Charles Bahr | Alcatel-lucent Bell Labs (original) (raw)
Papers by Charles Bahr
Chem Phys, 1995
We describe time-resolved modulated molecular beam measurements of the adsorption of NO2 on the G... more We describe time-resolved modulated molecular beam measurements of the adsorption of NO2 on the GaAs(110) surface. These measurements provide a quantitative description of the gas-surface interaction; thermal NO2 has a nearly unity sticking probability, dissociating to form NO (which quickly desorbs) and a surface oxide of ˜1/3 ML saturation coverage. The NO2 dissociation probability is strongly controlled by the defect density, the oxygen coverage, and the surface temperature. We have characterized the competition between the processes leading to oxygen deposition, i.e., desorption and diffusion of NO 2 to reactive defects. The barrier energies Ediff=6±2 kcal/mol and Edes=9±2 kcal/mol provide an interesting example of the transition from defect-dominated surface chemistry, typical of low Miller index metal surfaces to defect- insensitive diffusion-limited chemistry, which we argue is generic to corrugated semiconductor surfaces.
Physical Review A
The energies and intensities of the 2 /sup 3/P/sub 1/..-->..1 /sup 1/S/sub 0/, and 2 /sup 1/P/... more The energies and intensities of the 2 /sup 3/P/sub 1/..-->..1 /sup 1/S/sub 0/, and 2 /sup 1/P/sub 1/..-->..1 /sup 1/S/sub 0/ transitions in He-like sulfur ions, and of the 2/sup 2/P..-->..1/sup 2/S transition in H-like sulfur ions have been studied as a function of the thickness and electron density of the solid through which the ions travel. The thickness dependence of the x-ray intensities was analyzed in terms of a three-component model description of K-shell vacancy production and decay. Cross sections for electron excitation or ionization and capture deduced from this analysis were used to establish the energies of the x-ray peaks for complete emission in vacuum (i.e., outside the target). Energy shifts were obtained by comparing the peak energies for emission in thick targets to those for emission in vacuum. The results show that the energy shifts increase approximately linearly with the square root of the valence electron density of the target and are in good agreeme...
Spectra of K..cap alpha.. x rays from 64-MeV sulfur ions incident on a wide range of thick elemen... more Spectra of K..cap alpha.. x rays from 64-MeV sulfur ions incident on a wide range of thick elemental targets have been measured with a plane crystal spectrometer. Various features of the spectra were found to be sensitive to the atomic structure of the targets. In particular, the K..cap alpha.. satellite and hypersatellite intensity distributions displayed oscillatory variations approx. 14% as a function of target atomic number. This behavior is satisfactorily accounted for by the systematics of electron-capture cross sections. Considerable line broadening was observed, and the Lorentzian component of the total linewidth was found to increase linearly with the product of the target atomic number and the atomic concentration.
Thesis (Ph. D. in Chemistry)--University of California, Berkeley, May 1986. Includes bibliographi... more Thesis (Ph. D. in Chemistry)--University of California, Berkeley, May 1986. Includes bibliographical references (leaf 164).
We describe a method of surface structure determination based on oscillations in core-level photo... more We describe a method of surface structure determination based on oscillations in core-level photoemission intensity - Angle-Resolved Photoemission Extended Fine Structure - with particular emphasis on the use of Fourier transformation. Qualitative comparisons of Fourier power spectra reveal adsorption sites and shortcomings in theoretical calculations; quantitative backtransformation analysis allows accurate bond lengths and bond angles to be determined. Examples are drawn from these similar atomic adsorption systems: c(2x2)S/Ni(100), p(2x2)S/Cu(100) and c(2x2)S/Ni(110).
Nuclear Instruments and Methods in Physics Research, 1983
A new high intensity beam line with a wiggler magnet source is described. This project, in final ... more A new high intensity beam line with a wiggler magnet source is described. This project, in final stages of design, is a joint effort between Lawrence
Phys. Rev. A, 1980
ABSTRACT Spectra of Kα x rays from 64-MeV sulfur ions incident on a wide range of thick elemental... more ABSTRACT Spectra of Kα x rays from 64-MeV sulfur ions incident on a wide range of thick elemental targets have been measured with a plane crystal spectrometer. Various features of the spectra were found to be sensitive to the atomic structure of the targets. In particular, the Kα satellite and hypersatellite intensity distributions displayed oscillatory variations ∼ 14% as a function of target atomic number. This behavior is satisfactorily accounted for by the systematics of electron-capture cross sections. Considerable line broadening was observed, and the Lorentzian component of the total linewidth was found to increase linearly with the product of the target atomic number and the atomic concentration.
