摘要:

Auger photoelectron coincidence spectroscopy has been used to study theM4,5VVAuger spectra of GaAs(110). Using this technique, the Ga and As spectra can be separated and studied independently. The line shape of the As‐M4,5VVmeasured in coincidence with the As 3dphotoemission line differs significantly from the conventional Auger spectrum. This is attributed to the surface electronic properties of the system. In addition, it is found that thesscomponent of the As spectrum is more intense than expected based on calculations using atomic matrix elements. The Ga‐M4,5VVspectrum, of which only theppcomponent is observed, agrees well with that expected from an independent electron model. A first principles electronic structure calculation of a five‐layer GaAs(110) slab has been performed to aid in the interpretation of the Auger spectra.

ISSN:0734-2101    

DOI:10.1116/1.577542

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The interaction of ammonia with clean and CO‐covered Re(0001) surface was investigated, utilizing optical second harmonic generation (SHG), temperature programmed desorption (TPD), and work function change (Δφ) techniques. The surface coverage of ammonia was continuously monitored during both isothermal desorption and equilibrium measurements as a function of crystal temperature and ambient NH3pressure. A fast decrease of the activation energy for desorption from 21±1 to 9±1 kcal/mol as coverage increases from 0.2 to 0.9 of the saturation coverage was found. This indicates the existence of very strong repulsive interactions among adsorbed ammonia molecules. The pre‐exponent factor was also determined as a function of coverage, for the first time for any ammonia–metal system. Its fast decrease with increasing coverage, reveals an example for a compensation effect in desorption. The coadsorption system of CO and NH3was also studied. It was found that while ammonia effectively blocks the adsorption of CO on Re(0001) when it is preadsorbed, the opposite order of adsorption, namely preadsorption of CO, hardly affects the adsorption–desorption kinetics of NH3. It was also found that decrease of work function caused by ammonia of 2.4 eV at saturation coverage, shifts upward exactly by the increase of Δφ by CO on the clean surface if the latter is preadsorbed. These minor effects of CO if preadsorbed, are in sharp contrast to the dramatic decrease of the second harmonic (SH) response due to ammonia on Re(0001), if only small amounts of CO are present on the surface. It is suggested that submonolayer coverage of CO are enough to remove any electronic interaction which otherwise leads to a strong SH response. A large SHG quenching cross section of CO is introduced, implying a nonlocal effect of CO on the adsorbed NH3on Re(0001). If ammonia is preadsorbed, much of the quenching effect due to the coadsorbed CO is removed. This is discussed in terms of NH3islands formation.

ISSN:0734-2101    

DOI:10.1116/1.577543

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We presentN7VV(4f7/25d5d) Auger spectra from TaC(111) taken in time coincidence with photoemitted bulk and surface‐shifted 4f7/2core‐level electrons, and compare these data with self‐convolutions of calculated valence bandddensities of states (SCDOS) for the surface (first) and bulk (exponential sum of second, fourth, and sixth subsurface) atomic layers of a 13‐layer slab. The Ta 4flevels of TaC(111) and Ta(100) both exhibit large surface core level shifts, but of opposite sign. Therefore, we compare the TaC(111) data and calculations with analogousN7VVAuger coincidence spectra and calculated SCDOS that we have previously published for Ta(100). The bulk Ta(100) and surface TaC(111) spectra show favorable agreement with the calculations; the surface Ta(100) and bulk TaC(111) spectra, on the other hand, exhibit marked disagreement with the calculations. These discrepancies are consistent with and provide added support for the presence of our proposed1surface‐to‐bulk (bulk‐to‐surface) interatomic Auger decay mode which adds a bulk (surface) contribution to the surface (bulk) Auger line shape of Ta(100) (TaC(111)). These conclusions are supported by comparison of our data to incoherent sums of bulk and surface SCDOS.

ISSN:0734-2101    

DOI:10.1116/1.577544

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The field of surface magnetism is defined and a variety of contemporary issues are outlined and illustrated using examples predominantly from magneto‐optic Kerr‐effect studies. The issues involve the existence of magnetism in two dimensions (2D), trends in ordering temperatures with film thickness, the magnitude of the magnetic moments, the surface magnetic anisotropy, the 2D magnetic phase transition and critical behavior, and the existence of new and altered phases stabilized via epitaxy.

