, M. N.; Land, D. P. Surf. Sci. 1999, 436, L702. (4) Sun, L.; McCash, E. M. Surf. Sci. 1999, 422, 77. (5) Zaera, F. Surf. Sci. 1991, 255, 280. (6) Trushelm, M. R.; Jackson, R. L. J. Phys. Chem. 1983, 87, 1910. (7) Sato, S.; Tanaka, S. Appl. Surf. Sci. 1998, 135, 83. (8) Sato, S.; Minoura, S.; Urisu, T.; Takasu, Y. Appl. Surf. Sci. 1995, 90, 29. (9) Sato, S.; Suzuki, T. J. Phys. Chem. 1996, 100, 14769. (10) Henderson, M. A.; Ramsier, R. D.; Yates, J. T., Jr. Surf. Sci. 1991, 259, 173. (11) Foord, J. S.; Jackman, R. B. Surf. Sci. 1986, 171, 197.

films. The surfaces employed are Au(111)/mica and methyl-terminated organothiol self-assembled monolayers (SAMs) chemisorbed onto Au(111)/mica substrates. These surfaces were selected because they are chemically inert toward most adsorbates and the ease with which the organic film can be modified to vary the adsorption properties; a long-term motivation is our interest in using the decomposition of organometallic species under electron impact to produce metallic overlayers on organic SAMs, with possible applications in molecular electronics.12-14 To preclude probe-induced molecular damage, we have used infrared reflection-absorption spectroscopy (IRRAS) to probe the structural characteristics of the deposited film; the strong optical absorption by the two distinguishable types of carbonyl groups of IPC are a definite advantage in this regard. Adsorbed IPC has been studied by numerous previous workers on metal,4,5,7,9,10,15-21 semiconducting,2,8,11,22,23 and insulating6,8 supports. For reactive supports [e.g., Pd(111),3 Cu(111),4 and Pt(111)5], thermal molecular decomposition can be induced, often (12) Wang, W.; Lee, T.; Reed, M. A. Phys. Rev. B 2003, 68, 35416. (13) Kuan, C.-C.; Guo, H. Nano. Lett. 2003, 3, 1521. (14) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103. (15) Sato, S.; Suzuki, T. J. Electron Spectrosc. Relat. Phenom. 1997, 83, 85. (16) Sato, S.; Suzuki, T. Appl. Spectrosc. 1997, 51, 1170. (17) Sato, S.; Ukisu, Y.; Ogawa, H.; Takasu, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 4387. (18) Sato, S.; Ukisu, Y.; Ogawa, H.; Takasu, Y. Appl. Surf. Sci. 1994, 79, 428. (19) Henderson, M. A.; Ramsier, R. D.; Yates, J. T., Jr. J. Vac. Sci. Technol., A 1991, 9, 1563. (20) Henderson, M. A.; Ramsier, R. D.; Yates, J. T., Jr. Surf. Sci. 1992, 275, 297. (21) Xu, M.; Zaera, F. Surf. Sci. 1994, 315, 40. (22) Gluck, N. S.; Ying, Z.; Bartosch, C. E.; Ho, W. J. Chem. Phys. 1987, 86, 4957. (23) Swanson, J. R.; Friend, C. M.; Chabal, Y. J. J. Chem. Phys. 1987, 87, 5028.

