Aug 17, 2010 - of Ethene (C2H4) Chemisorbed on the Bare and Roughened. Cu(110) and Cu(111) ..... Formation of ethylene dimers. Spectrochimica Acta ...
O 36.24: Infrared Reflection Absorption Spectroscopy (IRRAS) of Ethene (C2H4) Chemisorbed on the Bare and Roughened Cu(110) and Cu(111) Surfaces Jan Pischel, Olaf Skibbe, and Annemarie Pucci
Kirchhoff-Institut für Physik Im Neuenheimer Feld 227 D-69120 Heidelberg
U NIVERSITÄT H EIDELBERG
Introduction
IRRAS
AlAlAl Al AlAlAlAlAl Al Al Al AlAl AlAlAl AlAlAlAl Al Al Al Al Al AlAlAl Al AlAl AlAlAlAlAl Al Al Al Al Al AlAl AlAl AlAl Al AlAlAlAlAl Al Al AlAlAlAlAl AlAlAl AlAlAl Al Al Al Al Al Al AlAlAlAl AlAlAlAlAlAlAl Al AlAlAl Al Al Al Al Al Al AlAlAl Al AlAlAl AlAlAlAlAl Al Al AlAl AlAlAl AlAlAl Al AlAlAlAlAlAl Al AlAl Al Al AlAlAlAl Al Al Al AlAl AlAl Al AlAlAl Al AlAl Al Al AlAl Al Al
[110]
incoming p-polarized IR-beam (diameter ≈ 2 mm) near gracing incidence (θ ≈ 85°) reflected by metal surface
Tab. 1: Some of the normal modes of gas phase ethene (nomenclature according to ref. [6]). Obeying the Raman-IR exclusion principle, the modes are either Raman active (R), infrared active (IR) or silent (s). The corresponding frequencies of solid ethene and of ethene adsorbed on the Cu(110) and Cu(111) surfaces are given in cm−1. mode
ν2
ν3
ν7
ν8
ν10
reference spectrum: smooth or roughened surface, bare
of adsorbates
ν12
only IR-active vibrations of adsorbates with dynamic
dipole moment perpendicular to the surface may be excited
visualization [6]
[001]
name activity frequency (solid) [7]
Fig. 1: The highly anisotropic Cu(110) surface (left) and the close-packed Cu(111) surface (right). Images generated by [1].
As a model for an organic-metal interface, the chemisorption systems C2H4/Cu(111) and C2H4/Cu(110) have attracted attention because of their remarkable absorption properties in infrared (IR) spectroscopy [2, 3, 4, 5]. Not only were two gas phase Raman active modes observed that are expected to be IR forbidden by the Raman-IR exclusion principle for centrosymmetric molecules, but also the only IR active mode allowed by the metal surface selection rule seemed to disappear at higher coverages on Cu(110). No final conclusion concerning these findings has been drawn so far. This brought us to reinvestigate the system C2H4/Cu(110) by means of IRRAS.
CC-stretch CH2-deformation CH2-wagging CH2-wagging CH2-rocking CH2-deformation R R IR R IR IR 1615, 1622 1329, 1348 942, 951 942, 952 820, 826 1436, 1440 C2H4/Cu(110) low coverage 1532, 1552 high coverage 1522 low coverage high coverage 1522 1534 low coverage 1535 high coverage 1523 ice layer 1518
ref.[2] ref.[3] ref. [4] this work
1274, 1285 1261 1275 1257 1275 1275 1260 1256
916 906 904 907 850 847, 957
-
821
1440
evaporator Omicron EFM3
1535 1539
sputtergun: Ar+ , 2 keV
UHV: p ≤ 6·10-11 mbar gases: C2H4 , CO, O2, ...
FTIR-spectrometer
1285 1283
910 916
-
-
QMB
40K ≤ T ≤ 900K
C2H4/Cu(111) ref. [8] ref. [9]
sample
IRRAS
-
detector
Fig. 2: Experimental setup.
C2H4/Cu(110) 1.9 L
1519 c
c
887
99,0
0.67 L
1.00 L
0.51 L 0.58 L
c c
c
c
98,5
1.67 L
1550
957
CC-stretch
0.30 L
0.44 L
[%] 0
rel. reflectance R/R
1535
1275
0
907
1440
c
rel. reflectance R/R
0.16 L 99,5
1560
c
annealed
1256
1540
1530 1523
1520 c
1.91 L
c
0.2 %
1260
98,0
1500
1600
-1
800
900
1000
1200
1300
1400
1500
wavenumber [cm ]
1600
-1
wavenumber [cm ]
Fig. 3: IRRAS spectra of C2H4/Cu(110) at 60 K and different exposures. The plane of incidence is oriented parallel to the [1¯10] direction. With increasing exposures, the Raman active modes develop and shift to lower frequencies whereas the IR active mode broadens significantly. At exposures higher than 1.4 Lc, multilayer adsorption occurs (ice peak at 957 cm−1).
