Reversible Changes in the Vibrational Structure of Tetra-tert

Felix Leyssner , Sebastian Hagen , László Óvári , Jadranka Dokić , Peter Saalfrank , Maike V. Peters , Stefan Hecht , Tillmann Klamroth and Petra...
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15370

J. Phys. Chem. C 2007, 111, 15370-15374

Reversible Changes in the Vibrational Structure of Tetra-tert-butylazobenzene on a Au(111) Surface Induced by Light and Thermal Activation La´ szlo´ O Ä va´ ri, Martin Wolf, and Petra Tegeder* Freie UniVersita¨t Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin, Germany ReceiVed: July 6, 2007; In Final Form: August 10, 2007

High-resolution electron energy loss spectroscopy (HREELS) is employed to analyze reversible changes in the geometrical structure of the molecular switch tetra-tert-butylazobenzene (TBA) adsorbed on Au(111), which are induced by UV-light and thermal activation. In the submonolayer regime, TBA adsorbs in the planar trans configuration. UV-light exposure at 3.5 eV leads to pronounced changes in the vibrational structure of the TBA molecules in direct contact with the Au(111) surface, which we assign to a trans to cis isomerization. The reverse process, that is, the cis to trans isomerization, can be stimulated by thermal activation. An intensity decrease of vibrational modes as a function of photon dose allows calculation of an effective cross section of σeff ≈ 2 × 10-21 cm2 for the trans to cis isomerization.

I. Introduction Understanding and controlling the switching of functional molecular properties via conformational changes of molecules in direct contact with (metal) surfaces is a major goal for the development of new technologies with possible applications in molecular electronics.1 Molecular switches like azobenzenes represent promising systems for molecular electronic devices1-3 due to their ability to undergo a reversible photoinduced conformational change between the trans and cis isomer.4-6 In the electronic ground state, the two configurations are energetically different, with the trans isomer lower in energy by approximately 0.6 eV. While the trans isomer is nearly planar with a vanishing dipole moment, the cis form is threedimensional, with a dipole moment of 3.2 Debye. Whereas the switching behavior of azobenzene and its derivatives in the liquid phase is well studied, it is an open question how the switching properties change when the molecule is bound to a metal surface. Besides knowledge about the electronic structure of the molecules in contact with metal substrates, detailed insights into the adsorption geometry (molecular orientation) are essential. In this contribution, we examine UV-light-induced changes in the geometrical structure of 3,3′,5,5′-tetra-tert-butylazobenzene (TBA, see Figure 1) adsorbed on Au(111) and the reverse process stimulated by thermal activation using surface vibrational spectroscopy, namely, high-resolution electron energy loss spectroscopy (HREELS). We choose this molecule because the four lateral tert-butyl groups act as a “spacer leg”7-9 to reduce the electronic coupling between the optically active molecular orbitals and the metal substrate. Furthermore, the substituent does not significantly govern the electronic structure of the chromophore and does not mediate steric hindrance upon the isomerization process. The molecules exhibit in solution the photochemical and thermal isomerization behavior typical for azobenzene derivatives.4-6,9 Low-temperature scanning tunneling microscopy (STM) experiments have shown that the trans/cis isomerization of * To whom correspondence should be addressed. Phone: +49-30-83856234. Fax: +49-30-838-56059. E-mail: [email protected].

various azobenzene derivatives adsorbed on Au(111) can be induced by excitation with a STM tip.9-12 Herein, different excitation mechanisms like resonant11 and inelastic tunneling10 as well as the stimulation by the applied electric field9 have been proposed. Also, light-induced switching of TBA molecules absorbed on Au(111) has been achieved.13,14 Using STM, Crommie and co-workers13 observed the trans-cis and cistrans isomerization of TBA due to UV-light exposure at 375 nm (3.3 eV). Recently, we have shown that TBA can be switched bidirectionally by UV light (4.1 and 4.4 eV) and thermal activation. The switching process resulted in reversible changes in the electronic structure of TBA on Au(111) investigated with two-photon photoemission spectroscopy (2PPE).14 In this work, we apply HREELS to demonstrate the reversible switching, that is, trans/cis isomerization induced by UV light at 355 nm (3.5 eV) and thermal activation, of TBA adsorbed on Au(111) in the submonolayer regime. Pronounced changes in the vibrational structure of TBA due to light exposure are observed, which allows a determination of an effective cross section for the trans to cis isomerization. II. Experimental Section The experiments were performed in an ultrahigh vacuum (UHV) apparatus consisting of two chambers separated by a gate valve. The upper chamber with a base pressure of 2 × 10-10 mbar was used for sample preparation and contained facilities for low-energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), deposition of the molecules, and surface cleaning by noble gas ion sputtering. The lower chamber with a base pressure of 6 × 10-11 mbar housed a highresolution electron energy loss (HREEL) spectrometer for recording vibrational spectra. The Au(111) crystal was mounted on a liquid-nitrogen-cooled cryostat, which, in conjunction with resistive heating, enables temperature control from 90 to 750 K. The crystal was cleaned by cycles of Ar+ sputtering (600 eV kinetic energy) and annealing up to 800 K. The TBA was dosed by means of a home-built effusion cell held at 380 K at a crystal temperature

