- Email: [email protected]
Linear dichroism electron scattering from chiral surfaces
30 November 2001
Chemical Physics Letters 349 (2001) 167±171 www.elsevier.com/locate/cplett
Linear dichroism electron scattering from chiral surfaces Q. Chen *, D.J. Frankel, C.W. Lee 1, N.V. Richardson School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, UK Received 14 August 2001; in ®nal form 1 October 2001
Abstract The adsorption of R-phenylglycine on Cu(1 1 0) surfaces shows that the periodic superstructure of the chiral overlayer destroys the substrate mirror plane symmetry. Here, we report on the novel linear dichroism electron scattering method to determine the chirality, based on the unique symmetry of the chiral surface. The details of the molecular structure, with the absolute surface chirality, are derived from the azimuthal variation of intensity of individual vibrational modes. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Molecules adsorbed at surfaces have a restricted set of symmetry properties. The point group of an isolated molecule on a single crystal surface or the factor group associated with a 2D array of adsorbed species is constrained to be one of C1 , Cs , C2 , C2v , C3 , C3v , C4 , C4v , C6 , C6v . In the case of the adsorption of a chiral molecule [1±5], which by de®nition cannot have a mirror plane of symmetry, the options are restricted to C1 , C2 , C3 , C4 and C6 . Furthermore, if this adsorption takes place on the (1 1 0) face of an fcc substrate, rotational axes greater than two are not permitted, so only C1 and C2 remain i.e., the only permitted symmetry element is a C2 rotation. Although individual chiral
*
Corresponding author. Fax: +44-1334-467285. E-mail address: [email protected] (Q. Chen). 1 Permanent address: Department of Physics, Kookmin University, Seoul 136-702, Korea.
domain may have only C1 symmetry, co-existing rotational domains will ensure a C2 symmetry in any experiment whose sample areas are larger than the domain size on the fcc (1 1 0) surface. Here, we want to emphasise the uniqueness of C2 symmetry in periodic structures formed by chiral adsorbates. If the isolated adsorbate is achiral or even prochiral, local chiral domains may be observed if mirror planes are lost on adsorption. However, such a chiral domain is always present in a racemic mixture with its enantiomeric partner domain. Because the enantiomer domains are always correlated through the mirror planes of the substrate, the overall diraction pattern or spectroscopic signature will always have C2v symmetry on fcc (1 1 0) surfaces. The diraction patterns corresponding to an ordered chiral array of R-phenylglycine on Cu(1 1 0) are shown in Fig. 1, achieved by dosing to saturation at room temperature, followed by annealing to 450 K. The adsorbed moiety is the anionic species, NH2 CH RCO2 R C6 H5 ,
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 1 7 0 - 8
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Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
5 3 periodicity of 4 1 R-phenylglycine on Cu{1 1 0} surface with a primary energy of Fig. 1. (a) The LEED pattern of
26 eV. (b) Overlayer reciprocal lattice cell indicated by the dotted oblique lines. The geometry for azimuthal dependent impact scattering is also marked.
