19136
J. Phys. Chem. 1996, 100, 19136-19140
Structure and Dynamics of C2H4 at Submonolayer Coverages on a Copper(001) Surface Studied by Helium Atom Scattering A. P. Graham, M. F. Bertino, F. Hofmann, and J. P. Toennies* M.P.I. fu¨ r Stro¨ mungsforschung, Bunsenstrasse 10, D-37073 Go¨ ttingen, Germany ReceiVed: June 11, 1996; In Final Form: August 13, 1996X
The sub-monolayer (Θ < 0.5) adsorption structures and vibrational dynamics of ethene molecules (C2H4) on copper(001) have been studied with high-resolution helium atom scattering at Ts ) 50 K. Diffraction rings observed in the angular distributions indicate that the ethene molecules repel each other on the surface at intermediate exposures, thereby forming ordered, nearly hexagonal structures. The time of flight spectra at Θ ) 0.038 reveal two peaks, one at 6.3 meV (51 cm-1), which is attributed to the perpendicular stretch vibrational mode (S) of the adsorbate, and another mode at a somewhat lower frequency, which is attributed to the frustrated translational vibration parallel to the surface (T-mode). The T-mode frequency increased significantly with increasing coverage from 4.2 meV (34 cm-1) at Θ ) 0.014-5.3 meV (46 cm-1) for the monolayer (Θ ) 0.5), whereas the S-mode frequency was virtually independent of ethene exposure. The shift in the T-mode frequency with ethene exposure is attributed to a long-range substrate mediated interaction between adsorbed ethene molecules.
I. Introduction The adsorption and dynamics of hydrocarbon molecules on metal surfaces has recently received significant attention due to the importance of these interactions in catalysis1,2 and lubrication.3 Considerable progress has been made in the understanding of bonding geometries, fragmentation, and bond character.1,4-6 In particular, a number of recent studies have focused on the internal vibrational mode frequencies of the adsorbed molecules to elucidate the strength of the bond to the surface and the molecular surface fragments existing under certain conditions.6 However, the internal molecular modes usually have rather high frequencies (J100 meV (800 cm-1)) and are consequently expected to play a minor role in thermodynamic processes since they are not appreciably populated at room temperature. The external vibrations of the molecules, including frustrated translations and rotations, have, on the other hand, much lower frequencies (j10 meV (80 cm-1))7 in the range of thermal energies. In particular, the molecule-surface stretch mode (S-mode) provides important information concerning the attempt frequency for adsorbate desorption, while the parallel frustrated translation mode (Tmode) is important for an understanding of adsorbate diffusion and friction.3 The external mode frequencies usually lie too low to be observed using traditional surface specific spectroscopic probes, such as infrared spectroscopy and electron energy loss spectroscopy (EELS), and they are only accessible using inelastic helium scattering.8 So far, only a few studies of hydrocarbons have been reported.7,8 In addition, little is known about intermolecular interactions between adsorbed hydrocarbon molecules except at high densities, where steric effects become important in determining overlayer structures.9 Copper surfaces have been extensively used as a model for molecular adsorption on metal surfaces due to their relatively weak bonding and the large amount of information already available for the low index faces. In addition to the helium atom scattering (HAS) studies of external vibrational modes on the saturated hydrocarbons,7,10 the internal modes of several unsaturated hydrocarbons have been investigated on copper X
Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)01713-3 CCC: $12.00
surfaces.11-14 Ethene is known to adsorb in 4-fold hollow sites15 on the Cu(001) surface with the C-C bond parallel to either the [110] or [11h0] directions.11 It is generally believed that bonding is mediated through donation of charge from the π molecular orbital to the surface and back-donation of charge into the π* anti-bonding orbitals, resulting in an out-of-plane tilting of the hydrogen atoms away from the surface.16 This results in the formation of an electric dipole in the ethene molecule which explains the work-function decrease of ∆Φ ) -0.90 ( 0.05 eV observed at completion of a monolayer.14 HAS has been used in the present study to measure the structure and low-frequency external vibrational modes of ethene on a Cu(001) surface in the sub-monolayer regime. The helium angular distributions show at very low coverages that the ethene molecules are initially randomly distributed. Then at higher coverages it appears that the repulsive interaction between neighboring ethene molecules, resulting from the adsorption induced electric dipole, is strong enough to produce diffraction ring structures, indicating a hexagonal fcc(111) structure. The latter behavior is similar to observations of alkali metal atoms on metal surfaces.17 The complete C2H4 monolayer was found to have a p(2 × 2) diffraction structure. The S-mode frequency for ethene was determined from time-of-flight (TOF) measurements to be 6.3 meV (51 cm-1), independent of coverage, while the T-mode frequency shows a strong coverage dependence between 4.2 meV (34 cm-1) and 5.3 meV (46 cm-1). A simlar coverage dependence was recently found for the T-mode frequency of sodium on Cu(001).18 In both cases the evidence points to a long-range substrate mediated interaction. The present paper is organized in the following way: Following a brief description of the experimental details the helium scattering results are presented in section III. The helium scattering results are discussed in section IV, and the paper closes with a presentation of the conclusions drawn in section V. II. Experimental Details The high-resolution helium atom time-of-flight apparatus, described in detail elsewhere,19 has a fixed total scattering angle © 1996 American Chemical Society
C2H2 at Submonolayer Coverages on a Cu(001) Surface of θSD ) 95.8°. Different parallel wave vector transfers are accessed by rotating the crystal about an axis perpendicular to the plane of the incident and detected beams. The Cu(001) single crystal was aligned to an accuracy of better than 0.25°, was mechanically polished, and was mounted on a six-axis sample manipulator. The sample was cleaned with repeated cycles of argon ion sputtering and annealing to 750 K until no residual contamination could be detected with Auger spectroscopy (