10.1021/jp075274o CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

Reversible Changes in the Vibrational Structure of TBA

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15371

Figure 1. Scheme of the experimental setup for the high-resolution electron energy loss spectroscopy (HREELS). The arrangement allows illumination of the sample and carrying out HREELS without changing the sample position. Right: the 3,3′,5,5′-tetra-tert-butylazobenzene (TBA) molecule.

of 250 K. The coverage was analyzed with TDS. In the TDS, three desorption features (R1-R3) peaking around 314 (R1), 400 (R2), and 525 K (R3) were observed, which were assigned to the desorption from the multilayer (R1) and the first monolayer (ML) (R2 + R3), respectively.14 Thereby, the monolayer regime contained two desorption peaks (R2,R3), where R2 represented the desorption of ≈10% of the monolayer coverage (for details, see ref 14). All measurements presented below were performed at a coverage of 0.9 ML, which was prepared by heating the multilayer-covered surface to 420 K.14,15 The HREEL spectrometer (Ibach-type) consists of two subsystems, the electron monochromator and analyzer, both with double-pass, 127° cylindrical deflectors16 (see Figure 1). HREEL spectra were recorded at a sample temperature of 90 K and in both specular (θi ) θr ) 60°) and off-specular (θi ) 60°; θr ) 50.8°) scattering geometries. The energy of the primary electrons was set to 3.7 eV, with an overall resolution of e4 meV, measured as the full-width at half-maximum (fwhm) of the elastic peak. In the specular spectra of HREELS, the signals contained both dipole- and impact-scattering components.17 The selection rule for dipole scattering, that is, only vibrations with a component of the dipole moment change normal to the surface are observable, is the same as that for infrared reflection absorption spectroscopy.18,19 Therefore, it is useful for characterizing the geometrical structure and predominant orientation of adsorbates. To separate the dipole-scattering components, offspecular spectra consisting of impact-scattering components were measured at a detection angle of 50.8° (corresponding to 9.2° off-specular). A viewport at the spectrometer level allows illumination of the sample at the position at which also the HREEL spectra are recorded (see Figure 1). For the illumination of the TBA-covered Au(111), a pulsed (10 Hz, pulse length: 5 ns) Nd:YAG laser (Quanta-Ray GCR-150, Spectra Physics) at a wavelength of 355 nm (3.5 eV) was used. The output power of the laser beam was set to 140 mW/cm2. The laser spatial profile was characterized by a CCD camera located at a position outside of the UHV chamber, which is equivalent to the sample position. A rather large spot size with a diameter of ≈5 mm was chosen to guarantee an overlap between the illuminated area and the electron beam in the HREEL spectrometer. The annealing experiments were carried out by heating up the sample from 90 to 300 K, staying there for 5 min, and subsequently cooling down to 90 K. Changes in the vibrational structure of TBA upon UV-light exposure and annealing were monitored by HREELS. A Fourier transform infrared (FTIR) study was performed on the TBA powder pressed into a pellet in a KBr matrix with a pure KBr pellet as a reference. The data were recorded with

Figure 2. (a) HREEL spectrum of 0.9 ML TBA adsorbed on Au(111) recorded in specular geometry with a primary electron energy of 3.7 eV. The fwhm of the elastic peak is 30 cm-1 (3.7 meV). (b) Fourier transform infrared spectrum of condensed TBA (KBr pellet) measured with a resolution of 4 cm-1.

