VOLUME 69, NUMBER 22 P H YSICAL R EV I EW LETTERS 30 NOVEMBER 1992 Chemical Shift Photoelectron Diffraction from Molecular Adsorbates K. -U. Weiss, R. Dippel, K.-M. Schindler, P. Gardner, V. Fritzsche, ' and A. M. Bradshaw Fritz Ha-ber Ins-titut der Max Pla-nck Ges-ellschaft, Faradayweg 4-6, D W-l0-00 Berlin 33, Germany A. L. D. Kilcoyne and D. P. Woodruff Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom (Received 25 June 1992) By using the chemical shift in core-level photoemission from adsorbed species, we apply the technique of energy-scan photoelectron diAraction to molecules containing nonequivalent atoms of the same ele- ment. As an example, we demonstrate the complete structural characterization of the surface acetate species on Cu[110] including independent determination of the sites of the two inequivalent carbon atoms. PACS numbers: 68. 35. — p, 61.16. — d, 79. 60. — i Since the majority of molecular adsorption systems form disordered overlayers and are thus not accessible to conventional diff'raction techniques, there is considerable interest in surface structural probes which do not require the presence of long-range order [1-3]. In this context, we have shown recently that energy-scan photoelectron difl'raction is particularly useful for determining the structural parameters of small, adsorbed molecules con- taining C, N, and 0 atoms (e. g. , Refs. [4-6]). The inten- sity of a core-level photoelectron peak, corresponding to an atom of the adsorbate, is measured at a selected emis- sion angle as a function of photon energy and thus of the photoelectron kinetic energy. In the plot of peak intensity versus kinetic energy ("the photoelectron difl'raction spec- trum") modulations occur as a result of interference be- tween the primary photoelectron wave and the secondary waves elastically scattered at surrounding atoms. These modulations provide information on the scattering path lengths and thus on the local bonding geometry, i. e. , on the surface site and the distance to neighboring substrate atoms. In the case of larger molecules adsorbed on surfaces there are often several atoms of the same element present, as, for example, in hydrocarbons. If these atoms are chemically distinct and have different core-level bind- ing energies in the photoelectron spectrum, then the possibility exists of performing chemical shift photoelec- tron diffraction. The modulations in the corresponding photoelectron diffraction spectrum of each atom will be different, if the surface site and/or the separation from the surface is different. In some cases, atoms which are equivalent in the free molecule will become chemically distinct on the surface due to the bonding geometry, e.g. , in adsorbed molecular fragments resulting from hetero- geneous reactions. By performing appropriate scattering calculations the photoelectron diffraction spectra can be simulated and the site of each atom determined separate- ly relative to the surface. An example of an adsorbed molecule containing non- equivalent atoms of the same element is the surface ace- tate species (CH3COO-) which can be formed on a Cu [110] surface by decomposition of acetic acid. The po- larization dependence of the oxygen and carbon K-edge absorption spectra [7] shows that the molecular plane is perpendicular to the Cu surface in the (110) azimuth. Further, vibrational spectra of the same species on the Cu{100] surface [8] indicate a symmetric configuration with two equivalent oxygen atoms. A plausible bonding geometry based on that of the corresponding surface for- mate species [5] on Cu[110] is shown as an inset in Fig. 1. Note that in this proposed structure, which is in fact confirmed by the present study (see below), the so-called aligned bridge site [5] is occupied. From literature data [9] on acetic acid and acetates it is known that the car- boxyl carbon atom has a 3.5-4.0-eV higher 1s photoelec- tron binding energy than the methyl carbon atom. Figure 1(a) shows that this is also the case for the surface ace- tate species on Cuj110}. In the corresponding trifi'uoro acetate species (CF3COO-) on the same surface the bind- ing energy of the outermost carbon atom increases by -6 eV (spectrum not shown in Fig. 1) and the peak thus shifts to the low kinetic-energy side of the peak from the carboxyl carbon, again in agreement with literature data [9]. (The surface trifluoroacetate species, in which the H atoms are replaced by F atoms, can be prepared analo- gously to the acetate species by the decomposition of trifluoroacetic acid. ) Thus the surface acetate is ideally suited to demonstrate the potential of chemical shift pho- toelectron diffraction. At the same time, we obtain valu- able structural information on an interesting chemisorp- tion system. The experiments were performed in a purpose-built ul- trahigh vacuum spectrometer on the HE-TGM 1 mono- chromator [10] at the BESSY synchrotron radiation source. The Cu[110] sample was cleaned with the usual methods. The acetate and triAuoroacetate species were prepared in submonolayer concentration by exposing the surface to 1. 5x10 mbars acetic acid at 100 K and sub- sequently warming to 380 K. A VG Scientific 152-mm mean radius 150' electrostatic deflection analyzer (with 3196