Nuclear Instruments and Methods in Physics Research, 1983
Abstract The performance of a number of monochromator crystals for the energy region 550–5000 eV ... more Abstract The performance of a number of monochromator crystals for the energy region 550–5000 eV has been tested with regard to the resolution, the intensity, the sensitivity to radiation damage, and the contribution of higher orders.
Phys. Rev. Lett., 1983
... Determination with Photoelectron Diffraction JJ Barton, CC Bahr, Z. Hussain, (a) SW Robey, JG... more ... Determination with Photoelectron Diffraction JJ Barton, CC Bahr, Z. Hussain, (a) SW Robey, JG Tobin, LE Klebanoff, and DA Shirley Materials and ... ranges,5 and ex-perimental curves over short ranges,6 were Fourier analyzed to yield peaks at distances close to adsorbate ...
The Structure of Surfaces, 1985
Phys. Rev. A, 1985
Synchrotron radiation was directed at neon gas, creating Ne/sup +/ ions in ''sa... more Synchrotron radiation was directed at neon gas, creating Ne/sup +/ ions in ''satellite'' states characterized by the configurations 1s²2s²2pâ´nl, in addition to the main-line final states: 2sâ»Â¹ and 2pâ»Â¹. Satellite features appearing in the photoelectron spectrum were studied in the near-threshold region with photon energies from 55.2 to 99.7 eV. For three of the satellite peaks, the angular-distribution-asymmetry parameter ..beta../sub i/ varies with kinetic energy in much the same way as the asymmetry parameter of the 2p line. None of the satellites have a ..beta../sub i/ like that of the 2s line. All the satellite partial cross sections sigma/sub i/ have a kinetic-energy dependence similar to the partial cross section of the 2p main line. However, the sigma/sub i/ curves are not identical, and some deviation from the 2p intensity behavior is observed. These ..beta../sub i/ and sigma/sub i/ results are used to confirm the previous assignments of the satellite final states in neon and to consider, in general, the energy-dependent behavior of satellites near threshold.
Phys. Rev. B, 1986
... J. J.Barton, CC Bahr, S.%.Robey, Z. Hussain ... Steps on the crystal surface, impurities, or ... more ... J. J.Barton, CC Bahr, S.%.Robey, Z. Hussain ... Steps on the crystal surface, impurities, or animproper dose of HzS Atoms will lead to S photoemission notrepresentative of an ordered over-layer; thiswill typically reduce the measured oscillations as they are expressed as a fraction ...
Phys. Rev. B, 1987
... CC Bahr, * JJ Barton, Z. Hussain, ~ S. W. Robey, J. G. Tobin, ~ and D. A. Shirley Materials a... more ... CC Bahr, * JJ Barton, Z. Hussain, ~ S. W. Robey, J. G. Tobin, ~ and D. A. Shirley Materials and Molecular Research Division, Lawrence Berkeley ... This work will address not only the sulfur adsorption site, but also the positions of nearby copper atoms in the first and second layers. ...
Phys. Rev. B, 1987
Measurements of the extended fine structure in the photoemission intensity from the S(1s) core le... more Measurements of the extended fine structure in the photoemission intensity from the S(1s) core level were performed for a (2×2) overlayer of S on Ge(111). This is the first application of angle-resolved photoemission fine structure (ARPEFS) to study an adsorbate on a semiconductor substrate to our knowledge. The adsorption site and local geometry were determined from the ARPEFS with use
Phys. Rev. B, 1990
ABSTRACT The oxidation of the GaAs(110) surface with NO2 has been studied by Fourier-transform in... more ABSTRACT The oxidation of the GaAs(110) surface with NO2 has been studied by Fourier-transform infrared spectroscopy, molecular-beam techniques, and transport measurements. We find that ambient NO2 dissociatively adsorbs with an average low-coverage probability of 0.03 at room temperature and 0.14 at liquid-nitrogen temperature. In contrast, the molecular-sticking probability is greater than 0.5. We have identified the vibrational modes related to oxygen atoms, nitrogen atoms, NO, N2O, and dimerized NO2 adsorbed on the (110) surface. We find significantly different vibrational frequency distributions for oxygen atoms directly deposited on the surface compared to those derived from NO2 dissociative adsorption. Based on a variety of measurements we argue that NO2 dissociation requires steps or defects. Both vibrational spectra and transport properties indicate that a small fraction of the adsorbed oxygen atoms migrates into the bulk of the GaAs crystal, creating an electronic level in the band gap.
Phys. Rev. B, 1990
We report an example of the effects of surface chemistry on bulk electronic properties. We study ... more We report an example of the effects of surface chemistry on bulk electronic properties. We study the dissociative adsorption of NO2 on a high-resistance GaAs(110) crystal surface. We show that the dissociation can be accurately followed with high sensitivity by monitoring the sample conductance. We use infrared spectroscopy, He specular scattering, and Auger spectroscopy, to specify the chemical processes involved.