10.1021/la050593x CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005

Fe(CO)5 Thin Films Adsorbed on Au(111) and SAMs

proceeding by the formation of a structurally distorted intermediate species and CO transfer to the substrate. Less reactive surfaces such as Ag(111) and Au(111) usually7,9,10,15-19 reversibly adsorb and desorb IPC, thus facilitating the interpretation of the infrared results. Despite the apparent simplicity of the IPC/(Au, Ag) systems, however, there is considerable disagreement in the literature concerning the structure of the adsorbed layer; the uncertainty of the structural configuration of adsorbed IPC has important consequences in the interpretation of its dissociation dynamics. Sato et al.15 have reported the IRRAS spectra of multilayer IPC/Au(111) to be composed of a dominant narrow band at ∼2058 cm-1 with a weak shoulder at ∼2017 cm-1; these bands were identified as being due to the excitation of the axial and equatorial modes of IPC, respectively. On the basis of a structural motif of molecules tilted by ∼65° from the surface normal and the transmission spectra of IPC adsorbed on sapphire windows, they determined the optical constants for IPC layers and applied these to interpret the IRRAS results. This model was able to reproduce the broad features of their experimental results, but the predicted AX frequency was only 2048 cm-1, while the width and intensity distribution among the EQ and AX bands could not reproduce the experimental trends. The physical interpretation of the IRRAS spectroscopy remains problematic. As mentioned above, the decomposition of organometallic species at surfaces is a fundamental interest for our work, and IPC presents several attractive characteristics in this regard. Gas-phase IPC readily decomposes by CO elimination by electron irradiation even for incident electron energies below 2-3 eV,24 and this low-energy dissociation channel could be exploited to produce atomic/ metallic Fe on fragile substrates without leading to significant damage to the substrate. Electron irradiation of IPC/Ag(111) was explored by Henderson et al.10 using TPD measurements, but the cross-sections for CO elimination were found to be rather low (∼1 Å2 below 8 eV). We have verified that under the conditions of their experiment the cross-sections for IPC degradation on Au(111)/mica are indeed very small; in an effort to understand why the dissociation dynamics are so strongly perturbed by the adsorbed state, we studied the structure of these films and explored the sensitivity of the electron-induced processes to the film structure. The structural issues for as-deposited films are described in this work, while the effects of thermal processing are explored in detail in a companion paper, herein referred to as II.25 The central result of II is that as-deposited films of IPC on Au(111) and alkanethiol SAMs do not form thermodynamically stable structures, despite (1) the apparently crystalline simplicity of the IRRAS results for these systems and (2) the common assumption that the as-deposited films are homogeneous.15 This fundamental observation has important consequences; as will be shown, the thermal processing of these films can have profound effects on the structure of the adsorbed material, which in turn, we believe, is the underlying cause of the low dissociation cross-sections reported previously. Equally important, the sensitivity of the film structure to the thermal history of the sample may be an underlying cause for the inability of previous spectroscopic models to accurately reproduce the IRRAS results for the as-deposited films. By exploring both the as-deposited films (in this paper) and the (24) Pignataro, S.; Foffani, A.; Grasso, F.; Cantone, B. Z. Phys. Chim. Neue Folge 1965, 47, 106. (25) Hauchard, C.; Pepin, C.; Rowntree, P. Langmuir 2005, 21, 91669175.

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structural changes induced by sample heating (II), a more comprehensive understanding of the film structure and its consequences for molecular decomposition processes can be developed. Experimental Procedures The experiments reported herein were performed in an ultrahigh vacuum chamber evacuated by a 220 L/s ion pump (Physical Electronics) to a base pressure of 5 × 10-11 Torr. The chamber was equipped with load-lock evacuated by a 65 L/s turbomolecular pump (Balzers TPU-065), a residual gas analyzer (Stanford Research Systems RGA-200), and a custom-built sample manipulator that supported the surfaces in the vertical plane with five independent degrees of motion. The sample holder was connected to an electrically isolated closed-cycle helium cryostat (APD-202B) by an OFHC copper braid, allowing sample temperatures to be adjusted from 25 to 500 K. Individual samples consisted of 1.0 × 2.0 cm Au/mica films (see below) secured to a 2.5 mm thick copper carrier with copper straps providing a thermal and electrical contact directly to the front surface of the metal film. The lower edge of this copper sample carrier had an extension that was immersed in a Ga-filled well of the sample holder during measurements; cooling the sample holder below the normal melting point of Ga (27 °C) created reliable mechanical, electrical, and thermal contacts between the sample carrier and the sample holder. A Chromel versus AuFe (0.07%) thermocouple embedded in the sample holder and a conventional temperature controller (Omega 1402) stabilized the temperature within 1 K of the set point; the accuracy of the thermocouple and controller system is estimated to be (1 K based on 4.2 and 77 K calibration points in our laboratory. No thermal hysteresis or lag was detected in the spectral data for the slow heating or cooling cycles employed in this study. The plane-parallel infrared beam from a Fourier transform infrared spectrometer (Nicolet Magna 550) was focused through a differentially pumped NaCl window onto the sample at an angle of incidence of 85° with respect to the surface normal. The diverging reflected infrared beam exited the chamber through a second differentially pumped NaCl window and was refocused onto a 77 K mercury cadmium telluride detector (MCT-A) using an ellipsoidal mirror. The spectra shown herein were collected with a 2 cm-1 resolution (data point spacing of 1 cm-1), and each spectrum was the result of averaging 64 or 128 mirror scans. Moving mirror speeds of 1.98 cm/s (resulting in a typical spectrum acquisition time of 60 to 120 s) were used throughout, although the results show no sensitivity to such instrumental parameters. In measuring temperature-dependent spectra during heating or cooling cycles, the quoted temperature is the average measured during the spectrum acquisition, with each spectrum being signalaveraged across a ∼2-3 K temperature range. Peak positions were obtained by fitting a parabolic function to the peak maximum and extracting the maximum from the fitted function; this permitted the reproducible and objective evaluation of the peak position (and intensity) with an estimated precision of ∼0.25 cm-1, while the absolute accuracy of our results is estimated to be (1 cm-1. Reference reflectivities for absorbance spectra are obtained using pristine substrate prior to sample deposition or by deflecting the incident beam such that it passes in front of the sample; no significant differences are seen with this “direct beam” reference as compared to using the reflectivity of pristine Au/ mica surfaces. ∆Absorbance spectra are obtained by referencing to the reflectivity of the surface just prior to molecular adsorption, which may already support molecular adsorbates. The only data manipulation that has been applied to these data is a linear baseline correction, defined by two points outside the pertinent spectral region. For the IRRAS experiments, the substrates used were 200 nm gold (Johnson-Mathey, 99.999%) films evaporated on freshly cleaved mica sheets (ASTM-V2, ProScience/Techniglass). The mica surfaces were degassed by heating to 300 °C for 12 h prior to deposition, using a modified version of the protocols described by DeRose et al.26 The deposition was performed at a rate of ∼0.2 (26) DeRose, J. A.; Thundat, T.; Nagahara; L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102.