similar to other a-amino acids such as glycine R H [6±8] and alanine R CH3 [4]. The LEED patterns were recorded on a rear view LEED optics. Here, some care is required since front-view LEED optics cause a re¯ection of the image. The high symmetry azimuths, together with the spots present in the clean substrate, are also indicated in Fig. 1. The corresponding Bravais lattice, 5 3 ; 4 1 is oblique and signi®cantly, the primitive vectors are not aligned along high symmetry directions of
the underlying Cu lattice: in other words, there is no plane of mirror symmetry either in the Bravais lattice or in the adsorbed molecule. The macroscopic 2D structures, including the contribution from independent, translational and possible C2 rotational domains, as well as the real space unit cells are therefore chiral. This chirality of the 2D periodic structure is clearly inherited from the chirality of the adsorbate and made particularly obvious by the oblique Bravais lattice. In principle, a chiral molecule could form a rectangular Bravais lattice on an fcc (1 1 0) surface giving rise to a rectangular LEED pattern. The absence of mirror plane symmetry demanded by a chiral adsorbate would then be revealed only in the diering I=V curves of spots whose positions are related by mirror planes e.g., the 0, 1 spot from the 0, )1. Here, we report a novel spectroscopic method to determine the structure of chiral adsorbates, based on the unique symmetry of the chiral surface. Since the adsorption of the chiral molecules on an fcc (1 1 0) surface has at most twofold rotational symmetry, this is manifest not only by LEED but also by inelastic electron scattering processes, such as impact scattering, as found in vibrational electron energy loss spectroscopy (EELS) [9]. In the impact scattering mechanism, vibrational modes having the same symmetry as a vector in the scattering plane (formed by the incident beam and the surface normal) can be excited o-specular in the scattering plane, in addition to the totally symmetry modes [9]. Impact scattering is particularly strong for those modes involving displacement of H atoms parallel to the scattering plane. For a chiral surface with only C2 symmetry, the impact scattering along the h and h azimuthal directions will show a dierence in the intensity distribution, simply due to the absence of the mirror plane symmetry. EELS experiments of this type, essentially based on a linear dichroic response, therefore probe directly the chirality of surface species. They may provide a simpler alternative to techniques based on circular dichroism using photoemission with circularly polarised light from chiral adsorbates. Vibrational circular dichroism and UV absorption circular dichroism are likely to be too weak to observe for monolayer adsorbates on metal surfaces. Of course, linear
Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
dichroism in photoemission is well established in angle resolved studies from adsorbates but this has not been applied to determine the structure of chiral adsorbates. In addition, the connection between structure and angular resolved photoemission is less direct than in EELS by impact scattering. Here, the details of the relative intensity variations of dierent vibrational modes for chiral molecules, such as phenylglycine, depend directly on and allow determination of the azimuthal orientation, the internal molecular structure and the absolute chiral conformation of the chiral adsorbate. Furthermore, the domain averaging (of two C2 rotational domains) does not perturb the difference of the scattering along the h directions. Fig. 2a,b shows impact scattering spectra along the azimuths 20° with respect to the [0 0 1] direction as de®ned in Fig. 1. The spectra are taken at 5° o-specular with the analyser at 45° and monochromator 50° relative to the surface normal. The EELS spectra have been normalised against
Fig. 2. The result of impact scattering of R-phenylglycine (a) at 20°, and (b) at 20° with respect to the [0 0 1] direction. (c) The dierence spectrum b a. The spectra have been normalised against the elastic peak intensity. The kinetic energy of primary electrons is 8 eV.
169
the elastic peak with Razor baseline subtraction [10]. The vibrational modes at 315 and 511 cm 1 are assigned to modes with NH2 wagging coupled to the C±C±N bend, while the modes at 620 and 750 cm 1 are assigned to phenyl ring, out-of-plane C±H bending modes. All modes between 1000 and 1500 cm 1 are assigned to either in-plane CH bending or the C±C stretching vibrations. The peak at 1604 cm 1 is the NH2 scissor mode and the peaks at 3000 and 3076 cm 1 are the C±H stretching modes characteristic of aromatic molecules. The mode at 1420 cm 1 is assigned to the carboxylate symmetric stretch. The relatively weak intensity of both NH2 and phenyl ring out-ofplane bending modes suggests that the NH2 and phenyl ring planes are close to the surface normal, while the low intensity of the carboxylate stretch, in comparison to other carboxylic acids, which are known to adsorb with the carboxylate plane perpendicular to the surface, indicates that the carboxylate group, while not parallel to the substrate surface, is tilted out of the surface plane by less than 30°. The dierence spectrum, that is taken at 20° subtracted from that at 20°, is shown in Fig. 2c. It is clear that the peaks associated with NH2 wagging show a positive deviation, while the NH2 in-plane scissor mode at 1604 cm 1 has a negative excursion. However, for the phenyl ring, the situation is reversed: the out-of-plane mode has a negative value while in-plane modes have a positive value, with the exception of the CH stretching modes. These observations strongly suggest that the NH2 group is aligned with its plane in the h quadrants while the plane of phenyl ring plane lies in the h quadrant. Previous studies of the bonding geometry of glycine on Cu(1 1 0) [6±8] indicate almost on-top adsorption of both O atoms and the N atom. With the additional spectral information described here, a more detailed structural model focusing on the azimuthal orientation of the functional groups of R-phenylglycine adsorbed on Cu(1 1 0) is proposed in Fig. 3. The negative value in the dierence spectrum in the CH stretching region around 3000 cm 1 arises because these modes are excited less by impact scattering than by the excitation of a negative ion resonance.
