J. Phys. Chem. 1985, 89, 3842-3845
3842
Orientation of CHBNCAdsorbed on Ag(311) S.
T. Ceyer*t and J. T . Yates, Jr.g
Surface Science Division, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: March 1 , 1985; In Final Form: April 12, 1985)
The adsorption of methyl isocyanide on the Ag(311) surface at 95 K has been studied by high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), and thermal desorption spectroscopy. Examination of the intensities of the loss features in the HREEL spectrum within the selection rule for a dipolar scattering mechanism and the behavior of the ESDIAD patterns as a function of coverage lead to the conclusion that CH3NC adsorbs on Ag(3 11) with two orientations. The C,,axis of the more strongly bound CH3NC molecule is tilted toward the macroscopic surface. The more weakly bound state interacts through the u orbital on the - N S end of the molecule leading to the CH3NC molecule bound with its C3, axis perpendicular to the macroscopic surface. The thermal desorption data exhibit two peaks which correlate with these bonding geometries.
Introduction Carbon monoxide and isocyanide are often interchangeable ligands in transition-metal chemistry.' This is a result of the electronic similarity of their frontier orbitals, the 5a and 27r* of end of the isocyanide C O and the u and r* orbitals of the -N= ligand. As part of an ongoing program2,, to investigate the applicability of this analogy to surface chemistry, we have attempted to study the chemisorption of CO and CH3NC on Ag(311). Both the C O and CH3NC molecules have the potential for u bonding through the carbon end and bonding to the surface through the "-type orbitals. Interaction with the surface through the a-type orbitals is the more prevalent kind of bonding that is observed. However, bonding through the "-type orbitals has recently been observed for ethylene on Ag( 110): ethylene5 and pyridine and benzene6on Ag(l1 l), and C O on Cu(31 l).' The Ag(311) surface has a corrugated structure consisting of channels bounded by Ag( 11 1) and Ag( 100) microfacets. Since the electron density in the channels is expected to be larger than on the ridges or the corresponding flat surface and since Ag is an electron-rich s-metal, the Ag(311) surface is a reasonable choice for observation of bonding through the "-type orbitals. For both C O and CH,NC, the two types of bonding lead to very different orientations of the molecule relative to the surface. For d y p e bonding the C, and C,,axes of the C O and CH3NC molecules, respectively, would be oriented perpendicularly to the surface while for "-type bonding these axes would be parallel to the surface. Therefore, we have used the normal dipole selection rule in high resolution electron energy loss (HREEL) spectroscopy to examine and relate the intensities of the loss features to the type of bonding for CH3NC on Ag(311). We have also employed electron stimulated desorption ion angular distribution (ESDIAD) measurements to probe the orientation and hence the bonding of CH3NC to Ag(311). Experimental Section The experiments were carried out in a ultrahigh-vacuum chamber pumped by an ion pump and a liquid nitrogen cooled titanium sublimator pump. The routine base pressure was 3 X lo-" torr. The crystal, mounted on a manipulator described previously,8 was positioned on the long axis of the cylindrical chamber. The crystal was translated 18 cm between a level on which the high resolution, 127' sector electron spectrometer was mounted and the level on which a LEED/ESDIAD a p p a r a t ~ s , ~ a single pass cylindrical mirror Auger analyzer with integral electron gun, a quadrupole mass spectrometer, and an ion sputtering gun were mounted. A calibrated microcapillary array gas doser was mounted between the two levels. Present address: 6-229, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139. 'Present address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260.
The full width at half-maximum intensity of the 3-eV electron beam used for the energy loss measurements was typically 60-70 cm-I. The incident angle of the beam was typically 60' measured from the surface normal. The ESDIAD patterns were observed with an electron current of about 5 pA at the sample and an electron beam energy of 100 eV. The heating rate in the thermal desorption spectra shown here was about 10 K/s. Methyl isocyanide was synthesized by established methods'O and was stored in a dry ice-acetone bath. The Ag sample was oriented to within 1' of the (31 1) direction by the back-reflection Laue method and was then spark cut. The roughly cut sample was then reoriented on a X-ray diffractometer to within 0.1 ' of the (311) plane and ground and polished to this orientation. The final polishing step involved a brief chemical etch with a mixture of equal parts of 47 wt % of NH3(aq), 3 wt % of HzOz,and water. A Pt wire was wrapped in a groove spark cut around the perimeter of the sample. This wire provided both mechanical support of and electrical contact to the crystal. A W-5% Re/W-26% Re thermocouple was wedged in between this wire and the Ag surface. The sample was cleaned by argon ion bombardment (typically 20 min at 600 eV with an ion current of 10 pA) and then annealed at 500 K.