Chem Phys, 1995
We describe time-resolved modulated molecular beam measurements of the adsorption of NO2 on the G... more We describe time-resolved modulated molecular beam measurements of the adsorption of NO2 on the GaAs(110) surface. These measurements provide a quantitative description of the gas-surface interaction; thermal NO2 has a nearly unity sticking probability, dissociating to form NO (which quickly desorbs) and a surface oxide of ˜1/3 ML saturation coverage. The NO2 dissociation probability is strongly controlled by the defect density, the oxygen coverage, and the surface temperature. We have characterized the competition between the processes leading to oxygen deposition, i.e., desorption and diffusion of NO 2 to reactive defects. The barrier energies Ediff=6±2 kcal/mol and Edes=9±2 kcal/mol provide an interesting example of the transition from defect-dominated surface chemistry, typical of low Miller index metal surfaces to defect- insensitive diffusion-limited chemistry, which we argue is generic to corrugated semiconductor surfaces.
Physical Review A
The energies and intensities of the 2 /sup 3/P/sub 1/..-->..1 /sup 1/S/sub 0/, and 2 /sup 1/P/... more The energies and intensities of the 2 /sup 3/P/sub 1/..-->..1 /sup 1/S/sub 0/, and 2 /sup 1/P/sub 1/..-->..1 /sup 1/S/sub 0/ transitions in He-like sulfur ions, and of the 2/sup 2/P..-->..1/sup 2/S transition in H-like sulfur ions have been studied as a function of the thickness and electron density of the solid through which the ions travel. The thickness dependence of the x-ray intensities was analyzed in terms of a three-component model description of K-shell vacancy production and decay. Cross sections for electron excitation or ionization and capture deduced from this analysis were used to establish the energies of the x-ray peaks for complete emission in vacuum (i.e., outside the target). Energy shifts were obtained by comparing the peak energies for emission in thick targets to those for emission in vacuum. The results show that the energy shifts increase approximately linearly with the square root of the valence electron density of the target and are in good agreeme...
Spectra of K..cap alpha.. x rays from 64-MeV sulfur ions incident on a wide range of thick elemen... more Spectra of K..cap alpha.. x rays from 64-MeV sulfur ions incident on a wide range of thick elemental targets have been measured with a plane crystal spectrometer. Various features of the spectra were found to be sensitive to the atomic structure of the targets. In particular, the K..cap alpha.. satellite and hypersatellite intensity distributions displayed oscillatory variations approx. 14% as a function of target atomic number. This behavior is satisfactorily accounted for by the systematics of electron-capture cross sections. Considerable line broadening was observed, and the Lorentzian component of the total linewidth was found to increase linearly with the product of the target atomic number and the atomic concentration.
Thesis (Ph. D. in Chemistry)--University of California, Berkeley, May 1986. Includes bibliographi... more Thesis (Ph. D. in Chemistry)--University of California, Berkeley, May 1986. Includes bibliographical references (leaf 164).
We describe a method of surface structure determination based on oscillations in core-level photo... more We describe a method of surface structure determination based on oscillations in core-level photoemission intensity - Angle-Resolved Photoemission Extended Fine Structure - with particular emphasis on the use of Fourier transformation. Qualitative comparisons of Fourier power spectra reveal adsorption sites and shortcomings in theoretical calculations; quantitative backtransformation analysis allows accurate bond lengths and bond angles to be determined. Examples are drawn from these similar atomic adsorption systems: c(2x2)S/Ni(100), p(2x2)S/Cu(100) and c(2x2)S/Ni(110).
Nuclear Instruments and Methods in Physics Research, 1983
A new high intensity beam line with a wiggler magnet source is described. This project, in final ... more A new high intensity beam line with a wiggler magnet source is described. This project, in final stages of design, is a joint effort between Lawrence
Phys. Rev. A, 1980
ABSTRACT Spectra of Kα x rays from 64-MeV sulfur ions incident on a wide range of thick elemental... more ABSTRACT Spectra of Kα x rays from 64-MeV sulfur ions incident on a wide range of thick elemental targets have been measured with a plane crystal spectrometer. Various features of the spectra were found to be sensitive to the atomic structure of the targets. In particular, the Kα satellite and hypersatellite intensity distributions displayed oscillatory variations ∼ 14% as a function of target atomic number. This behavior is satisfactorily accounted for by the systematics of electron-capture cross sections. Considerable line broadening was observed, and the Lorentzian component of the total linewidth was found to increase linearly with the product of the target atomic number and the atomic concentration.
Nuclear Instruments and Methods in Physics Research, 1983
Abstract The performance of a number of monochromator crystals for the energy region 550–5000 eV ... more Abstract The performance of a number of monochromator crystals for the energy region 550–5000 eV has been tested with regard to the resolution, the intensity, the sensitivity to radiation damage, and the contribution of higher orders.