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Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
Fig. 3. A model of the structure of R-phenylglycine adsorbed on Cu(1 1 0).
formed by the adsorption of the chiral molecule, R-phenylglycine on the Cu(1 1 0) surface, has been studied with azimuthal dependent, impact scattering in EELS. The adsorption of pure R-phenylglycine results in a 2D structure periodicity, which reduces the substrate symmetry from C2v to C1 with two C2 related domains by lifting the mirror plane. Impact scattering shows the large dierences in intensity of the spectra measured at 20°. Both amino and phenyl ring planes are inclined well away from the surface towards the surface normal. The phenyl ring plane is azimuthally aligned in the h quadrant while the amino group is aligned in the h quadrant. Thus, the internal structure of chiral adsorbate and the absolute surface chirality is determined. 2. Experimental
All spectra were taken with a kinetic energy of 8 eV, at which the phenyl ring has strong negative ion resonances both into its r and p states [11]. Scattering along the molecular plane, only the r state can be accessed while scattering perpendicular to the phenyl ring, both the r and p states can be accessed. Thus, scattering along the h direction is likely to show a higher peak intensity for the CH stretch under this mechanism. The absolute chirality of this adsorbate induced superstructure can be easily revealed by a comparison between R- and S-phenylglycine. If the Rphenylglycine were replaced by S-phenylglycine in the structure of Fig. 3, with bonding to the same substrate atoms through both O atoms and the N atom, the phenyl ring would point towards the surface incurring a large steric repulsion. This is believed to bond the N atom to the other Cu atom allowing the phenyl ring to be directed away from the substrate, correlated with the previous Rphenylglycine structure shown in Fig. 3 by mirror planes along the h0 0 1i direction. In combining with the C2 symmetry related to domain averaging, the mirror plane correlation along the h1 1 0i direction is also recovered. In summary, the periodic structure, 5 3 ; 4 1
The experiments were performed in two dierent UHV systems. One is equipped with a rearview LEED and quadrupole mass spectrometer, while the other has a front-view LEED and HREELS. The diraction patterns observed in rear-view and front-view LEED optics are related by a re¯ection operation, while the diraction pattern recorded from the rear view LEED optics corresponds directly to the unit cell via the standard relationship linking the real and reciprocal space unit cells. Thus, care must be taken when an absolute chiral structure is to be determined, since the re¯ected image observed in front-view optics corresponds to the LEED pattern of the surface of opposite chirality, i.e., the other enantiomer of the adsorbate. Clean Cu{1 1 0} surfaces were obtained by Ar sputtering and annealing (800 K) cycles. The cleanliness of the Cu{1 1 0} surface was assessed by the appearance of a sharp 1 1 LEED pattern. The R-phenylglycine was sublimed several times on a gas line and attached to the chamber via a gate valve. The doser is made of a glass tube with heating wire and a thermal couple sensor, so that the dosing temperature is well controlled and the reproducibility is ensured. The monochromator in the HREELS can be rotated between 80° and 180° in a plane relative to
Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
the ®xed analyser. The sample is mounted on a carrier, with a thermocouple, resistive and direct heating facilities, which can be transferred between the two chambers and rotated freely to adjust the scattering azimuth. The assignment of the vibrational spectrum is assisted by the results of ab initio calculations using GA U S S I A N 98 [12]. The structure of a free molecular anion was optimised and followed by a calculation of the vibrational frequencies with a 631g basis set using the hybrid density functional theory (DFT) with the nonlocal, Becke's three parameter function (B3LYP) [13,14] to describe the exchange and correlation energy.