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nm/s, while the mica surfaces were maintained at 300 °C. The Au/mica surfaces were heated for an additional 1-6 h after deposition to ensure sample uniformity and high substrate quality; samples were then cooled to ambient temperature under vacuum for 6-12 h. STM images of these Au/mica surfaces showed the expected terraced topography with 100-200 nm domain sizes and atomically resolved steps characteristic of the Au(111) surfaces; the long-range 22x3 reconstruction of the Au(111) surface are frequently observed in our laboratory using these films, demonstrating the high quality of the substrates. The substrates were then cleaned with sulfochromic acid for 10 min to remove any adventitiously adsorbed organic impurities on the surface and rinsed with copious quantities of Nanopure water prior to being introduced into the UHV environment via the load lock. In situ and ex situ IRRAS measurements confirm that these surfaces are contamination free, while STM images show that the sulfochromic acid introduces only isolated oneatom deep hole defects into the surface structure. For the experiments carried out using organic self-assembled monolayers as substrates, acid-cleaned Au/mica substrates were immersed in 1 mM solution of the desired alkanethiol (Aldrich) in methanol for 24 h, rinsed with fresh methanol, dried with nitrogen, and introduced in the vacuum chamber. IRRAS and STM measurements in our laboratory demonstrate that these organic films are highly ordered and highly uniform, as discussed in detail elsewhere.27-29 Transmission experiments described in II also used a sapphire window (Meller Optics, 12.5 mm diameter, 1 mm thick) as a deposition substrate. The sapphire window was supported in a copper frame that included the thermal contact with the Ga well of the sample holder; because of the geometric constraints of the system, the sapphire was supported at an angle of 70° relative to the direction of propagation of the infrared beam. The sample cooling system and measurement electronics were as described above. IPC (Aldrich, 99.999%) was stored at 5 °C in a dark glass bottle covered with aluminum foil and freeze-pumped-thawed before each use to eliminate trace CO gas resulting from decomposition. The liquid sample was replaced every week or if evidence of decomposition appeared. Dosing was done by increasing the main chamber pressure using a UHV leak valve; IPC was detected in the chamber using the RGA only while dosing, and all mass peaks observed were due to IPC cracking; the signal observed on the parent mass is extremely weak because of extensive fragmentation in the ion source. The exposures are reported in Langmuirs (1 L ) 10-6 Torr s) at a sample temperature of 45-50 K and 2 × 10-8 Torr equivalent N2 pressure; no corrections for ion-gauge sensitivities have been applied. No changes were observed in the as-deposited spectra using deposition temperatures from 45 to 100 K [45-125 K for Au(111) substrates], by changes in the deposition rate or by changes in the sample holder orientation with respect to the gas source. The results have been reproduced at least 10 times for each type of substrate, using different batches of Au/mica substrates and different samples of IPC. No evidence of gaseous impurities was detected by the mass spectrometer during deposition or desorption, and the background gases detected during the experiment were consistent with a UHV environment at 5 × 10-11 Torr operating pressure. No unidentified peaks were detected using IRRAS before, during, or after the IPC experiments.

