Steven L. Hulbert

X‐ray spectromicroscopy with a zone plate generated microprobe

The scanning photoelectron microscope at the National Synchrotron Light Source (NSLS) has recently recorded micrographs with a resolution below half a micron. To demonstrate elemental and chemical sensitivity at the submicron level, an artificial structure consisting of Al and SiO2 lines on a boron‐doped silicon substrate was examined. Al 2p and Si 2p primary photoelectrons as well as O KVV Auger electrons were used for image formation. Contrast reversal between the the Si and SiO2 areas was observed in images formed from Si 2p and oxide‐shifted Si 2p photoelectrons. The soft x‐ray undulator at the NSLS provides coherent illumination of a zone plate to produce the microprobe. The sample is mechanically scanned through the beam allowing the formation of images from photoelectrons detected by a single‐pass cylindrical mirror analyzer, or a more complete spectroscopic examination of a selected area of the sample.

Influence of substrate temperature on epitaxial copper phthalocyanines studied by photoemission spectroscopy

We report the formation of heteroepitaxial copper phthalocyanine (CuPc) overlayers on a “5×20” reconstructed Au(001) substrate, with thicknesses ranging from 0.8 to 26.9 A. The adsorbed CuPc at room temperature forms a quasiepitaxial overlayer, incommensurate with the substrate lattice, aligned with the 〈110〉 and 〈110〉 axes of the Au(001) surface. At elevated substrate temperatures (ET) between 200 and 250 °C, the deposition of CuPc results in a 13.8 A square overlayer structure commensurate with the substrate lattice. The electronic structures of CuPc overlayers grown at both RT and ET are also investigated. While the films grown at both temperatures generally exhibit lowering of the vacuum level, consistent with the presence of an interfacial dipole, it is the ET-grown CuPc overlayer that exhibits a sharp decrease of the surface vacuum level upon completion of a monolayer. The experimental CuPc valence band energies are compared with the calculated ionization potentials of a CuPc molecule. Using the tu...

Auger photoelectron coincidence spectroscopy using synchrotron radiation

The technique of Auger photoelectron coincidence spectroscopy (APECS) using synchrotron radiation is discussed. Technical considerations and experimental details are emphasized. Results from Cu(100), Ta(100), and Al(111) are presented to show the kinds of new information that APECS can provide.

Interaction of metallophthalocyanines (MPc,M=Co,Ni) on Au(001): Ultraviolet photoemission spectroscopy and low energy electron diffraction study

Thin films of metallophthalocyanine (MPc,M=Co,Ni) evaporated onto a “5×20” reconstructed Au(001) substrate at room temperature have been investigated by employing low energy electron diffraction (LEED) and ultraviolet photoelectron spectroscopy (UPS). The LEED images from NiPc thin films show that the overlayers are highly ordered with a square unit cell of 12.8×12.8A2 aligned along the ⟨110⟩ and ⟨11¯0⟩ axes of the Au(001) substrate. For CoPc, the LEED pattern reveals the superposition of multiple rotationally equivalent domains of a 12.9×12.9A2 square lattice which are rotated by 16° with respect to each other. The contrast between NiPc and CoPc on Au(001) is further demonstrated in the interfacial electronic structure. UPS studies of the interfacial layers of NiPc deposited on the hexagonally reconstructed gold substrate indicate that NiPc physisorbs on the gold surface as evidenced by a uniform molecular orbital (MO) shift. The CoPc MO’s, on the other hand, indicates a charge transfer at the interface,...

Illumination for coherent soft X-ray applications: the new X1A beamline at the NSLS.

The X1A soft X-ray undulator beamline at the NSLS has been rebuilt to serve two microscopy stations operating simultaneously. Separate spherical-grating monochromators provide the resolving power required for XANES spectroscopy at the C, N and O absorption edges. The exit slits are fixed and define the coherent source for the experiments. The optical design and the operational performance are described.

Spin‐polarized photoemission spectroscopy of magnetic surfaces using undulator radiation

A beamline has been established at the National Synchrotron Light Source to perform angle‐resolved photoemission experiments on magnetic surfaces with spin sensitivity. The system has two novel features: it uses a miniature electron‐spin polarization analyzer and it also uses synchrotron radiation from an undulator rather than a bending magnet.

In situ reactive glow discharge cleaning of x‐ray optical surfaces

We report the first implementation of an in situ glow discharge cleaning procedure for the removal of carbon contamination from x‐ray optical surfaces. In situ cleaning obviates the need for the time‐consuming and costly process of element removal, cleaning or repolishing, remounting, and realignment usually necessary to restore x‐ray optics to their original condition. The apparatus required is quite simple and can easily be fitted to most existing UHV (ultrahigh vacuum) mirror boxes or monochromators. Results with several monochromators at the Brookhaven National Labs National Synchrotron Light Source (NSLS) show that better than original performance can be obtained at the carbon K edge, since the elements are not exposed to atmosphere after in situ cleaning and do not accumulate the adventitious carbon layer associated with more traditional methods. Possible improvements and extensions of the technique are also described.

Normal incidence grating spectrometer designed for inverse photoemission studies in the range 10–30 eV

We describe a normal incidence grating spectrometer for use in inverse photoemission studies of surfaces. We show that by operating in a configuration where the source and image are displaced from the Rowland Circle it is possible to construct a useful spectrometer that can be attached to any existing surface science chamber. The device covers the range 10–30 eV in photon energy with a typical energy resolution of 150 meV. Capable of operating with relatively low incident currents, this instrument will be an excellent choice for spin‐polarized inverse photoemission experiments.

X1A: Second-generation undulator beamlines serving soft x-ray spectromicroscopy experiments at the NSLS

The X1A undulator beamline is being rebuilt with two separate monochromators on its two branches. The new arrangement will deliver spatially coherent beams to imaging experiments, with spectral resolving power of up to 5000, and the capability to optimize the resolving power versus flux. The beamlines will operate simultaneously, and each will use 15 percent of the undulator beam, yet deliver high coherent flux. Because of the small beam divergence, the spherical grating monochromators can operate with fixed exit arms throughout the 250–800 eV range.

Auger photoelectron coincidence spectroscopy using synchrontron radiation

The technique of Auger-photoelectron coincidence spectroscopy (APECS) is described and illustrated with a case study of the Cu(100) 3p and M23VV spectra. APECS offers many advantages over the conventional singles spectroscopy such as isolating overlapping spectral features, reducing secondary electron background, and revealing new decay modes. In the coincidence Cu Auger spectra discussed here, the multiplet structure of the quasi-atomic 3d8 Auger final state is clearly observed, as well as different intensities for the multiplet components for the p12 and p32 transitions. Furthermore, the spectra reveal evidence for a Coster-Kronig decay channel for 3p12 core holes, and illustrate that the sum of the Auger electron and photoelectron kinetic energies is conserved. Possible technical improvements that can increase the counting efficiency are also discussed.