Results N o evidence of carbon monoxide adsorption with a residence time of greater than 5 min at 95 K was found after a 500-langmuir exposure. Figure 1 presents the high resolution electron energy loss spectra measured at the specular angle as a function of CH3NC exposure on a Ag(311) surface at 95 K. The loss features at 950, 1080, 1460, 2190, and 3000 cm-' do not shift in frequency as the exposure is changed. However, two new features appear at 125 and 550 cm-' for very high exposures. The most striking development in this series of spectra is the change in the relative intensities of the 950-, 1460-, and 2190-cm-' loss features. The 950- and 2 190-cm-' features grow more rapidly than the 1460-cm-' feature (1) Malatesta, L.; Bonati, F. 'Isocyanide Complexes of Metals"; WileyInterscience: New York, 1969. (2) Cavanagh, R. R.; Yates, Jr., J. T.J . Chem. Phys. 1981, 75, 1551. (3) Semancik, S.;Haller, G. L.; Yates, Jr., J. T. J. Chem. Phys. 1983, 78,
-_
m7n
(4) B a c k , C.; de Groot, C. P.; Biloen, P. Appl. Surf.Sci. 1981, 6, 256. (5) Felter, T.E.;Weinberg, W. H.; Zhdan, P. A.; Boreskov, G. K. Surf.
Sci. 1980, 97, L313.
(6) Demuth, J. E.; Sanda, P. N.; Warlaumont, J. M.; Tsang, J. C.; Christmann, K. "Proceedings of the 2nd International Conference on Surface Vibrations, Namur, Belgium, Sept, 1980"; Plenum: New York, 1981. (7) Shinn, N. D.; Trenary, M.; McClellan, M. R.; McFeely, F. R. J. Chem. Phys. 1981, 75, 3142. (8) Pararas, A,; Ceyer, S. T.; Yates, Jr., J. T. J . VQC.Sci. Techno/. 1982, 21, 1031. (9)Madey, T. E.; Yates, Jr., J. T. Surf.Sci. 1977, 63, 203. (10) Casanova, Jr., J.; Schuster, R. E.; Werner, N. D. J . Chem. Soc. 1963, 4280. (1 1) Herzberg, G. 'Molecular spectra and Molecular Structure 11"; Van Nostrand Reinhold: New York, 1945.
This article not subject to US.Copyright. Published 1985 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 18, 1985 3843
Orientation of C H 3 N C Adsorbed on Ag(3 11) CH,NC/Ag ,
(311)
125
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a
CH,NC/Ag
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950
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, 0
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16
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Figure 3. Peak intensities of selected loss features vs. angle from the specular angle. Angle of incidence is constant.
n V
0
1000
3000
2000
Frequency 1 c m - I )
Thermal Desorption Spectra
Figure 1. High resolution electron energy loss spectra as a function of
CH,NC / Ag (311)
CH3NC exposure on Ag(311).
u)
1.6
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5
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0
a
-
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Figure 4. Thermal desorption spectra as a function of CH3NC exposure. The peak at 95 K results from desorption of CH,NC from the leads on the crystal.
I
-
Figure 2. Ratios of the peak intensities of the loss features at 2190 and 950 cm-I to the loss feature at 1460 cm-' vs. exposure of CH3NC.
pearance of a second peak near 160 K. The larger area under the TDS curve at 0.8-langmuir exposure relative to that at 0.6 langmuir probably reflects an increase in effective CH3NC flux at the crystal due to less adsorption on the walls of the doser at the longer exposure time. The peak at 160 K increases in intensity and gradually shifts to 140 K as the exposure is increased to 4 langmuirs. No desorption of Hz, HCN, or N 2 is observed. Electron energy loss spectra taken after heating a saturated surface to several temperatures between 155 and 200 K and then cooling to 95 K reflect only changes in coverage. N o new loss features are observed. N o ordered LEED pattern of the adsorbed overlayer was observed at any coverage. N o proton intensity was detected in the ESDIAD measurements for low exposures (