References [1] Q. Chen, C.W. Lee, D.J. Frankel, et al., Phys. Chem. Commun. 9 (1999).
171
[2] C.F. McFadden, P.S. Cremer, A.J. Gellman, LANGMUIR 12 (1996) 2483. [3] K.H. Ernst, M. B ohringer, C.F. McFadden, et al., Nanotechanology 10 (1999) 355. [4] J. Williams, S. Haq, R. Raval, Surf. Sci. 368 (1996) 303. [5] M.O. Lorenzo, C.J. Baddeley, C. Muryn, et al., Nature 404 (2000) 376. [6] S.M. Barlow, K.J. Kitching, S. Haq, et al., Surf. Sci. 401 (1998) 322. [7] N.A. Booth, D.P. Woodru, O. Scha, et al., Surf. Sci. 397 (1998) 258. [8] J. Hasselstrom, O. Karis, M. Weinelt, et al., Surf. Sci. 407 (1998) 221. [9] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. [10] B.B. Frederick, Ron Unwin, Scienti®c Consultant, 10 Constable Dr. Wilmslow, Cheshire SK9 2NS, UK, 1994. [11] Q. Chen, B.G. Frederick, N.V. Richardson, J. Chem. Phys. 108 (1998) 5942. [12] M.J. Frisch, et al., Gaussian Inc., Pittsburgh, PA, 1998. [13] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [14] A.D. Becke, Phys. Rev. A 38 (1988) 3098.
Chemical Physics Letters 349 (2001) 167±171 www.elsevier.com/locate/cplett
Linear dichroism electron scattering from chiral surfaces Q. Chen *, D.J. Frankel, C.W. Lee 1, N.V. Richardson School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, UK Received 14 August 2001; in ®nal form 1 October 2001
Abstract The adsorption of R-phenylglycine on Cu(1 1 0) surfaces shows that the periodic superstructure of the chiral overlayer destroys the substrate mirror plane symmetry. Here, we report on the novel linear dichroism electron scattering method to determine the chirality, based on the unique symmetry of the chiral surface. The details of the molecular structure, with the absolute surface chirality, are derived from the azimuthal variation of intensity of individual vibrational modes. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Molecules adsorbed at surfaces have a restricted set of symmetry properties. The point group of an isolated molecule on a single crystal surface or the factor group associated with a 2D array of adsorbed species is constrained to be one of C1 , Cs , C2 , C2v , C3 , C3v , C4 , C4v , C6 , C6v . In the case of the adsorption of a chiral molecule [1±5], which by de®nition cannot have a mirror plane of symmetry, the options are restricted to C1 , C2 , C3 , C4 and C6 . Furthermore, if this adsorption takes place on the (1 1 0) face of an fcc substrate, rotational axes greater than two are not permitted, so only C1 and C2 remain i.e., the only permitted symmetry element is a C2 rotation. Although individual chiral
*
Corresponding author. Fax: +44-1334-467285. E-mail address: [email protected] (Q. Chen). 1 Permanent address: Department of Physics, Kookmin University, Seoul 136-702, Korea.