a resolution of 4 cm-1 using a Nicolet 8700 Fourier transform spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector. III. Results and Discussion Figure 2a shows the HREEL spectrum of 0.9 ML TBA adsorbed on Au(111) recorded in the specular scattering geometry. For comparison, the solid-state (in KBr) infrared (IR) data are displayed in Figure 2b. The vibrational frequencies and their assignments for both the adsorbed and condensed TBA are listed in Table 1 together with the literature values of

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15372 J. Phys. Chem. C, Vol. 111, No. 42, 2007 TABLE 1: Vibrational Frequencies (in cm-1) and Assignments for 0.9 ML TBA Adsorbed on Au(111) vibrational modea δ(C-N) τ(C-C), Ar τ(C-C), Ar; δ(C-N) δas(CC3), t-butyl τ(C-C), Ar δs(CC3), t-butyl δ(C-N) δ(C-C) Ar; τ(C-C), Ar τ(C-C), Ar τ(C-C), Ar γ(C-H), δ(C-C), Ar υ(C-C), Ar F(CH3) υ(C-N) δ(C-N) δs(H-C-H)(CH3) δas(H-C-H)(CH3), υ(C-C), Ar υ(C-C), Ar υs(C-H)(CH3) υas(C-H)(CH3) υ(C-H), Ar

0.9 ML TBAb

cond. TBAc

204 275 299 352 403 (da)h 451 512 543 696 (da) 879 (da) 921 sh 1008 1153 (da) 1189 1230 1359 (da), 1385 sh 1453 1584 2878 sh (da) 2951 3057

536 599 705 885 910 1024 1188 1205 1245 1365, 1395 1463 1602 2865, 2900 2962 3066

trans-azod f

219 (ra) 251 (ra) f 298g 403 (ra) f 521 s 545 s 689 s 776 s 927 s, 1000 w 1020 m 1223 m 1300 m 1456 s, 1486 s 1585 m 3065

cond. isobutanee 367 (ra) 433 (ra) 1173 1365, 1389 1459 2879 2965 -

a s indicates symmetric; as indicates asymmetric; υ is stretch; δs,as are the bending of the t-butyl; δ indicates an in-plane bend; γ is an out-ofplane bend; τ is torsion; F is rocking; Ar indicates an aromatic ring. b Obtained by HREELS; present study. c IR spectrum recorded in KBr. d IR spectrum of trans-azobenzene recorded in KBr adapted from ref 20. Abbreviations: w, weak; m, medium; s, strong; sh, shoulder. e IR and Raman data adapted from ref 21. f Raman spectrum recorded in CCl4, adapted from ref 22. g Calculated value adapted from ref 23. h The da indicates a strong dipole activity.

vibrational modes of trans-azobenzene20 and isobutane.22 Although the trans-azobenzene is nearly planar, the TBA probably will not have a horizontal symmetry plane. Nevertheless, it is reasonable to assume that the azobenzene part of the molecule remains planar. In this sense, it is intended to use an “in-plane” and “out-of-plane” in the assignments of the phenyl ring modes. The HREEL spectrum for the adsorbed TBA agrees well with the IR data of the condensed-phase TBA; only small shifts toward lower energies are observed for some vibrations due to adsorption on Au(111). While in the condensed phase the asymmetric CH3 stretch mode (υas: 2962 cm-1) and the CH3 deformation modes (δas: 1365 cm-1, δs: 1463 cm-1) of the tert-butyl-groups result in the most intense infrared absorption, in the HREEL, the torsion modes (out-of-plane) of the phenyl rings (τ(C-C)) at 696 and 879 cm-1 show the highest intensity. The stretch modes of the aromatic rings (ν(C-C)) at 1602 and 1024 cm-1 of the condensed TBA are less intense in the adsorbed TBA, and the absorption peak at 1245 cm-1, which can be assigned to the in-plane C-N deformation, is barley visible for the adsorbed species. In order to obtain insights into the excitation mechanism, that is, dipole versus impact scattering, and to analyze the adsorption geometry of TBA on Au(111), we performed angular dependence measurements. Figure 3 shows a comparison of HREEL spectra recorded in specular and 9.2° off-specular geometries for 0.9 ML TBA/Au(111). Most striking is the huge intensity decrease of the out-of-plane torsion modes of the phenyl rings ((τ(C-C)) at 696 and 879 cm-1 in the off-specular spectrum, indicating that their intensity originates mostly from dipole scattering in the specular spectrum (see Table 1 for the assignment of the dipole active modes). The strong dipole activity of the phenyl ring torsion modes points toward a preferential orientation of the TBA parallel to the Au(111) surface, namely, the planar trans geometry, since in this orientation, these modes have a strong dipole moment change upon vibration perpendicular to the surface. This interpretation is supported by STM measurements, which show that in the low-coverage regime, TBA forms well-ordered islands with the