Phys. Rev. Lett., 1983
... Determination with Photoelectron Diffraction JJ Barton, CC Bahr, Z. Hussain, (a) SW Robey, JG... more ... Determination with Photoelectron Diffraction JJ Barton, CC Bahr, Z. Hussain, (a) SW Robey, JG Tobin, LE Klebanoff, and DA Shirley Materials and ... ranges,5 and ex-perimental curves over short ranges,6 were Fourier analyzed to yield peaks at distances close to adsorbate ...
The Structure of Surfaces, 1985
Phys. Rev. A, 1985
Synchrotron radiation was directed at neon gas, creating Ne/sup +/ ions in ''sa... more Synchrotron radiation was directed at neon gas, creating Ne/sup +/ ions in ''satellite'' states characterized by the configurations 1s²2s²2pâ´nl, in addition to the main-line final states: 2sâ»Â¹ and 2pâ»Â¹. Satellite features appearing in the photoelectron spectrum were studied in the near-threshold region with photon energies from 55.2 to 99.7 eV. For three of the satellite peaks, the angular-distribution-asymmetry parameter ..beta../sub i/ varies with kinetic energy in much the same way as the asymmetry parameter of the 2p line. None of the satellites have a ..beta../sub i/ like that of the 2s line. All the satellite partial cross sections sigma/sub i/ have a kinetic-energy dependence similar to the partial cross section of the 2p main line. However, the sigma/sub i/ curves are not identical, and some deviation from the 2p intensity behavior is observed. These ..beta../sub i/ and sigma/sub i/ results are used to confirm the previous assignments of the satellite final states in neon and to consider, in general, the energy-dependent behavior of satellites near threshold.
Phys. Rev. B, 1986
... J. J.Barton, CC Bahr, S.%.Robey, Z. Hussain ... Steps on the crystal surface, impurities, or ... more ... J. J.Barton, CC Bahr, S.%.Robey, Z. Hussain ... Steps on the crystal surface, impurities, or animproper dose of HzS Atoms will lead to S photoemission notrepresentative of an ordered over-layer; thiswill typically reduce the measured oscillations as they are expressed as a fraction ...
Phys. Rev. B, 1987
... CC Bahr, * JJ Barton, Z. Hussain, ~ S. W. Robey, J. G. Tobin, ~ and D. A. Shirley Materials a... more ... CC Bahr, * JJ Barton, Z. Hussain, ~ S. W. Robey, J. G. Tobin, ~ and D. A. Shirley Materials and Molecular Research Division, Lawrence Berkeley ... This work will address not only the sulfur adsorption site, but also the positions of nearby copper atoms in the first and second layers. ...
Phys. Rev. B, 1987
Measurements of the extended fine structure in the photoemission intensity from the S(1s) core le... more Measurements of the extended fine structure in the photoemission intensity from the S(1s) core level were performed for a (2×2) overlayer of S on Ge(111). This is the first application of angle-resolved photoemission fine structure (ARPEFS) to study an adsorbate on a semiconductor substrate to our knowledge. The adsorption site and local geometry were determined from the ARPEFS with use
Phys. Rev. B, 1990
ABSTRACT The oxidation of the GaAs(110) surface with NO2 has been studied by Fourier-transform in... more ABSTRACT The oxidation of the GaAs(110) surface with NO2 has been studied by Fourier-transform infrared spectroscopy, molecular-beam techniques, and transport measurements. We find that ambient NO2 dissociatively adsorbs with an average low-coverage probability of 0.03 at room temperature and 0.14 at liquid-nitrogen temperature. In contrast, the molecular-sticking probability is greater than 0.5. We have identified the vibrational modes related to oxygen atoms, nitrogen atoms, NO, N2O, and dimerized NO2 adsorbed on the (110) surface. We find significantly different vibrational frequency distributions for oxygen atoms directly deposited on the surface compared to those derived from NO2 dissociative adsorption. Based on a variety of measurements we argue that NO2 dissociation requires steps or defects. Both vibrational spectra and transport properties indicate that a small fraction of the adsorbed oxygen atoms migrates into the bulk of the GaAs crystal, creating an electronic level in the band gap.
Phys. Rev. B, 1990
We report an example of the effects of surface chemistry on bulk electronic properties. We study ... more We report an example of the effects of surface chemistry on bulk electronic properties. We study the dissociative adsorption of NO2 on a high-resistance GaAs(110) crystal surface. We show that the dissociation can be accurately followed with high sensitivity by monitoring the sample conductance. We use infrared spectroscopy, He specular scattering, and Auger spectroscopy, to specify the chemical processes involved.