domain may have only C1 symmetry, co-existing rotational domains will ensure a C2 symmetry in any experiment whose sample areas are larger than the domain size on the fcc (1 1 0) surface. Here, we want to emphasise the uniqueness of C2 symmetry in periodic structures formed by chiral adsorbates. If the isolated adsorbate is achiral or even prochiral, local chiral domains may be observed if mirror planes are lost on adsorption. However, such a chiral domain is always present in a racemic mixture with its enantiomeric partner domain. Because the enantiomer domains are always correlated through the mirror planes of the substrate, the overall diraction pattern or spectroscopic signature will always have C2v symmetry on fcc (1 1 0) surfaces. The diraction patterns corresponding to an ordered chiral array of R-phenylglycine on Cu(1 1 0) are shown in Fig. 1, achieved by dosing to saturation at room temperature, followed by annealing to 450 K. The adsorbed moiety is the anionic species, NH2 CH RCO2 R C6 H5 ,
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 1 7 0 - 8
168
Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
5 3 periodicity of 4 1 R-phenylglycine on Cu{1 1 0} surface with a primary energy of Fig. 1. (a) The LEED pattern of
26 eV. (b) Overlayer reciprocal lattice cell indicated by the dotted oblique lines. The geometry for azimuthal dependent impact scattering is also marked.
similar to other a-amino acids such as glycine R H [6±8] and alanine R CH3 [4]. The LEED patterns were recorded on a rear view LEED optics. Here, some care is required since front-view LEED optics cause a re¯ection of the image. The high symmetry azimuths, together with the spots present in the clean substrate, are also indicated in Fig. 1. The corresponding Bravais lattice, 5 3 ; 4 1 is oblique and signi®cantly, the primitive vectors are not aligned along high symmetry directions of
the underlying Cu lattice: in other words, there is no plane of mirror symmetry either in the Bravais lattice or in the adsorbed molecule. The macroscopic 2D structures, including the contribution from independent, translational and possible C2 rotational domains, as well as the real space unit cells are therefore chiral. This chirality of the 2D periodic structure is clearly inherited from the chirality of the adsorbate and made particularly obvious by the oblique Bravais lattice. In principle, a chiral molecule could form a rectangular Bravais lattice on an fcc (1 1 0) surface giving rise to a rectangular LEED pattern. The absence of mirror plane symmetry demanded by a chiral adsorbate would then be revealed only in the diering I=V curves of spots whose positions are related by mirror planes e.g., the 0, 1 spot from the 0, )1. Here, we report a novel spectroscopic method to determine the structure of chiral adsorbates, based on the unique symmetry of the chiral surface. Since the adsorption of the chiral molecules on an fcc (1 1 0) surface has at most twofold rotational symmetry, this is manifest not only by LEED but also by inelastic electron scattering processes, such as impact scattering, as found in vibrational electron energy loss spectroscopy (EELS) [9]. In the impact scattering mechanism, vibrational modes having the same symmetry as a vector in the scattering plane (formed by the incident beam and the surface normal) can be excited o-specular in the scattering plane, in addition to the totally symmetry modes [9]. Impact scattering is particularly strong for those modes involving displacement of H atoms parallel to the scattering plane. For a chiral surface with only C2 symmetry, the impact scattering along the h and h azimuthal directions will show a dierence in the intensity distribution, simply due to the absence of the mirror plane symmetry. EELS experiments of this type, essentially based on a linear dichroic response, therefore probe directly the chirality of surface species. They may provide a simpler alternative to techniques based on circular dichroism using photoemission with circularly polarised light from chiral adsorbates. Vibrational circular dichroism and UV absorption circular dichroism are likely to be too weak to observe for monolayer adsorbates on metal surfaces. Of course, linear
Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
dichroism in photoemission is well established in angle resolved studies from adsorbates but this has not been applied to determine the structure of chiral adsorbates. In addition, the connection between structure and angular resolved photoemission is less direct than in EELS by impact scattering. Here, the details of the relative intensity variations of dierent vibrational modes for chiral molecules, such as phenylglycine, depend directly on and allow determination of the azimuthal orientation, the internal molecular structure and the absolute chiral conformation of the chiral adsorbate. Furthermore, the domain averaging (of two C2 rotational domains) does not perturb the difference of the scattering along the h directions. Fig. 2a,b shows impact scattering spectra along the azimuths 20° with respect to the [0 0 1] direction as de®ned in Fig. 1. The spectra are taken at 5° o-specular with the analyser at 45° and monochromator 50° relative to the surface normal. The EELS spectra have been normalised against
Fig. 2. The result of impact scattering of R-phenylglycine (a) at 20°, and (b) at 20° with respect to the [0 0 1] direction. (c) The dierence spectrum b a. The spectra have been normalised against the elastic peak intensity. The kinetic energy of primary electrons is 8 eV.
