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A tilted precursor for CO dissociation on the Fe(100) surface
L123
Surface Science 180 (1987) L123-L128 North-Holland. Amsterdam
SURFACE
SCIENCE
LETTERS
A TILTED PRECURSOR FOR CO DISSOCIATION ON THE Fe(100) SURFACE
‘, F. ZAERA 3, W. EBERHARDT D.W. MOON ‘, S,. CAMERON R. CARR 4, S.L. BERNASEK ‘, J.L. GLAND ’ and D.J. DWYER
2, ’
’ Chemistty Department, Princeton University, NJ 08544, USA ’ Exxon Research and Engineering Company, Annandale, NJ 08801, USA -’ Brookhaven National Laboratory, NSLS Department, 510E Upton, NY 11973, USA * Stanford Synchrotron Radiation Laboratory, Menlo Park, CA 94304, USA Received
14 January
1986; accepted
for publication
22 July 1986
Near edge X-ray absorption fine structure (NFXAFS) has been used to study the molecular orientation of the 01s state of CO on the Fe(100) surface. It is found that the molecule is tilted by 45O + loo with respect to the surface normal, allowing direct interaction of the oxygen end of the molecule with the iron surface. The C-O bond is found to be elongated by 0.07 i 0.02 A in the a3 state, relative to the other molecularly adsorbed CO states on this surface.
The chemistry of CO molecules on iron surfaces has attracted special attention due to its relevance to Fischer-Tropsch synthesis. Recently, we reported an observation of a CO((Y~) molecular state on Fe(100) which is the precursor state for CO dissociation. This state was studied by X-ray photoelectron spectroscopy, temperature programmed desorption, and high resolution electron energy loss spectrometry [1,2]. Competitive dissociation and desorption of the CO(cws) molecules occurs near 440 K. The carbon-oxygen stretching frequency for this precursor to dissociation is exceptionally low, which desorb molecularly have 1210 cm-‘. The (or and (Ye states carbon-oxygen stretching frequencies of 2010 and 2070 cm-‘. Low carbon-oxygen stretching frequencies on unmodified transition metal surfaces have been reported recently for CO on Cr(ll0) [3] and CO on Mo(100) [4]. For CO on Cr(ll0) the low stretching frequency is explained by CO +rr bonding in a configuration parallel to the surface based on electron stimulated desorption ion angular distribution (ESDIAD) and photoemission measurements [3]. Here we report the results of a near edge X-ray absorption fine structure (NEXAFS) study of the molecular orientation and the bond length of the CO((Y,) molecules on Fe(lOO). We have observed that the CO(a,) bond is tilted by 45” f loo with respect to the surface normal so that the oxygen atoms of the CO(a,) molecule can interact directly with the iron surface. 0039-6028/87/$03.50
(North-Holland
0 Elsevier
Physics
Publishing
Science Publishers Division)
B.V.
