Graeme L. Nyberg

Symmetry analysis of angle-resolved photoemission: Polarization dependence and lateral interactions in chemisorbed benzene

Abstract Using an oriented free-molecule model, exact selection rules are used to determine which bands are allowed in both normal and off-normal emission as a function of polarization direction. An approximate angular analysis of the final states is then applied to give a qualitative account of the emission angle dependence. The consequences of both inter- and intra-ad sorbate unit cell lateral interactions are considered, with effects which are new to adsorbate photoemission. When applied to benzene-on-Pd(100) angle-resolved spectra these concepts are shown to give a very adequate description of the results. Previous assignments are confirmed with the exception that the second π-orbital is now located on the leading edge of the C-band, thus implying that both π-orbitals are subject to similar adsorbate-substrate binding shifts. In conjunction with LEED data a specific adsorbate geometry is proposed.

A vibrational and TDS study of sulfur adsorbates on Cu(100): Evidence for CH3S species

Abstract Vibrational (EELS) and TDS data for methyl mercaptan (CH 3 SH), dimethyl sulfide (CH 3 ) 2 S and dimethyl disulfide (CH 3 S) 2 are analyzed to determine the nature of the adsorption states on Cu(100). Dimethyl sulfide is reversibly adsorbed on Cu(100); no dissociation (CS bond breaking) was found. By contrast, methyl mercaptan and dimethyl disulfide dissociate below 300 K to form adsorbed CH 3 S (methyl mercaptide) species. Depending on the coverage, two orientations of methyl mercaptide are found: linear and bent. The two different orientations can be distinguished via the surface dipole selection rule by different intensities of the methyl rocking and deformation vibrations. By contrast with the methoxy species, which on Cu(100) decomposes to formaldehyde, no H 2 C=S is liberated during decomposition of CH 3 S. The mercaptide is stable to ∼ 350 K, but decomposes at higher temperatures to form adsorbed sulfur and recombinant methane, hydrogen and ethane. The methane appears to be formed by methyl-hydrogen recombination when the C-S bond scission occurs. TDS results show that sulfur released from the decomposition poisons the surface toward further adsorption. In addition, the selectivity toward methane versus ethane can be altered by pre-titrating the adsorbed hydrogen with oxygen, thereby changing the relative methyl-hydrogen and methyl-methyl recombination probabilities.

An electron spectroscopic study of the adsorption of benzenethiol and 1,2-benzenedithiol on the Cu(110) surface

Abstract The adsorption of benzenethiol and 1,2-benzenedithiol on Cu(110) is studied over wide exposure and temperature ranges using UPS, Δφ, XPS and vibrational-EELS. Both compounds lose their thiol hydrogen atom(s) when chemisorbed on Cu, forming monoand di-phenyl mercaptide respectively. At room temperature and low surface coverage the chemisorbed phenyl mercaptide species further undergoes limited desulfurization, with the build-up of the resulting chemisorbed atomic sulfur on the surface self-inhibiting continued C-S bond dissociation. At low exposure the phenyl ring of both the adsorbates lies flat on the Cu surface, but takes a standing-up (though not strictly perpendicular) orientation at high exposures. Whereas the monomercaptide adsorbate appears highly ordered, the dimercaptide does not. UPS was employed to monitor the adsorbate thermo-evolution. Although direct observation of the interaction between the first adsorbate monolayer and the substrate was not possible, analysis of the spectral changes indicates that the loss of the thiol hydrogen of the first layer benzenethiol and 1,2-benzenedithiol were complete at −112 and −90°C, respectively, significantly below the final multilayer desorption temperatures of −89 and −46°C.

UPS of CH3OH and C2H5OH adsorbed on Cu(410): Band assignment and chemisorption bonding model

Abstract Through using the fractional-difference technique applied to the angle-resolved He I UP spectra of methanol and ethanol adsorbed on Cu(410), the structure of the adsorbate-covered surface in the d-band region is resolved into a number of components. A previously unrecognised band on the Fermi-level side of the d-band is assigned as the antibonding counterpart of the adsorbate-induced bonding band immediately to the high binding energy side. The subsequent, higher-binding-energy, molecular-like bands also are all assigned — in this case through a novel use of molecular-orbital calculations. In these, an appropriate model for the preadsorbed species is derived through distortion of the parent molecule. These assignments, together with the observation that it is the Cu 3d xz , yz band which again (as with adsorbed oxygen and sulfur) is the substrate component most affected by the adsorption, have allowed the formulation of a specific, diatomic-like, adsorbate-substrate bonding scheme for this (100)-type surface. The end result is a chemisorption bonding model which is not only easy to apply but which also fully accounts for all bands across the entire spectrum.