Spectroscopic Background In the gas, solid, and liquid phases, individual IPC molecules have a trigonal bipyramid form of D3h symmetry;30 the diffraction results of Braga et al.31 show that the axial and equatorial Fe-C bond lengths are similar (1.811 versus 1.803 Å, respectively), as are the C-O bond lengths (1.117 and 1.133 Å, respectively). Jonas and Thiel’s BP86/ECP2 DFT calculations have confirmed these (27) Paradis, E.; Rowntree, P. J. Electroanal. Chem. 2003, 550, 175. (28) Kang, J.; Rowntree, P. A. Langmuir 1996, 12, 2813. (29) Garand, E.; Picard, J.-F.; Rowntree, P. J. Phys. Chem. B 2004, 108, 8182. (30) Hanson, A. W. Acta Crystallogr. 1962, 15, 930. (31) Braga, D.; Grepioni, F.; Orpen, A. G. Organometallics 1993, 7, 1481.

Hauchard et al. structures32 and have reproduced the experimental vibrational progressions and infrared band intensities for the dominant absorptions; the DZP/BP86 and DZP/B3LYP DFT results of Jang et al.33 have also reproduced the gas-phase structural parameters. The disagreement in the infrared intensities for the equatorial mode reported in these two works may be due to different handling of the degeneracy of the mode. A total of 18 fundamental vibrations are expected for the D3h structure (i.e., in the absence of C or O isotopic variations).34 In the gas phase, two strong bands (2014 and 2034 cm-1) are observed in the C-O stretching region;34 the more intense band at 2014 cm-1 corresponds to the doubly degenerate motion of the equatorial carbonyls with E′ symmetry, while the 2034 cm-1 band corresponds to the singly degenerate out-of-phase motion of the axial carbonyls with A2′′ symmetry.35-37 To avoid confusion because of differing mode identifications,15,32 they are referred to herein as being due to the equatorial (EQ) or axial (AX) modes. These principal bands red shift by 10-15 cm-1 in Ar, Kr, and Xe matrixes.38-41 Matrix phase infrared spectra of IPC also exhibit significant splittings of the AX and EQ bands that has been attributed to molecular distortions and site inhomogeneities.38 All C-O stretching modes are infrared active in the solid crystalline phase. Frequencies at 2115, 2033, and 2003 cm-1 and the doublet at 1982 and 1977 cm-1 were assigned to the modes ν1, ν2, ν6, and ν10, respectively.37 Cn alkanethiol SAMs (where Cn identifies an alkanethiol with n carbon atoms per chain, chemisorbed to the Au/mica substrate by the terminal thiol functionality) exhibit an infrared spectrum in the C-H stretching region (2800-3000 cm-1) with bands identified as d+ (CH2 symmetric stretch; ∼2850 cm-1), r+ (CH3 symmetric stretch; ∼2877 cm-1), d- (CH2 asymmetric stretch; ∼2921 cm-1), r+FR (Fermi-resonance-coupled symmetric CH3 stretch; ∼2936 cm-1), and rb-/ra- (in- and out-of-plane asymmetric CH3 stretch; ∼2955-2970 cm-1). These assignments have been described in detail elsewhere42-44 for 300 K samples. The present work will focus on the use of Cn surfaces under cryogenic conditions, which introduces subtle yet systematic band changes. Studies of the temperature dependence of the infrared spectra of long-chain alkanethiols (C16-C22)29,44,45 and n-alkanes46-48 show that, as the temperature decreases to 77 K, (i) the band intensities increase by as much as 100%, (ii) the peaks shift to lower frequencies by ∼1-3 cm-1, and (iii) the ra- and rb- bands become more clearly resolved than at 300 K, principally by the growth of the lower frequency rb- peak. These thermal effects in the 2D SAMs have been attributed to coupling of the C-H stretches with low-frequency modes of the layer, with the magnitude of the inter- and intramolecular contributions varying with the vibrational mode.29 Slight conformational changes within the films or changes in the intrinsic absorption coefficients of these transitions may also be involved.44,49-51 All temperature(32) Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474. (33) Jang, J. H.; Lee, J. G.; Lee, H.; Xie, Y.; Schaefer, H. F., III J. Phys. Chem. 1998, 102, 5298. (34) Jones, L. H.; McDowell, R. S.; Goldblatt, M.; Swanson, B. I. J. Chem. Phys. 1972, 37, 2050. (35) Edgell , W. F.; Wilson, W. E.; Summitt, R. Spectrochim. Acta 1963, 19, 863. (36) Pistorius, C. W. F. T.; Haarhoff, P. C. Chem. Phys. Lett. 1950, 31, 1439. (37) Cataliotti, R.; Foffani, A.; Marchetti, L. Inorg. Chem. 1971, 10, 1594. (38) Poliakoff, M.; Turner, J. J. J. Chem. Soc., Dalton Trans. 1973, 13, 1351. (39) Poliakoff, M. J. Chem. Soc., Dalton Trans. 1974, 210. (40) Poliakoff, M.; Turner, J. J. J. Chem. Soc., Dalton Trans. 1974, 2276. (41) Poliakoff, M.; Turner, J. J. J. Chem. Soc., Faraday Trans. 2 1974, 70, 93. (42) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (43) Truong, K. D.; Rowntree, P. A. J. Phys. Chem. 1996, 100, 19917. (44) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (45) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. J. Vac. Sci. Technol., A 1995, 13, 1331. (46) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623. (47) Macphail, R. A.; Snyder, R. G.; Strauss, H. L. J. Chem. Phys. 1982, 77, 1118. (48) Kodati, V. R.; El-Jastimi, R.; Lafleur, M. J. Phys. Chem. 1994, 98, 12191. (49) Hautman, J.; Klein, M. L. J. Chem. Phys. 1990, 93, 7483.