L124
D. W. Moon et al. / A tilted precursor
for CO
dissoctatron
on Fe(lO0)
From a previously established correlation [6,7] between the u shape resonance position and the CO bond length, it is estimated that the bond length of the CO(a,) molecule is increased by 0.07 + 0.02 A relative to the bond length of the CO((Y,) and CO((Y~) molecules. NEXAFS spectra were recorded with the use of the “grasshopper” monochromator (1200 lines/mm holographic grating) on beam line III at the Stanford Synchrotron Radiation Laboratory (SSRL) by means of partial electron yield detection in an UHV system of base pressure 2 X lo-” Torr. The Fe(lOO) surface was cleaned by Ar+ bombardment and checked for cleanliness by Auger spectroscopy. Carbon monoxide was adsorbed at 150 K on the clean Fe(lOO) surface. The measured partial electron yield spectra were normalized using the yield from the clean surface and were scaled by the magnitude of the edge jump into continuum states at 315 eV. In fig. la we show the NEXAFS spectra at the C K-edge for the CO saturated Fe(lOO) at normal X-ray incidence (6’ = 90”, E parallel to the surface) and at glancing X-ray incidence (13= 30 O, E close to surface normal). The surface prepared in this manner contains CO in three molecular states (ai, a1 and (Ye) [1,2]. Two of these states (a, and (Ye) are thought to be “normal” CO with stretching frequencies of 2010 and 2070 cm-l and the third is “abnormal” CO with an unusually weak carbon-oxygen bond (1210 cm-‘). Interpretation of the NEXAFS data in fig. la is complicated by this mixture of states but the weak angular dependence of the 7~ resonance clearly indicates that the orientation of CO is quite different than that observed on other transition metals [5]. It is possible to prepare a surface that only contains CO in the unusual low stretching frequency state (aj) by annealing the CO saturated surface to 340 K or by limiting the CO dose to 0.5 L at 170 K. Both methods of preparation of the CO(+) state gave the same spectrum within our experimental uncertainty. The NEXAFS spectra at the C K-edge for CO(a3) on Fe(lOO) at the two incidence angles are shown in fig. lb. The NEXAFS spectra for CO( CUE)at both angles in fig. 2b exhibit a distinct peak A (n resonance), which is known to be due to the C Is -+ 2a* excitation [5]. A very weak peak B (a shape resonance) is also observed at both angles of incidence. The rr resonance intensity for CO(a,) normalized by the overall jump at the C K-edge does not show any significant dependence on the incidence angle. This indicates that the tilt angle of the CO((Y,) molecule on Fe(lOO) is near the “magic angle” (54.7O) from the surface normal [5]. The intensity of the u shape resonance in the NEXAFS spectra for CO(a3) for both glancing and normal incidence angles in fig. lb is exceptionally weak. This again suggests that the molecular orientation of CO(a,) is neither perpendicular nor parallel to the surface. However, we do observe a slightly increased u shape resonance intensity at the glancing incidence angle. Using the previously determined polarization-dependence of the X-ray absorption [5], the ratio of the u shape resonance intensity between the two incidence
D. W. Moon Ed al. / A tilted precursor for CO dissociation on Fe(lO0)
280
290 300 Photon Energy (eV)
L125
310
Fig. 1. (a) C K-edge NEXAFS spectra recorded at 30’ X-ray incidence (top) and at normal incidence (bottom) for the Fe(lOO) saturated with CO. XPS denotes the Cls binding energies relative to the Fermi level measured by photoemission. (b) C K-edge NEXAFS spectra for CO( a,) on Fe(100) recorded for the same conditions as in (a).
angles is more sensitive to the tilt angle than the ratio of the 7~ resonance as plotted in fig. 2. Therefore, within our experimental uncertainty, we assign a range for the CO(a3) tilt angle to be 45O f 10” as indicated on the plot in fig. 2. The position of the u shape resonance at 8 = 30” in the NEXAFS spectrum for CO(a,) in fig. lb is shifted by 2.2 eV from that for the saturated CO in fig. la. This shift indicates that the CO(a,) bond is longer than that of the CO(a,) and CO(a,) states. Using the relationship between the u shape resonance position variation (Au) and the bond length variation (AR), (Au = aA R, a = 32 + 2 eV/& [6,7] we estimate the difference in bond length to be 0.07 f 0.02 A between the two CO types. The stretched CO bond length of the CO(a,)
L126
D. W. Moon et al. / A
0.
-
t
-1. 0.
-,’
)
tiltedprecursorfor CO dissoctutron on Fe(100)
CM
o E=90°
20.
40. Tilt Angle
60.
60.
