A.M. Bradshaw

SMART: a planned ultrahigh-resolution spectromicroscope for BESSY II

Abstract A new UHV spectromicroscope called SMART (spectromicroscope for all relevant techniques) is currently under construction for a soft X-ray undulator beamline at BESSY II. The instrument consists of a plane-grating monochromator with an aspherical focusing mirror and an ultrahigh-resolution, low-energy electron microscope containing an energy filter. It can be used as a photoemission microscope for a variety of electron spectroscopies (XAS, XPS, UPS, XAES) and has a calculated spatial resolution of better than 1 nm. A maximum energy resolution of about 0.1 eV will be provided by a corrected omega filter. The high lateral resolution of the electron microscope will be achieved through the correction of the chromatic and spherical aberrations of the objective lens by means of an electrostatic mirror in combination with a corrected magnetic beam separator. An additional electron source placed on the other side of the beam separator opposite the electrostatic mirror will also allow LEEM, MEM and small-spot LEED investigations to be carried out. The basic ideas, the various modes of operation and the electron optical design of the instrument are outlined.

The adsorption and incorporation of oxygen on Cu(100) at T ⩾ 300 K

Abstract The initial uptake of oxygen on Cu(100) results in the consecutive formation of (√2 × √2)R45° and (√2 × 2√2)R45° surface structures. The appearance of the extra LEED spots indicative of these structures is accelerated by increasing the crystal temperature. The formation of the (√2 × √2)R45° structure is accompanied by an increase in the work function, which then decreases again during the formation of the (√2 × 2√2)R45° structure. The work function decrease follows to a good approximation first order kinetics and possesses a small activation energy of ~4 kcal mol −1 . The appearance of the (√2 × 2√2)R45° structure and the concomitant reversal in direction of the work function change are evidence for a reconstruction or incorporation process, which appears not to be associated with oxide formation.

Structure determination of ammonia on Cu(110) — a low-symmetry adsorption site

Abstract The local adsorption structure of ammonia on Cu(110) has been determined in a quantitative fashion using N 1s scanned-energy mode photoelectron diffraction. While inversion of the photoelectron diffraction spectra using a direct method indicates that the adsorbed NH 3 molecules are near to atop sites, a fuller multiple scattering analysis shows that the molecule is actually displaced 0.37 A off the atop site in a 〈100〉 azimuth. The result is ascribed to adsorbate-adsorbate interactions (steric hindrance) similar to those found in (2 × 1)ue5f8CO (pmg) structures seen on several fcc (110) surfaces, although in the case of ammonia, it occurs at coverages well below saturation, implying that adsorbate-adsorbate attraction also occurs. These general conclusions are entirely consistent with those of a recent ESDIAD study of this system.

The interaction of oxygen with aluminium single crystal surfaces: Mainly Δφ aspects

Abstract The initial uptake of oxygen by aluminium (100), (110) and (111) surfaces has been studied with several experimental techniques, including photoelectron, Auger and electron loss spectrscopies as well as by measuring the changes in work function (Δφ) and ellipsometric parameters. This paper is concerned mainly with the Δφ results, where considerable differences between the free surfaces were observed. The results from the (111) surface are correlated with recent core level photoelectron spectra and lead to an incorporation/chemisorption interpretation for the initial interaction.

The interaction of oxygen with aluminium (111)

Abstract The initial uptake of oxygen by an aluminium (111) surface at 300 K has been studied by several experimental techniques including XPS, AES, Δφ, ellipsometry and surface plasmon spectroscopy. The interaction falls into two stages. Oxygen exposure up to 30–50 L in the first stage results in a chemisorbed layer with oxygen atoms located in the immediate surface region. In the second stage increased exposure produces a thin, relatively homogeneous oxide film.

The effect of a resonance on vibrational structure in the photoelectron spectrum of acetylene

Abstract A minimum in the partial photoionisation cross section for the 2 Π u state of C 2 H 2 at hv ≈ 14 eV is accompanied by a significant departure of the vibrational intensities from the Franck-Condon distribution. Using an explicit formula for resonant photoemission we show with model calculations that a resonance ≈3 eV above the first ionisation potential is responsible for this effect.

Vibrational excitation in C 1s and O 1s photoionization of CO

The C 1s and O 1s photoelectron spectra of the CO molecule have been measured with high resolution. The vibrational spacing of the state is significantly lower than in the state and in the molecular ground state. Compared to the ground-state equilibrium C - O distance of 1.128 A, we extract from the analysis of the Franck - Condon factors bond lengths of 1.079(2) A and 1.167(4) A for the C 1s and O 1s core-ionized states, respectively.

Plasmon coupling to core hole excitations in carbon

The results from soft X-ray appearance potential spectroscopy (SXAPS), photon-induced Auger appearance spectroscopy h(cross) omega -APS), X-ray photoelectron spectroscopy (XPS) and ionization loss investigations on the carbon K level in graphite are compared. It is shown that plasmon excitation accompanies core hole creation and that the coupling in XPS is much smaller than in SXAPS, in sharp contrast to the situation pertaining in the simple metals. Plasmon coupling probably takes place in the case of h(cross) omega -APS and of ionization loss, but it is difficult to assess its magnitude. The transition probabilities for the various experiments are calculated within the framework of a simple model. The coupling constants for core and conduction electrons are different in graphite, leading to the observed anomalous behaviour.

One-electron versus multielectron effects in the near-threshold C 1s photoionization of acetylene

The C 1s partial photoionization cross section and asymmetry parameter β of the C2H2 molecule have been determined with high-energy resolution between threshold and 360 eV. The C 1s shake-up satellite spectrum is richly structured; the cross section of several satellites increases strongly near threshold, which indicates that there is a conjugate contribution to their intensity. It is shown that the large enhancement in the total photoabsorption cross section between 305 and 320 eV, previously attributed to a σ* shape resonance, is largely due to this photon energy dependence of the shake-up transitions.

Vibronic coupling in the K shell excitation of ethyne

The C 1s−π∗ band in the high-resolution photoabsorption spectra of the ethyne isotopomers C2H2, C2D2 and C2HD is found to be dominated by vibronic coupling effects. There is quantitative agreement between experiment and a recent theoretical model which treats vibronic coupling in both the core and valence orbital spaces. The theoretical analysis shows that a strong excitation of the bending modes dominates the observed spectra.