Fe(CO)5 Thin Films Adsorbed on Au(111) and SAMs

Figure 1. IRRAS spectra of the deposition of IPC on Au/mica at 50 K. Each curve is identified with the exposure to IPC in Langmuirs (uncorrected for gauge sensitivity).

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Figure 2. IRRAS spectra of the deposition of IPC on C8/Au/ mica at 50 K. Each curve is identified with the exposure to IPC in Langmuirs (uncorrected for gauge sensitivity).

induced changes in the organic films spectra are highly reproducible and are completely reversible in the sub-300 K regime. Permanent broadening and intensity changes are observed in the organic films’ absorption spectra only if heated above ∼375 K,45 providing evidence of gauche defect creation and possible S-S dimerization processes,52,53 but the films of the present work are never subjected to these conditions.

Experimental Results To clarify the interpretation of these experiments, this paper is organized with a presentation of results followed by their collective interpretation. IPC was deposited and characterized on five types of substrates: Au(111), C4, C8, C12, and C16 SAMs, all at 50 K. All of the Cn SAM substrates have common well-defined 3D structures with a collective molecular tilt of 25-35° from the surface normal42,51 and present highly uniform methyl terminations to the incoming IPC in a similar c(4×2) reconstruction;27,51 the principal effect of increasing the Cn SAM chain length is to increase the IPC-Au(111) separation by ∼1-2 nm. In all cases, results obtained using C4, C8, and C12 substrates (i.e., band positions, relative and absolute shifts, intensities, etc.) were intermediate between those of Au(111)/ mica and those of C16 SAMs. To clarify the presentation, therefore, not all data are presented herein, but all will be discussed as required. As discussed below, we believe that 1 ML surface coverage corresponds to ∼5 L exposure to IPC vapor at 40-50 K, for all substrates considered herein. Figures 1-3 show the evolution of the CO stretching spectra as the IPC coverage increases from 0 to 40 L on the Au(111), C8, and C16 substrates, respectively, at 50 K. The infrared spectra obtained for low exposures (0-6 L) are shown in the insets of Figures 1-3. In this low coverage regime, two-four distinct structures are observed in the C-O stretching region at approximately 2000, 2010-2012, 2033, and 2051-2058 cm-1; the relative intensities of these low-coverage bands are sensitive to both the coverage and to the nature of the substrate. We identify the principal bands at 2010-2012 and 2051-2058 cm-1 that are displayed most clearly in the 6 L spectra as the EQ and (50) Batia, R.; Garrison, B. J. Langmuir 1997, 13, 765. (51) Zhang, L.; Goddard, W. A., III; Jiang, S. J. Chem. Phys. 2002, 117, 7342. (52) Kluth, G. J.; Carraro, C.; Moboudian, R. Phys. Rev. B 1999, 59, 10449. (53) Fenter, P.; Eberhardt, A.; Eisenberger, A. Science 1994, 266, 1216.

Figure 3. IRRAS spectra of the deposition of IPC on C16/Au/ mica surfaces. Each curve is identified with the exposure to IPC in Langmuirs (uncorrected for gauge sensitivity).

AX bands, respectively.9,15 In the low-coverage regime [