Figure 3. HREEL spectra of 0.9 ML TBA adsorbed on Au(111) recorded in specular and 9.2° off-specular geometries, with a primary electron energy of 3.7 eV.

molecules adsorbed in a planar configuration, which is assigned to the trans isomer.9,13 Also in the liquid phase, the trans isomer is established to be the energetically favorable configuration.4-6 Note that the planar adsorption geometry also explains the changes in the peak intensities of, for example, the phenyl torsion modes between the adsorbed and condensed species due to their strong dipole activity (see Figure 2). Figure 4 shows HREEL spectra recorded before and after illumination of the TBA/Au(111) system with UV light (hν ) 3.5 eV). UV-light exposure leads to a significant intensity decrease of the elastic peak and all dipole active modes, most demonstrative for the phenyl torsion vibrations (at 696 and 878 cm -1), while the impact-scattering components do not change (see inset of Figure 4). The only exception is the intensity decrease of the non-dipole-active C-H stretch mode at 3057 cm-1 in both specular and off-specular scattering geometries. The intensity drop in the dipole-scattering components could, in principle, be due to an order-disorder phase transition or a structural change in the ordered layer. A possible disorder could be generated either by a local transient heating effect of the

Reversible Changes in the Vibrational Structure of TBA

Figure 4. HREEL spectra of 0.9 ML TBA adsorbed on Au(111) before and after UV-light exposure at 355 nm with a photon dose of 1 × 1021 photons/cm2 recorded in specular geometry at a primary electron energy of 3.7 eV. Inset: 9.2° off-specular spectra before and after illumination at 355 nm.

laser pulses (i.e., the TBA molecules in its trans form but with a random adsorption geometry) or by electronic excitation of the TBA molecules, leading to a conformational change (e.g., trans to cis isomerization). However, a disorder created by a higher transient lattice temperature is implausible since heating normally causes ordering in the adsorbate layers. This was also our experience; deposition of TBA at a surface temperature of 90 K and subsequent heating to 250 K resulted in a higher intensity of the elastic peak and all dipole active modes compared to the intensities observed at 90 K. Note that during UV-light exposure, the transient lattice temperature, estimated by using the equation proposed by Burgess et al.,24 is kept below 300 K in order to exclude desorption of TBA molecules. Hence, we conclude that the process leading to the pronounced decrease in the intensity of the elastic peak and the dipole active modes is driven by an electronic excitation of the TBA molecules followed by a conformational change, namely, trans/cis isomerization. Note that exposure of the sample to the electron beam at an electron energy of 3.7 eV in the absence of light does not induce any change in the vibrational features. One would expect the appearance of new vibrational features, which are associated with the TBA molecules in its modified configuration (cis-form). Vibrational (IR20 and Raman22) spectroscopy of the trans- and cis-azobenzene in the condensed phase determined only small differences in the vibrational structure between the two isomers. The main difference is a shift of the NdN stretching mode to higher energies, from 1443 to 1511 cm-1, when going from the trans to the cis form. However, the intense CH3 bending modes of the tert-butyl groups in the TBA adsorbed on Au(111) are also located in this frequency range, which inhibits the discrimination between the trans and cis isomer via the detection of the NdN stretch mode. A possibility to overcome this difficulty is to use specific marker groups, for example, cyano (-CtN) or methoxy (-OCH3) groups, in the meta and/or para position of both phenyl rings, which might change their dipole activity due to the isomerization process. On the basis of the observed STM images and a structural model in the light-induced trans to cis isomerization of individual TBA molecules on Au(111), Crommie and co-

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Figure 5. Reversible switching of TBA molecules in direct contact with the Au(111) surface, induced by light and thermal activation. HREEL spectra of 0.9 ML TBA adsorbed on Au(111) (a) before and (b) after UV-light exposure at 355 nm as well as (c) after annealing the illuminated sample to 300 K; recorded with a primary electron energy of 3.7 eV.