90.
a
Fig. 2. Plots of the v resonance and 0 shape resonance peak intensities as a function of the tilt angle for glancing incidence (0 = 30°) and normal incidence (e = 90”) angles. Shaded area indicates the range of tilt angles consistent with NEXAFS data.
molecules is consistent with the bond weakening reflected in the exceptionally low CO stretching observed by HREELS [l]. Our determination of the tilt angle and the bond length for this state provides insight into the role of the CO(+) molecule as the precursor for CO dissociation on this surface. These measurements are also consistent with the fact that the CO stretching frequency is exceptionally low, 1210 cm-‘. The tilted configuration in the fourfold hollow site allows direct interaction of the CO ~7orbitals and the iron d valence electrons, resulting in increased hybridization of the 7~ electron system and a substantially weakened CO bond. In addition to the weakened CO bond, the direct interaction of the oxygen end of the CO(rws) molecule with the iron surface would be expected to decrease the activation barrier for the CO dissociation, assuming this tilted configuration to be similar to the transition state for CO dissociation. The tilted configuration of CO(cw,) on Fe(lOO) is consistent with recently reported ESDIAD measurement on this system [ll]. In the ESDIAD experiment, the O+ emission normal to the surface is found to be extremely low. For the CO(ar,) molecule adsorbed in the fourfold hollow site on Fe(lOO) with 45” k loo tilt angle, the emission of the O+ ions to surface normal is expected
D. W. Moon et al. / A tilted precursor for CO dissociation on Fe(lO0)
L127
to be very low due to the tilted configuration and the image force increase associated with the close proximity of the oxygen atom to the surface [12]. Very low CO stretching frequencies have been observed for a Nb cluster [8], ($-C,H,)Nb,(CO), (1330 cm-‘), and a Fe cluster [9], [Fe,(AuPEt,)(CO),,](1380-1430 cm-‘). In both complexes, metal atoms directly interact with the r-electron system of the CO ligand. Exceptionally low CO stretching frequencies have also been observed for clean Cr(lOO) [3] (1150-1330 cm-‘), Mo(100) [4] (1065-1235 cm-‘), and potassium-promoted Ru(OO1) [lo] (- 1460 cm-‘). For all three systems, “lying-down” or “bent” CO configurations were suggested, allowing both the carbon and oxygen atoms to be coordinated to the metal atoms. It is interesting that a structural analogy can be found between the CO(cu,) on Fe(lOO) and the m-CO ligand in the iron “butterfly” cluster, [Fe,(AuPEt,)(CO),,][9]. The bond length of the m-CO ligand interacting with all four of the iron atoms in the Fe, “butterfly” cluster is ‘0.08 A longer than that of the terminal CO ligands. This change of CO bond length is very similar to the increase in the bond length (0.07 + 0.02 A) of the CO(a3) relative to that of the CO( al) and CO( (Ye). An analogy can be also seen in the CO stretching frequency. The CO stretching frequency of the m-CO in the “butterfly” iron cluster is very low, 1380-1430 cm-‘, which is comparable to that of CO(&) on Fe(lOO), 1210 cm-‘. A simple model based on space filling hard spheres was investigated for this proposed CO(a,) configuration. The Fe-C and Fe-O bond lengths were taken from the structure of 7 bonded CO in the Fe, “butterfly” cluster [9]. This modeling indicates that the tilt angle of a CO molecule in the fourfold hollow site on Fe(lOO) would be - 40° for the [llO] CO azimuthal orientation and 55” from the surface normal for the [loo] CO azimuthal orientation. The measured tilt angle of CO(cus), 45O f loo obtained from our NEXAFS measurement is quite similar to the values obtained from this simple hard sphere modeling. However, we cannot differentiate between the two azimuthal orientations within our experimental uncertainty. In summary, the NEXAFS investigation indicates that the precursor state for CO dissociation on Fe(100) occupies the fourfold hollow site and is tilted by 45” f 10” from the surface normal. The carbon-oxygen bond for this species is longer by 0.07 k 0.02 A than those in the CO(a,) and CO((Y*) “normal” bonding configurations. The unusual bond weakening reflected by the low stretching frequency (1210 cm-‘) and the facile dissociation of this state is believed to be due to r bonding and resulting in direct interaction of the oxygen end of the CO(a3) molecule with the iron surface. The tilted, weakened, and stretched bond of this state and the possible [loo] azimuthal orientation of the bond, combine to suggest that the reaction coordinate for dissociative adsorption of CO may be across the saddle point separating adjacent fourfold hollow sites.
