D.R. Lloyd

Selection rules in photoemission from adsorbates: Pyridine adsorbed on copper

Abstract We have investigated the adsorption of pyridine on a Cu(110) single crystal face at room temperature using angular-resolved photoemission. By comparison with the emission from adsorbed benzene and application of the selection rules which govern adsorbate photoemission, it has been possible to show that the interaction takes place through the nitrogen lone-pair rather than the π-electrons of the aromatic ring. The differential bonding shift in the lone-pair orbital is ca. 0.9 eV.

Compressed benzene on Ni(110)

Abstract The adsorption of benzene on Ni(110) provides an example of benzene in a significantly crowded surface configuration. This is shown from studies with angle resolved photoemission, near-edge X ray absorption fine structure, LEED and thermal desorption spectroscopy. The benzene molecules are incorporated in a well-ordered, densely packed c(4 × 2) overlayer with preferential azimuthal orientation. It is suggested that compressive intermolecular interaction induces C 2v symmetry of the flat lying molecules. The high packing density in the overlayer leads to a considerable orbital overlap, which is reflected in a large C-H band dispersion of 0.75 eV.

Adsorption of CN on Pd(110): A study with angle-resolved ultraviolet photoelectron spectroscopy and LEED

Abstract Saturation adsorption of C 2 N 2 on Pd(110) at room temperature gives a partially ordered CN layer; heating to 150° C gives a sharp c(2 × 2) LEED pattern with spot splitting, heating to 425° C converts this to a (2 × 3) pattern with missing spots. Possible structures are proposed. Analysis of ARUPS data shows that all the four expected ionisations for CN are present, and application of selection rules with polarised synchrotron radiation allows an assignment of these bands. The CN groups are oriented along [1−10], parallel to the surface.

Cyanogen adsorption on Pd(110) at low temperatures: A study with angle-resolved photoemission, leed and thermal desorption

Abstract The adsorption of cyanogen, C 2 N 2 , on Pd(110) has been studied as a function of temperature using thermal desorption spectroscopy (TDS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), LEED and work function measurements. At temperatures below 100 K multilayers are formed; flashing of these to successively higher temperatures gives several different states of the surface. At 200 K a state which shows a c(2 × 2) LEED pattern and which is assigned as a monolayer of parallel-bonded C 2 N 2 molecules is obtained. The molecules are predominantly aligned perpendicular to the close-packed rows of Pd atoms, i.e. along [001]. Six valence orbital ionisations are observed and assigned; these assignments indicate strong π interactions with the surface. Above 200 K dissociation to CN occurs. Desorption between 100 and 200 K is complex; there is some evidence which suggests a tilted molecular species here.

The electronic structure of the high temperature phase of ethylene adsorbed on Pd(111)

Abstract The room temperature phase of ethylene on Pd(111), previously assigned as ethylidyne CH 3 −C, has been studied by angle-resolved ultraviolet photoelectron spectroscopy. For the first time all the C 2p derived levels of ethylidyne on a surface have been resolved and there is good correspondence with the levels of ethylidyne in a metal complex; an adsorbate-induced feature of the metal is also observed. The ethylidyne species is slightly less thermally stable than on Pt(111).

Angular resolved photoemission studies of pyridine on Pt(111) — evidence for an α-pyridyl species

HeI angular resolved photoelectron spectra (ARUPS) of pyridine adsorption at room temperature on Pt(111) are very similar to those previously reported for adsorption on Ir(111) but are very different from those for adsorption on Cu(110). The angular variations suggest a ring orientation approximately perpendicular to the metal surfaces in all three systems, and it is argued that this information, supported by independent NEXAFS results for the Cu(110) and Pt(111) systems, is best interpreted in terms of the adsorbed species on both Pt(111) and Ir(111) being α -pyridyl. An interpretation of ESDIAD results for the Ir(111) system is proposed.

H2S adsorption on the (110) surfaces of III–V semiconductors

Abstract The adsorption of H 2 S on cleaved (110) surfaces of InP, GaP and GaAs has been studied by core level soft X-ray photoelectron spectroscopy (SXPS) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. H 2 S adsorption occurs at room temperature on InP and GaP. At low exposures (up to 5 L) the surface component of the cation spectra (In 4d, Ga 3d) is strongly reduced while that of the P 2p core level spectra is replaced by a component at higher binding energy. The S 2p spectrum shows a single component and the valence band spectra show sharp new features. This adsorbate can be removed by gentle annealing. Adsorption on GaAs does not occur at room temperature, but low temperature (200 K) adsorption yields SXPS and ARUPS spectra which indicate that this low exposure adsorption process is essentially the same for all three substrates. A model of dissociative adsorption consistent with these photoelectron data is presented.

Adsorbate band structure of bromine on Pd(111) studied by angle resolved ultraviolet photoemission

The photoemission spectra from Pd(111) and from the Pd(111)-Br(√3 × √3)R30° system are reported; some new features for the clean surface are detected and assigned. The principal effect of the Br overlayer on the direct transitions is a general intensity reduction. Three adsorbate derived features are detected; one at 4 eV with no dispersion is probably an adsorbate-induced feature of the metal, and the other two which disperse are assigned as Br 4px, (4.5 eV) and Br 4pz (6 eV) at \gG.

An angle resolved UPS study of benzene adsorption on Pt(111)

Abstract The results of an ARUPS study of benzene adsorption on Pt(111) at 300 K are presented. It is concluded that benzene is adsorbed on this surface with the ring parallel to the surface, and that the local symmetry of the adsorbed benzene is best described as C Зυ (σ d ). The n MOs of benzene are stabilized by ∼1.7 eV with respect to the σ M.O.S. on ehemisorption. These results are compared briefly with NEXAFS, HREELS and FT-i.r. data for benzene on this and other surfaces. The strength of the metal-benzene interaction on this surface appears to be greater than on Ni(1 11), Pd(1 1 1) or Rh(111) but less than on Ir(1 11), Os(0001) or Re(0001).

Angle-Resolved Ups Studies of Cn on Cu(110) and Cu(100)

Cyanogen, (CN) 2 , and HCN both react on the Cu(110) surface to give very similar photoemission spectra. HCN does not react with Cu(100), but reacts with an oxygen pre-covered surface to give spectra which are very similar to those from the reaction product of C 2 N 2 with Cu(100). The spectra on the two surfaces have substantial intensity differences, particularly at normal emission, but can all be assigned to CN. The spectra from the Cu(110)/CN system are interpreted as indicating that the CN axis is approximately parallel to the surface and to the [110] direction. On the Cu(100) surface and probably also on an oxygen pre-covered Cu(110) surface the CN axis appears to lie at an angle to the surface.