169
the elastic peak with Razor baseline subtraction [10]. The vibrational modes at 315 and 511 cm 1 are assigned to modes with NH2 wagging coupled to the C±C±N bend, while the modes at 620 and 750 cm 1 are assigned to phenyl ring, out-of-plane C±H bending modes. All modes between 1000 and 1500 cm 1 are assigned to either in-plane CH bending or the C±C stretching vibrations. The peak at 1604 cm 1 is the NH2 scissor mode and the peaks at 3000 and 3076 cm 1 are the C±H stretching modes characteristic of aromatic molecules. The mode at 1420 cm 1 is assigned to the carboxylate symmetric stretch. The relatively weak intensity of both NH2 and phenyl ring out-ofplane bending modes suggests that the NH2 and phenyl ring planes are close to the surface normal, while the low intensity of the carboxylate stretch, in comparison to other carboxylic acids, which are known to adsorb with the carboxylate plane perpendicular to the surface, indicates that the carboxylate group, while not parallel to the substrate surface, is tilted out of the surface plane by less than 30°. The dierence spectrum, that is taken at 20° subtracted from that at 20°, is shown in Fig. 2c. It is clear that the peaks associated with NH2 wagging show a positive deviation, while the NH2 in-plane scissor mode at 1604 cm 1 has a negative excursion. However, for the phenyl ring, the situation is reversed: the out-of-plane mode has a negative value while in-plane modes have a positive value, with the exception of the CH stretching modes. These observations strongly suggest that the NH2 group is aligned with its plane in the h quadrants while the plane of phenyl ring plane lies in the h quadrant. Previous studies of the bonding geometry of glycine on Cu(1 1 0) [6±8] indicate almost on-top adsorption of both O atoms and the N atom. With the additional spectral information described here, a more detailed structural model focusing on the azimuthal orientation of the functional groups of R-phenylglycine adsorbed on Cu(1 1 0) is proposed in Fig. 3. The negative value in the dierence spectrum in the CH stretching region around 3000 cm 1 arises because these modes are excited less by impact scattering than by the excitation of a negative ion resonance.
170
Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
Fig. 3. A model of the structure of R-phenylglycine adsorbed on Cu(1 1 0).