L128
D. W. Moon et al. / A trltedprecursorfor
CO dtssocmtton
on Fe(100)
References [l] [2] [3] [4] [5] [6] [7] [8] [9] lo] 111 121
D.W. Moon, S.L. Bemasek, D.J. Dwyer and J.L. Gland, J. Am. Chem. Sot. 107 (1985) 4363. D.W. Moon, S.L. Bemasek and D.J. Dwyer, Surface Sci. 163 (1985) 215. N.D. Shinn and T.E. Madey, Phys. Rev. Letters 53 (1985) 2481. F. Zaera, E. Kollin and J.L. Gland, Phys. Rev. Letters, submitted. J. Stijhr and R. Jaeger, Phys. Rev. B26 (1982) 4111. F. Sette, J. StBhr and A.P. Hitchcock, Chem. Phys. Letters 110 (1984) 517; J. Chem. Phys. 81 (1984) 4906. J. Stiihr, J.L. Gland, W. Eberhardt, D. Outka, R.J. Madix, F. Sette, R.J. Koestner and V. Dobler, Phys. Rev. Letters 51 (1983) 2414. W.A. Herrmann. H. Biersack, M.L. Ziegler, K. Weidenhammer, R. Siegel and D. Rehder, J. Am. Chem. Sot. 103 (1981) 1692. C.P. Horwitz, E.M. Holt, C.P. Brock and D.F. Shriver, to be published. F.M. Hoffmann and R.A. DePaola, Phys. Rev. Letters 52 (1984) 1697. C. Benndorf, B. Krtiger and F. Thieme, Surface Sci. 163 (1985) L675. Z. MiSkoviC, J. VukaniC and T.E. Madey. Surface Sci. 141 (1984) 285.
Surface Science 180 (1987) L123-L128 North-Holland. Amsterdam
SURFACE
SCIENCE
LETTERS
A TILTED PRECURSOR FOR CO DISSOCIATION ON THE Fe(100) SURFACE
‘, F. ZAERA 3, W. EBERHARDT D.W. MOON ‘, S,. CAMERON R. CARR 4, S.L. BERNASEK ‘, J.L. GLAND ’ and D.J. DWYER
2, ’
’ Chemistty Department, Princeton University, NJ 08544, USA ’ Exxon Research and Engineering Company, Annandale, NJ 08801, USA -’ Brookhaven National Laboratory, NSLS Department, 510E Upton, NY 11973, USA * Stanford Synchrotron Radiation Laboratory, Menlo Park, CA 94304, USA Received
14 January
1986; accepted
for publication
22 July 1986
Near edge X-ray absorption fine structure (NFXAFS) has been used to study the molecular orientation of the 01s state of CO on the Fe(100) surface. It is found that the molecule is tilted by 45O + loo with respect to the surface normal, allowing direct interaction of the oxygen end of the molecule with the iron surface. The C-O bond is found to be elongated by 0.07 i 0.02 A in the a3 state, relative to the other molecularly adsorbed CO states on this surface.