Figure 6. (a) Change in the intensity of the elastic peak and phenyl torsion modes at 696 and 879 cm-1, respectively, as a function of the photon dose. The solid lines represent an exponential fit using the saturation function eq 1.

workers13 proposed a molecular structure for the cis isomer. In the cis form, one phenyl moiety remains parallel to the surface, whereas the second phenyl ring is pointing upward, as schematically shown in Figure 5. This structure has also been suggested in the case of electric-field-induced isomerization of TBA adsorbed on Au(111) by STM.9 If we assume the same molecular configuration is existent in our experiment, then the strong intensity decrease of the dipole active modes, in particular the phenyl torsion vibrations, may be understandable because in the switched state, one phenyl ring per molecule is no longer orientated parallel to the surface. In addition, the tilted geometry of one phenyl ring in the cis-TBA leads to a less-ordered molecular film, resulting in a broader scattered elastic electron beam (diffuse scattering), which causes a decrease in the elastic peak intensity and consequently an intensity drop of all dipole active modes. Since it is known that the reverse process, that is, the cis to trans isomerization of azobenzene and its derivatives in the

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15374 J. Phys. Chem. C, Vol. 111, No. 42, 2007 liquid phase, can also be stimulated by thermal activation, annealing experiments were performed. Indeed, these measurements indicate that by annealing the sample to 300 K, the same HREEL spectrum with respect to intensity and shape can be received as that in the non-illuminated case, as can be seen in Figure 5. Note that the sample position during these measurements was not changed. We propose that the observed reversible changes in the vibrational structure of the TBA adsorbed on Au(111) are due to a conformational change of the adsorbate. The UV light induces the trans to cis isomerization (as it has been observed in ref 13), whereas the reverse process can be stimulated by thermal activation, as illustrated in Figure 5. In order to quantify the light-induced trans/cis isomerization, we evaluated an effective cross section (σeff) from the observed pronounced intensity decrease of the elastic peak and the phenyl torsion vibrations at 696 and 879 cm-1 (see Figure 4). Provided that the decrease correlates with the number of switched molecules and that the process is first-order with respect to the photon dose (intensity), one can estimate an effective cross section (σeff) by using an exponential saturation function

∆I ) ∆I∞[1 - exp(-σeff‚np)]

(1)

whereby ∆I is the change and ∆I∞ the asymptotic change of the peak intensity, and np is the photon dose (number of photons per cm2). The change in the peak intensity as a function of the photon dose is displayed in Figure 6. The solid lines represent a fit using the exponential saturation function (eq 1). From this fit, one achieves an effective cross section on the order of σeff ≈ 2 × 10-21 cm2 for all features, the intensity change of the elastic peak, and the phenyl torsion modes. In comparison, in the trans/cis isomerization process of TBA on Au(111) observed by Crommie and co-workers,13 1 h of exposure at a wavelength of 375 nm (3.3 eV) and a power of 90 mW/cm2 induced switching of 4% of the monolayer. By using the equation ln(Nfinal/N0) ) -σ‚np, with Nfinal as the number of switched molecules and N0 as the initial number of molecules, one yields an absolute cross section of 5 × 10-21 cm2. This value is in the same range as σeff extracted from our measurements. The underlaying mechanism for the light-induced trans/cis isomerization is not yet resolved. Two possible scenarios have been discussed, the direct (intramolecular) electronic excitation within the adsorbate and the indirect excitation via attachment of excited substrate electrons to the molecules. From wavelengthdependent measurements, it has been suggested that the switching of the adsorbed molecules is not caused by direct intramolecular excitation as in the liquid phase but rather by an indirect mechanism.14 IV. Conclusion In summary, high-resolution electron energy loss spectroscopy (HREELS) has been used to investigate the adsorption geometry of the molecular switch 3,3′,5,5′-tetra-tert-butylazobenzene (TBA) in the submonolayer regime on Au(111). Furthermore, HREELS has been employed to study reversible changes in the vibrational structure of TBA, which are induced by UV-light and thermal activation. In the low-coverage regime (e0.9 ML), the phenyl ring torsion modes possess a strong dipole activity, indicating that TBA is adsorbed in the planar trans configuration. Illumination at 355 nm results in a pronounced intensity decrease of the elastic peak and all dipole active modes, in particular,