ISSN:0734-2101    

DOI:10.1116/1.577545

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The electronic excitations of Cr3+magnetic moments at an oxidized Cr(100) surface are studied by spin‐polarized electron energy loss spectroscopy. Sharp loss features are observed due to excitations from the spin‐quartet ground state to doublet states via exchange scattering (predominantly spin flip). The dipole allowed transitions (nonflip) are broadened by crystal field effects. The characteristic Cr3+excitations are observed from the very early stages of oxygen adsorption.

ISSN:0734-2101    

DOI:10.1116/1.577546

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Spin‐polarized electron energy‐loss spectroscopy has been used to investigate the probing depth of low energy (∼30 eV) electrons at metal surfaces and to study the growth and morphology of monolayer‐level metal films. Studies of the Cu(100)/Mo system show that the probing depth in molybdenum is small, ∼1 monolayer, and that molybdenum grows layer‐by‐layer on Cu(100). Data for the Cu(100)/Fe system are also reported that suggest that iron deposition on a room‐temperature Cu(100) substrate does not initially result in simple layer‐by‐layer growth.

ISSN:0734-2101    

DOI:10.1116/1.577547

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A molecular dynamics simulation of Pb(110), using embedded atom method energy functional and two‐body potential derived for the purpose, shows that near 400 K the three surface layers begin to disorder well before the bulk (interior) layers. Transfer of atoms from the first subsurface layer to the surface layer occurs. Disordering propagates beyond the top three layers above 550 K, accompanied by formation of an adlayer and vacancies in the top three layers. Behavior of the two dimensional layer structure factors indicates that disordering is anisotropic. Simulation results are consistent with experimental observations of surface roughening near 400 K and more extensive surface ordering above 525 K. Results are consistent with simulations for Ni and Al.

ISSN:0734-2101    

DOI:10.1116/1.577548

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Yb silicides have been grown epitaxially on the Si(111) surface using solid‐state epitaxy with annealing to 400 ° C. The amount of deposited Yb was varied from parts of a monolayer to films being about 40 monolayer thick. Auger‐ and photoelectron spectroscopy showed that Si segregates into the Yb overlayer at room temperature and that a strong reaction occurs during annealing to 400 ° C leading to the formation of thick silicides. The epitaxial silicides showed a 1×1 or a √3×√3 low‐energy electron diffraction pattern depending on the detailed preparation procedure. The composition of the silicide was found to be YbSi2−x.

ISSN:0734-2101    

DOI:10.1116/1.577549

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The 2×2 reconstruction of indium on the Si(100) 2×1 surface has been studied using scanning tunneling microscopy. For depositions below 150 °C, In lines up in rows perpendicular to the underlying Si dimer rows, with a two unit cell periodicity along each In row. In the 2×2 structure, these rows are spaced two unit cells apart. We investigate the evolution of the In(2×2) reconstruction from isolated rows at lowest coverage, to islands of 2×3 and 2×2 structure, to a surface entirely covered by 2×2 at 0.5 monolayers of In. The role of step edges in nucleating areas of In growth is also examined.

ISSN:0734-2101    

DOI:10.1116/1.577434

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Surface extended x‐ray adsorption fine structure (SEXAFS) has been used to investigate the structure of Sb on the Si(001) 2×1 surface. The coverage of Sb which remains after annealing thick layers at 375 °C, previously reported to be one monolayer (ML), is found in this work to form a disordered overlayer with three dimensional Sb clusters. This finding is concluded from the SbL3absorption spectra which are similar for this coverage to that of bulk Sb. After a 550 °C anneal, Auger electron spectroscopy, and scanning tunneling microscopy (STM) show that about 1 ML of Sb remains. Phase and amplitude analysis of the SbL3edge SEXAFS shows that the remaining Sb atoms occupy a modified bridge site with a Si–Sb bond length of 2.63±0.04 Å. The Sb atoms form dimers with a Sb–Sb bond length of 2.91±0.04 Å, which is almost identical to the bulk Sb–Sb bond length of 2.90 Å. The Sb atoms lie 1.74±0.06 Å above the Si(001) surface. STM confirms the dimer structure of the Sb overlayer.

ISSN:0734-2101    

DOI:10.1116/1.577433

出版商:American Vacuum Society    

年代:1991

数据来源: AIP