The chemistry of CO molecules on iron surfaces has attracted special attention due to its relevance to Fischer-Tropsch synthesis. Recently, we reported an observation of a CO((Y~) molecular state on Fe(100) which is the precursor state for CO dissociation. This state was studied by X-ray photoelectron spectroscopy, temperature programmed desorption, and high resolution electron energy loss spectrometry [1,2]. Competitive dissociation and desorption of the CO(cws) molecules occurs near 440 K. The carbon-oxygen stretching frequency for this precursor to dissociation is exceptionally low, which desorb molecularly have 1210 cm-‘. The (or and (Ye states carbon-oxygen stretching frequencies of 2010 and 2070 cm-‘. Low carbon-oxygen stretching frequencies on unmodified transition metal surfaces have been reported recently for CO on Cr(ll0) [3] and CO on Mo(100) [4]. For CO on Cr(ll0) the low stretching frequency is explained by CO +rr bonding in a configuration parallel to the surface based on electron stimulated desorption ion angular distribution (ESDIAD) and photoemission measurements [3]. Here we report the results of a near edge X-ray absorption fine structure (NEXAFS) study of the molecular orientation and the bond length of the CO((Y,) molecules on Fe(lOO). We have observed that the CO(a,) bond is tilted by 45” f loo with respect to the surface normal so that the oxygen atoms of the CO(a,) molecule can interact directly with the iron surface. 0039-6028/87/$03.50
(North-Holland
0 Elsevier
Physics
Publishing
Science Publishers Division)
B.V.
L124
D. W. Moon et al. / A tilted precursor
for CO
dissoctatron
on Fe(lO0)
From a previously established correlation [6,7] between the u shape resonance position and the CO bond length, it is estimated that the bond length of the CO(a,) molecule is increased by 0.07 + 0.02 A relative to the bond length of the CO((Y,) and CO((Y~) molecules. NEXAFS spectra were recorded with the use of the “grasshopper” monochromator (1200 lines/mm holographic grating) on beam line III at the Stanford Synchrotron Radiation Laboratory (SSRL) by means of partial electron yield detection in an UHV system of base pressure 2 X lo-” Torr. The Fe(lOO) surface was cleaned by Ar+ bombardment and checked for cleanliness by Auger spectroscopy. Carbon monoxide was adsorbed at 150 K on the clean Fe(lOO) surface. The measured partial electron yield spectra were normalized using the yield from the clean surface and were scaled by the magnitude of the edge jump into continuum states at 315 eV. In fig. la we show the NEXAFS spectra at the C K-edge for the CO saturated Fe(lOO) at normal X-ray incidence (6’ = 90”, E parallel to the surface) and at glancing X-ray incidence (13= 30 O, E close to surface normal). The surface prepared in this manner contains CO in three molecular states (ai, a1 and (Ye) [1,2]. Two of these states (a, and (Ye) are thought to be “normal” CO with stretching frequencies of 2010 and 2070 cm-l and the third is “abnormal” CO with an unusually weak carbon-oxygen bond (1210 cm-‘). Interpretation of the NEXAFS data in fig. la is complicated by this mixture of states but the weak angular dependence of the 7~ resonance clearly indicates that the orientation of CO is quite different than that observed on other transition metals [5]. It is possible to prepare a surface that only contains CO in the unusual low stretching frequency state (aj) by annealing the CO saturated surface to 340 K or by limiting the CO dose to 0.5 L at 170 K. Both methods of preparation of the CO(+) state gave the same spectrum within our experimental uncertainty. The NEXAFS spectra at the C K-edge for CO(a3) on Fe(lOO) at the two incidence angles are shown in fig. lb. The NEXAFS spectra for CO( CUE)at both angles in fig. 2b exhibit a distinct peak A (n resonance), which is known to be due to the C Is -+ 2a* excitation [5]. A very weak peak B (a shape resonance) is also observed at both angles of incidence. The rr resonance intensity for CO(a,) normalized by the overall jump at the C K-edge does not show any significant dependence on the incidence angle. This indicates that the tilt angle of the CO((Y,) molecule on Fe(lOO) is near the “magic angle” (54.7O) from the surface normal [5]. The intensity of the u shape resonance in the NEXAFS spectra for CO(a3) for both glancing and normal incidence angles in fig. lb is exceptionally weak. This again suggests that the molecular orientation of CO(a,) is neither perpendicular nor parallel to the surface. However, we do observe a slightly increased u shape resonance intensity at the glancing incidence angle. Using the previously determined polarization-dependence of the X-ray absorption [5], the ratio of the u shape resonance intensity between the two incidence
D. W. Moon Ed al. / A tilted precursor for CO dissociation on Fe(lO0)
280
290 300 Photon Energy (eV)
L125
310
Fig. 1. (a) C K-edge NEXAFS spectra recorded at 30’ X-ray incidence (top) and at normal incidence (bottom) for the Fe(lOO) saturated with CO. XPS denotes the Cls binding energies relative to the Fermi level measured by photoemission. (b) C K-edge NEXAFS spectra for CO( a,) on Fe(100) recorded for the same conditions as in (a).