the phenyl ring torsion vibrations. By thermal activation, these peaks are recovered with respect to the shape and intensity. We assign the reversible changes in the vibrational structure to trans/ cis isomerization of the TBA molecules. The intensity drop of the vibrational modes due to UV-light exposure is used to evaluate an effective cross section of σeff ≈ 2 × 10-21 cm2 for the trans to cis isomerization. No specific vibrational feature of the cis isomer due to the overlap with the vibrational modes of the tert-butyl groups is observed, which would allow determination of the adsorption geometry of this isomer. Therefore, further experiments, including investigations of different substituted TBA molecules with, for example, cyano (-CtN) or methoxy (-O-CH3) groups, which could act as marker groups for the vibrational spectroscopy, are in progress. Acknowledgment. This work has been supported by the Deutsche Forschungsgemeinschaft through the SFB 658. We thank S. Hecht and M. V. Peters (Humboldt Universita¨t Berlin) for the preparation of the azobenzene derivative and R. Haag (Freie Universita¨t Berlin) for the access to a FTIR spectrometer. References and Notes (1) Bryce, M. R.; Petty, M. C.; Bloor, D. Molecular Electronics; Oxford University Press: New York, 1995. (2) Irie, M., Ed. Photochromism: Memories and Switches, Chem. ReV. 2000, 100, 1683, special issue. (3) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, Germany, 2001. (4) Tamai, N.; Miyasaka, O. H. Chem. ReV. 2000, 100, 1875. (5) Rau, H. In Photochromism: Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, The Netherlands, 2003. (6) Fangha¨nel, D.; Timpe, G.; Orthman, V. In Organic Photochromes; El’tsov, A. V., Ed.; Consultants Bureau: New York, 1990, p 105. (7) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K. Nature 1997, 386, 696. (8) Moresco, F.; Meyer, G.; Rieder, K.-H.; Tang, H.; Gourdon, A.; Joachim, C. Phys. ReV. Lett. 2001, 86, 672. (9) Alemani, M.; Peters, M. V.; Hecht, S.; Rieder, K.-H.; Moresco, F.; Grill, L. J. Am. Chem. Soc. 2006, 128, 14446. (10) Henzl, J.; Mehlhorn, M.; Gawronski, H.; Rieder, K.-H.; Morgenstern, K. Angew. Chem., Int. Ed. 2006, 45, 603. (11) Choi, B.-Y.; Kahng, S.-J.; Kim, S.; Kim, H.; Kim, H. W.; Song, Y. J.; Ihm, J.; Kuk, Y. Phys. ReV. Lett. 2006, 96, 156106. (12) Henzl, J.; Bredow, T.; Morgenstern, K. Chem. Phys. Lett. 2007, 435, 278. (13) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Fre´chet, J. M. J.; Trauner, D.; Louie, S. G.; Crommie, M. F. Phys. ReV. Lett. 2007, 99, 038301. (14) Hagen, S.; Leyssner, F.; Nandi, D.; Wolf, M.; Tegeder, P. Chem. Phys. Lett. 2007, 444, 85. (15) Tegeder, P.; Hagen, S.; Leyssner, F.; Peters, M. V.; Hecht, S.; Klamroth, T.; Saalfrank, P.; Wolf, M. Appl. Phys. A 2007, 88, 465. (16) Ibach, H. Electron Energy Loss Spectrometers; Springer: Berlin, Germany, 1991. (17) Ibach, H; Mills, D. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982. (18) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211. (19) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107. (20) Ku¨bler, R.; Lu¨ttke, W.; Weckherlin, S. Z. Elektrochem. 1960, 64, 650. (21) Synder, R. G.; Schachtschneider, J. H. Specrochim. Acta 1965, 21, 1716. (22) Kellerer, B.; Hacker, H. H.; Brandmu¨ller, J. Ind. J. Pure Appl. Phys. 1971, 9, 903. (23) Biswas, N.; Umapathy, S. J. Phys. Chem. A 1997, 101, 5555. (24) Burgess, D., Jr.; Stair, P. C.; Weitz, E. J. Vac. Sci. Technol., A 1986, 4, 1362.