formed by the adsorption of the chiral molecule, R-phenylglycine on the Cu(1 1 0) surface, has been studied with azimuthal dependent, impact scattering in EELS. The adsorption of pure R-phenylglycine results in a 2D structure periodicity, which reduces the substrate symmetry from C2v to C1 with two C2 related domains by lifting the mirror plane. Impact scattering shows the large dierences in intensity of the spectra measured at 20°. Both amino and phenyl ring planes are inclined well away from the surface towards the surface normal. The phenyl ring plane is azimuthally aligned in the h quadrant while the amino group is aligned in the h quadrant. Thus, the internal structure of chiral adsorbate and the absolute surface chirality is determined. 2. Experimental
All spectra were taken with a kinetic energy of 8 eV, at which the phenyl ring has strong negative ion resonances both into its r and p states [11]. Scattering along the molecular plane, only the r state can be accessed while scattering perpendicular to the phenyl ring, both the r and p states can be accessed. Thus, scattering along the h direction is likely to show a higher peak intensity for the CH stretch under this mechanism. The absolute chirality of this adsorbate induced superstructure can be easily revealed by a comparison between R- and S-phenylglycine. If the Rphenylglycine were replaced by S-phenylglycine in the structure of Fig. 3, with bonding to the same substrate atoms through both O atoms and the N atom, the phenyl ring would point towards the surface incurring a large steric repulsion. This is believed to bond the N atom to the other Cu atom allowing the phenyl ring to be directed away from the substrate, correlated with the previous Rphenylglycine structure shown in Fig. 3 by mirror planes along the h0 0 1i direction. In combining with the C2 symmetry related to domain averaging, the mirror plane correlation along the h1 1 0i direction is also recovered. In summary, the periodic structure, 5 3 ; 4 1
The experiments were performed in two dierent UHV systems. One is equipped with a rearview LEED and quadrupole mass spectrometer, while the other has a front-view LEED and HREELS. The diraction patterns observed in rear-view and front-view LEED optics are related by a re¯ection operation, while the diraction pattern recorded from the rear view LEED optics corresponds directly to the unit cell via the standard relationship linking the real and reciprocal space unit cells. Thus, care must be taken when an absolute chiral structure is to be determined, since the re¯ected image observed in front-view optics corresponds to the LEED pattern of the surface of opposite chirality, i.e., the other enantiomer of the adsorbate. Clean Cu{1 1 0} surfaces were obtained by Ar sputtering and annealing (800 K) cycles. The cleanliness of the Cu{1 1 0} surface was assessed by the appearance of a sharp 1 1 LEED pattern. The R-phenylglycine was sublimed several times on a gas line and attached to the chamber via a gate valve. The doser is made of a glass tube with heating wire and a thermal couple sensor, so that the dosing temperature is well controlled and the reproducibility is ensured. The monochromator in the HREELS can be rotated between 80° and 180° in a plane relative to
Q. Chen et al. / Chemical Physics Letters 349 (2001) 167±171
the ®xed analyser. The sample is mounted on a carrier, with a thermocouple, resistive and direct heating facilities, which can be transferred between the two chambers and rotated freely to adjust the scattering azimuth. The assignment of the vibrational spectrum is assisted by the results of ab initio calculations using GA U S S I A N 98 [12]. The structure of a free molecular anion was optimised and followed by a calculation of the vibrational frequencies with a 631g basis set using the hybrid density functional theory (DFT) with the nonlocal, Becke's three parameter function (B3LYP) [13,14] to describe the exchange and correlation energy.
References [1] Q. Chen, C.W. Lee, D.J. Frankel, et al., Phys. Chem. Commun. 9 (1999).
171
[2] C.F. McFadden, P.S. Cremer, A.J. Gellman, LANGMUIR 12 (1996) 2483. [3] K.H. Ernst, M. B ohringer, C.F. McFadden, et al., Nanotechanology 10 (1999) 355. [4] J. Williams, S. Haq, R. Raval, Surf. Sci. 368 (1996) 303. [5] M.O. Lorenzo, C.J. Baddeley, C. Muryn, et al., Nature 404 (2000) 376. [6] S.M. Barlow, K.J. Kitching, S. Haq, et al., Surf. Sci. 401 (1998) 322. [7] N.A. Booth, D.P. Woodru, O. Scha, et al., Surf. Sci. 397 (1998) 258. [8] J. Hasselstrom, O. Karis, M. Weinelt, et al., Surf. Sci. 407 (1998) 221. [9] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. [10] B.B. Frederick, Ron Unwin, Scienti®c Consultant, 10 Constable Dr. Wilmslow, Cheshire SK9 2NS, UK, 1994. [11] Q. Chen, B.G. Frederick, N.V. Richardson, J. Chem. Phys. 108 (1998) 5942. [12] M.J. Frisch, et al., Gaussian Inc., Pittsburgh, PA, 1998. [13] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [14] A.D. Becke, Phys. Rev. A 38 (1988) 3098.