angles is more sensitive to the tilt angle than the ratio of the 7~ resonance as plotted in fig. 2. Therefore, within our experimental uncertainty, we assign a range for the CO(a3) tilt angle to be 45O f 10” as indicated on the plot in fig. 2. The position of the u shape resonance at 8 = 30” in the NEXAFS spectrum for CO(a,) in fig. lb is shifted by 2.2 eV from that for the saturated CO in fig. la. This shift indicates that the CO(a,) bond is longer than that of the CO(a,) and CO(a,) states. Using the relationship between the u shape resonance position variation (Au) and the bond length variation (AR), (Au = aA R, a = 32 + 2 eV/& [6,7] we estimate the difference in bond length to be 0.07 f 0.02 A between the two CO types. The stretched CO bond length of the CO(a,)
L126
D. W. Moon et al. / A
0.
-
t
-1. 0.
-,’
)
tiltedprecursorfor CO dissoctutron on Fe(100)
CM
o E=90°
20.
40. Tilt Angle
60.
60.
90.
a
Fig. 2. Plots of the v resonance and 0 shape resonance peak intensities as a function of the tilt angle for glancing incidence (0 = 30°) and normal incidence (e = 90”) angles. Shaded area indicates the range of tilt angles consistent with NEXAFS data.
molecules is consistent with the bond weakening reflected in the exceptionally low CO stretching observed by HREELS [l]. Our determination of the tilt angle and the bond length for this state provides insight into the role of the CO(+) molecule as the precursor for CO dissociation on this surface. These measurements are also consistent with the fact that the CO stretching frequency is exceptionally low, 1210 cm-‘. The tilted configuration in the fourfold hollow site allows direct interaction of the CO ~7orbitals and the iron d valence electrons, resulting in increased hybridization of the 7~ electron system and a substantially weakened CO bond. In addition to the weakened CO bond, the direct interaction of the oxygen end of the CO(rws) molecule with the iron surface would be expected to decrease the activation barrier for the CO dissociation, assuming this tilted configuration to be similar to the transition state for CO dissociation. The tilted configuration of CO(cw,) on Fe(lOO) is consistent with recently reported ESDIAD measurement on this system [ll]. In the ESDIAD experiment, the O+ emission normal to the surface is found to be extremely low. For the CO(ar,) molecule adsorbed in the fourfold hollow site on Fe(lOO) with 45” k loo tilt angle, the emission of the O+ ions to surface normal is expected
D. W. Moon et al. / A tilted precursor for CO dissociation on Fe(lO0)
L127
to be very low due to the tilted configuration and the image force increase associated with the close proximity of the oxygen atom to the surface [12]. Very low CO stretching frequencies have been observed for a Nb cluster [8], ($-C,H,)Nb,(CO), (1330 cm-‘), and a Fe cluster [9], [Fe,(AuPEt,)(CO),,](1380-1430 cm-‘). In both complexes, metal atoms directly interact with the r-electron system of the CO ligand. Exceptionally low CO stretching frequencies have also been observed for clean Cr(lOO) [3] (1150-1330 cm-‘), Mo(100) [4] (1065-1235 cm-‘), and potassium-promoted Ru(OO1) [lo] (- 1460 cm-‘). For all three systems, “lying-down” or “bent” CO configurations were suggested, allowing both the carbon and oxygen atoms to be coordinated to the metal atoms. It is interesting that a