The adsorption of silicon on Cu(111) and the initial stages of oxidation of the silicon-copper interface

We have used shallow-core level photoelectron spectroscopy from the Si 2p level to identify the presence of three silicon-containing species present when silane (SiH4) is adsorbed onto the Cu(111) surface at 300 K. These species have binding energies of 98.84, 99.11 and 99.40 eV. Two species are assigned to elemental silicon on the surface, the other to -SiH. We discuss the possible structural models for adsorption of silicon in three-fold hollow sires and in surface incorporation models. When the (root 3 x root 3)R30 degrees silicon layer is exposed to oxygen at 300 K, a broad photoemission feature at 102.50 eV binding energy appears. Heating to 600 K results in a shift of the oxidised stale binding energy to 103.80 eV, attributed to the Si4+ state found in SiO2. The width of this feature is similar to 1.5 eV, larger than the feature observed for the Si/SiO2 interface, and the increased width is attributed to inhomogeneities in the silicon oxide layer. If silane is adsorbed in the presence of preadsorbed oxygen, the photoemission features corresponding to elemental silicon, the -SiH species and oxidised silicon are observed. The binding energies are shifted to 99.11, 99.40 and 103.10 eV respectively. Heating to 800 K results in the complete extinction of any photoemission features from elemental silicon, while the oxidised silicon photoemission peak has a binding energy of 103.80 eV

The adsorption and bonding of methanethiol on aluminium

Abstract To further establish the nature of thiol adsorbate-species, and of their surface-bonding, the adsorption of methanethiol has been investigated on polycrystalline aluminium, using UPS, XPS and Δφ. The chemisorption proceeds through the loss of the thiol hydrogen, resulting in the corresponding mercaptide (as shown before also on copper). An assignment of the adsorbate-induced bands is made utilising previously developed techniques — an MO description of the “precursor” molecule, and the qualitative surface-molecule model for the adsorbate-substrate interaction. The latter is thus shown not to be confined just to copper, but to have general applicability. Comparison is also made of CH 3 S and CH 3 O bonding on Al and Cu, and its possible relationship to catalysis.

The reactivity and bonding of oxygen on the α-Cu17%Al(100) surface studied by UPS

Abstract The reactivity and the bonding of oxygen on the α-Cu17%Al(100) alloy are investigated using ultraviolet photoelectron spectroscopy (UPS), and the results are compared with those of Cu(100). In the first stage of the reaction, oxygen reacts with the Al atoms on the alloy surface. Since there is no Al enrichment on the clean alloy surface, only a relatively small exposure of oxygen is required to saturate these sites. This oxygen does not penetrate into the alloy surface, but remains as an overlayer with the oxygen atoms chemisorbed at the Al sites. As a result of this surface surface oxidation, a second stage of reaction follows, in which more Al atoms segregate to the surface at a very slow rate and continue the reaction with oxygen. The slow migration rate of Al atoms, which is difficult to observe within the normal experimental duration, is the rate-limiting process of the second stage of the reaction. However, given enough time, Al enriches at the alloy surface in multilayer quantities, forming Al 2 O 3 .

UPS of O2 and H2S adsorbed on Cu(410): Band resolution through fractional difference spectra

Abstract A fractional difference technique is described in which criteria are established for subtracting experimentally well-defined fractional multiples of the substrate UP-spectrum from that of the adsorbate-covered surface. Through application to oxygen and hydrogen sulfide adsorbed on copper, it is shown how this technique considerably enhances the observable structure within the d-band region. Through comparisons both among the present spectra, and with previous studies, a full assignment is made of all the component bands within the d-region. It is concluded that it is the substrate d xz , yz electrons which are chiefly involved in the adsorbate bonding.

The Determination of Adsorbate-Substrate Bonding via UPS

Understanding the nature of surface chemical bonding remains one of the central questions of chemisorption, and one which only UPS can answer. Some of the major difficulties in interpreting the adsorbate UP spectrum are: the lack of a comparative gas-phase UP spectrum for a dissociative adsorbate species; clearly identifying the adsorbate-induced changes to the metal substrate (d-band) spectrum; the absence of a well established theoretical framework within which to interpret the results. In tackling these difficulties we have attempted to divide the problem into successive stages: formulate a theoretical model for the dissociative adsorbate species, and thereby identify its ‘purely molecular’ bands; locate and then identify the lower-BE adsorbate-induced bands; calculate the appropriate symmetry-adapted substrate surface orbitals; formulate a qualitative surface-molecule adsoibate-substrate bonding model. Some techniques which have been of particular value in these procedures are: HF-MO calculations; the use of fractional-difference spectra; the choice of substrate (surface); polarization-dependent ARUPS. Each of these points is illustrated by application to methanethiol (CH3SH) adsorbed on Cu and Al.

Surface science lettersComment on “on the energy dependence of the angular distribution of photoelectrons from oriented molecules” by T. Gustafsson

Abstract A misunderstanding concerning the relationship between gas-phase photoelectron angular distributions and those of oriented (absorbed) molecules is clarified. The correct procedure for relating the two is outlined.