Fig. 5: IRRAS spectra of 1.9 Lc C2H4/Cu(110) at 50 K before and after annealing to 90 K for 100 s. The ice layer desorbs and the Raman active modes become sharper and deeper. 0.00 ML
1290
0.18 ML
1280
1270
2
1400
CH -scissors
1300
-1
1200
peak position [cm ]
1256
957
800 850 900 950 1000
1519
1260
0.5 % 1521
rel. reflectance R/R
960
0.12 ML
940
0.18 ML 0.23 ML 0.5 ML
920
900
3.0 ML
800
900
1000
1200
860
1300
0.2 %
880
1544
1277
894
1440 1518
all surfaces
2
0
c
4.2 ML
821 847
smooth surfaces
860
CH -wagging
average
rel. reflectance R/R
0.035 ML
rough surfaces
0
07/29/10
93 L
1258
[%]
08/17/10
1400
1500
1600
0,0
0,5
1,0
1,5
2,0
-1
957
wavenumber [cm ]
1256
dose [L ] c
800
1000
1200
1400
1600
-1
wavenumber [cm ]
Fig. 4: IRRAS spectra of 1.9 Lc C2H4/Cu(110) at about 50 K measured at two different days and their average (baseline corrected). The detail of a spectrum with very high exposure (93 Lc) is shown in order to identify the origin of the mode at 821 cm−1.
Fig. 6: IRRAS spectra of 1.9 Lc C2H4/Cu/Cu(110) at 50 K after annealing to 90 K for 100 s for various amounts of cold-evaporated copper. The quantity of evaporated atoms is given in units of monolayers (ML), where 1 ML represents the number of atoms within one atomic layer (although layer-by-layer growth is not necessarily taking place). With increasing surface roughness, the Raman peaks split up and the IR active mode appears near 900 cm−1.
Conclusions
C2H4/Cu(111) -1
Wavenumber [cm ] 0
1000
2000
3000
4000
0.00 ML
-1
115meV (920cm )
0.2 %
0.97 ML 1.3 ML
x300
800 850 900 950 1000
1200
1508
1300
1400
specular
1543
1500
erage on Cu(110) but appears to become broader and broader until it almost gets lost in the noise. Cu(110) and Cu(111) surfaces differ strongly: On Cu(110), the most prominent features are the IR forbidden Raman active modes ν2 and ν3, whereas on Cu(111) those modes are hardly distinguishable and the IR active mode is clearly visible.
off-specular
1.8 ML
1280
381meV(3048cm-1)
x300
371meV (2968cm-1)
0.64 ML
7meV
258meV (2064cm-1)
0.39 ML
230meV (1840cm-1)
0.23 ML
196meV (1568cm-1)
157meV (1256cm-1)
1551
intensity [a.u.]
rel. reflectance
0.04 ML
1241
The IR-active mode ν7 does not really disappear with increasing cov-
The spectra obtained for saturation coverage C2H4 on the bare
1284
915
Fig. 7: Schematic represantation of the dependence of the observed ethene peak positions on surface roughness and ethene exposure. The bars show the trend derived from several hundred spectra. Surfaces with less than 0.18 ML copper evaporated show similar properties, can be pooled and are called “smooth”. Analogously, those with higher amounts of cold-evaporated copper are called “rough”.
1600
-1
wavenumber [cm ]
0
100
200
300
400
500
Energy loss [meV]
Fig. 8: IRRAS spectra of C2H4/Cu/Cu(111) at saturation coverage (T = 95 K) for various amounts of cold-evaporated copper. Data taken from [5, 9], new compilation.
Fig. 9: High resolution electron energy loss (HREEL) spectra of 1.1 Lc C2H4/Cu(111) at 85 K (comparison between specular and 10° off-specular geometry) [5, 9].
References
[3] Kubota, J., J. N. Kondo, K. Domen, and C. Hirose: IRAS Studies of Adsorbed Ethene (C2H4) on Clean and Oxygen-Covered Cu(110) Surfaces. J. Phys. Chem., 98:7653-7656, 1994.
[1] Hermann, K. and F. Rammer: surface explorer. http://surfexp. fhi-berlin.mpg.de. [2] Jenks, C. J., B. E. Bent, N. Bernstein, and F. Zaera: Evidence for an unusual coordination geometry for ethylene on Cu(110). Surf. Sci. Lett., 277:L89-L94, 1994.
[4] Raval, R.: Probing the nature of molecular chemisorption using RAIRS. Surf. Sci., 343:201-210, 1995. [5] Skibbe, O., M. Binder, A. Otto, and A. Pucci: Electronic contributions to infrared spectra of adsorbate molecules on metal surfaces: Ethene on Cu(1 1 1). J. Chem. Phys., 128:194703-1 - 194703-6, 2008.
The intensity of ν2 and ν3 on Cu(110) is stronger or at least com-
parable with that of the IR-active mode ν7 on Cu(111). This could be explained if the modes of the adsorbed molecule on Cu(110) were found to possess a dynamic dipole moment. Upon roughening the surfaces, their adsorption properties concerning
ethene become similar as might be expected considering the fact that the single crystalline surfaces get covered and the same kind of defects govern the surface properties.
[6] Herzberg, G.: Infrared and Raman Spectra of Polyatmic Molecules. D. van Nostrand Company, Princeton, Reprint 1964. [7] Rytter, E. and D. M. Gruen: Infrared spectra of matrix isolated and solid ethylene. Formation of ethylene dimers. Spectrochimica Acta, 35A:199-207, 1979. [8] McCash, E. M.: 1989 C. R. Burch Prize: Surfaces and vibrations. Vacuum, 40(5), 1990. [9] Binder, M.: Untersuchung von Ethen auf der glatten und aufgerauten Cu(111)-Oberfl¨ache mit IRRAS und HREELS. Diploma Thesis, University of Heidelberg, 2006.