structural analogy can be found between the CO(cu,) on Fe(lOO) and the m-CO ligand in the iron “butterfly” cluster, [Fe,(AuPEt,)(CO),,][9]. The bond length of the m-CO ligand interacting with all four of the iron atoms in the Fe, “butterfly” cluster is ‘0.08 A longer than that of the terminal CO ligands. This change of CO bond length is very similar to the increase in the bond length (0.07 + 0.02 A) of the CO(a3) relative to that of the CO( al) and CO( (Ye). An analogy can be also seen in the CO stretching frequency. The CO stretching frequency of the m-CO in the “butterfly” iron cluster is very low, 1380-1430 cm-‘, which is comparable to that of CO(&) on Fe(lOO), 1210 cm-‘. A simple model based on space filling hard spheres was investigated for this proposed CO(a,) configuration. The Fe-C and Fe-O bond lengths were taken from the structure of 7 bonded CO in the Fe, “butterfly” cluster [9]. This modeling indicates that the tilt angle of a CO molecule in the fourfold hollow site on Fe(lOO) would be - 40° for the [llO] CO azimuthal orientation and 55” from the surface normal for the [loo] CO azimuthal orientation. The measured tilt angle of CO(cus), 45O f loo obtained from our NEXAFS measurement is quite similar to the values obtained from this simple hard sphere modeling. However, we cannot differentiate between the two azimuthal orientations within our experimental uncertainty. In summary, the NEXAFS investigation indicates that the precursor state for CO dissociation on Fe(100) occupies the fourfold hollow site and is tilted by 45” f 10” from the surface normal. The carbon-oxygen bond for this species is longer by 0.07 k 0.02 A than those in the CO(a,) and CO((Y*) “normal” bonding configurations. The unusual bond weakening reflected by the low stretching frequency (1210 cm-‘) and the facile dissociation of this state is believed to be due to r bonding and resulting in direct interaction of the oxygen end of the CO(a3) molecule with the iron surface. The tilted, weakened, and stretched bond of this state and the possible [loo] azimuthal orientation of the bond, combine to suggest that the reaction coordinate for dissociative adsorption of CO may be across the saddle point separating adjacent fourfold hollow sites.
L128
D. W. Moon et al. / A trltedprecursorfor
CO dtssocmtton
on Fe(100)
References [l] [2] [3] [4] [5] [6] [7] [8] [9] lo] 111 121
D.W. Moon, S.L. Bemasek, D.J. Dwyer and J.L. Gland, J. Am. Chem. Sot. 107 (1985) 4363. D.W. Moon, S.L. Bemasek and D.J. Dwyer, Surface Sci. 163 (1985) 215. N.D. Shinn and T.E. Madey, Phys. Rev. Letters 53 (1985) 2481. F. Zaera, E. Kollin and J.L. Gland, Phys. Rev. Letters, submitted. J. Stijhr and R. Jaeger, Phys. Rev. B26 (1982) 4111. F. Sette, J. StBhr and A.P. Hitchcock, Chem. Phys. Letters 110 (1984) 517; J. Chem. Phys. 81 (1984) 4906. J. Stiihr, J.L. Gland, W. Eberhardt, D. Outka, R.J. Madix, F. Sette, R.J. Koestner and V. Dobler, Phys. Rev. Letters 51 (1983) 2414. W.A. Herrmann. H. Biersack, M.L. Ziegler, K. Weidenhammer, R. Siegel and D. Rehder, J. Am. Chem. Sot. 103 (1981) 1692. C.P. Horwitz, E.M. Holt, C.P. Brock and D.F. Shriver, to be published. F.M. Hoffmann and R.A. DePaola, Phys. Rev. Letters 52 (1984) 1697. C. Benndorf, B. Krtiger and F. Thieme, Surface Sci. 163 (1985) L675. Z. MiSkoviC, J. VukaniC and T.E. Madey. Surface Sci. 141 (1984) 285.