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The bond-energy bond-order (LEBO) model of chemisorption
Classified abstracts 553-713 Classified abstracts 553-560
Editor’s
on this page
note
The labelimmediately following the title of each item denotes country of origin of publication, that at the end of each abstract indicates country of origin of work (where known).
I. General vacuum science and engineering 10. VACUUM
SCIENCE AND TECHNOLOGY
10 553. The many faceta of vacuum safety. (USA) M Tom Thomas, Abstract. (Proc 19th Nat Symp Am Vat Sot) J Vnc Sci Tech&, 10 (I), JanjFeb 1973,219. 16. GASES AND SOLIDS 16 554. Studies of adsorption kinetics by means of molecular lkw experiments. (Germany) An UHV-apparatus is described for the study of transient molecular flow of adsorbable gases through a single capillary. The simultaneous recording of different components of a gas mixture is achieved by means of a quadrupole mass-spectrometer. The application of the method is demonstrated for a CH4-CD4 gas mixture. The observed adsorption isotope effects are discussed. B A Gottwald et al, Vakuum-Tech, 22 (l), Feb 1973, 6-9 (in German). 16 555. Carbon monoxide adsorption on Ni(ll0).
(USA) The chemisorotion of CO on Ni(l10) has been studied at temoeratures down to--- 145”C, by means of LEED, Auger and work function measurements, and flash desorption. Since electron impact caused dissociation of the adsorbed molecules, precautions had to be taken to minimize spurious effects. The LEED patterns showed considerable disorder but could be described approximately by the sequence c(2 x 2) + (4 x 2) -+ ~(2 x 1). These structures suggest that both linear and bridged bonding of CO occurs on Ni(llO), and flash desorption revealed two bindings states separated by -0.2 eV. The isosteric heat of adsorption for the lower energy state was found to be 1.1 eV. The maximum work function change was 1.6 eV. T N Taylor and P J E&up, (Proc 19th Nat Symp Am Vat Sot) J Vat Sci Tech&, 10 (l), Jan/Feb 1973,26-30. 16 556. Orbital energy spectra of CO and Hg adsorbed on Ni(100). (USA) Orbital energy spectra of CO and Hg adsorbed individually and coadsorbed on Ni(100) have been determined by ion-neutralization spectroscopy (INS) and ultraviolet photoelectron spectroscopy (UPS) at 45” incidence. In the case of CO, two orbitals derived from the 50 and lr orbitals of free CO are observed 7.8 and 11.l eV below the Fermi level, respectively, by UPS, with INS revealing only the 7.8-eV orbital since 11.l eV is outside its accessible range. In the case of Hg, UPS and INS both reveal orbital peaks 7.8 and 9.7 eV below EF, identified, respectively, with the 5d,lz and 5d,,z states of Hg. The Hg(6s) orbital is not in evidence in the energy spectra obtained by either method. We have determined energy shifts of the observed orbitals with respect to their positions in the free molecule or atom and have discussed these shifts in terms of the effects of bonding and electric charge shifts in the metal-adsorbate complexes. This work permits a fairly detailed intercomoarison of INS and UPS. It also bears on the problem of peak shift‘s due to interference between bulk and surface photoemission and possible perturbation by the probing ion in the case of ion neutralization. G E Becker and H D Hagstrum (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Technol, 10 (l), Jrm/Feb 1973, 31-34. 16 557. The adsorption and decomposition of CO on Pt(ll1). (USA) A cylindrical mirror electron energy analyzer has been used to study the adsorption and decomposition of CO on an initially clean Pt(ll1) surface. The observed rate of adsorption and fractional surface coverage as a function of CO exposure are identical to those observed previously in this laboratory by flash filament adsorption technique. This result provides an absolute calibration of the Auger
and
spectrometry system for carbon and oxygen. Beam-induced decomposition of CO was observed at high incident fluxes. The surface oxygen coverage decreased exponentially to zero during electron bombardment, while surface carbon decreased and then levelled off at a finite value. CO adsorption studies on this carbon contaminated surface showed decreasing CO adsorption rate and decreasing saturation coverage with increasing carbon contamination. The adsorption rate approached zero at high carbon concentration. J M Mart&x and J B Hudson, (Proc 19th Nat Symp Am Vat Sot) J Vat Sci Tech&, 10 (l), JanjFeb 1973, 35-38. 16 558. Time-of-tight analysis of ions and excited neutrals released by ESD of CO on (100) W. (USA) The release of ions and excited neutrals by the interaction of low energy electrons with CO on (100) W is studied by a time-of-flight technique. The flight time distributions, approximate threshold energies, adsorption kinetics, and dependence behavior of the desorbed particle currents upon heating and heavy electron bombardment are presented for the excited neutrals, CO + and 0 + ions in the temperature range 110-600 K. The fact that the binding state producing the excited neutrals is identical with the state releasing CO + ions suggests that the excited neutrals are CO molecules. T G Newsham and D R Sandstrom (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Tecbnol, 10 (l), JanlFeb 1973, 39-42. 16 559. Auger line shape comparison of N and S in two different chemical environments. (USA) Line shapes of Auger electron spectra have been analyzed in the derivative mode dN/dE to obtain evidence of chemisorption bond characteristics of sulphur and nitrogen in two different chemical environments. Sulphur in a monolayer surface structure on nickel, Ni(llO)-c(2 x 2)s is characterized as having a residual d-band and two (molecular oribtal) resonances, as first depicted by ion-neutralization spectroscopy; while sulphur in a cleaved surface (1120) of CdS displays only filled band properties with no resolvable resonance structure. Nitrogen adsorbed on Ni(ll0) is characterized by a residual d-band and two resonances : one at N 12 eV and the other at N 25 eV below the vacuum zero. The analysis is consistent with data published earlier on a characteristic energy-loss description of Ni(llO)-(disordered)N. Nitrogen adsorbed on Si(ll1) appears to have similar resonances at 13 eV and 27 eV below the vacuum zero with no other intervening band structure. E N Sickafos and F Stelnrlsser (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Technof, 10 (l), Jan/Feb 1973,43-46. 16 560. The bond-energy bond-order (LEBO) model of chemisorption. (USA) The bond-energy bond-order (BEBO) model of chemisorption allows an estimate to be made of the interaction energy between a gaseous specie and a solid surface as a function of either bond length or bond order, ie, the length or order of either the gas-surface bond being formed or the bond of the gaseous molecule being broken. The relationship between bond energy and either bond length or bond order is deduced from spectroscopic correlations for gaseous molecules, and a linear relationship between bond energy and bond order is assumed for the surface-adsorbate interaction. The geometry of the surface orbitals is taken to be that predicted by the crystal field model. The model allows a prediction of several relevant quantities in gassurface interactions, namely: (1) binding energies for molecular adsorbed species, (2) binding energies to atomically adsorbed species, (3) activation energies to chemisorption, and (4) activation energies to dissociative chemisorption. The model is illustrated for the adsorption of HZ, CO, NO and O2 on Pt, W and Ni surfaces. W H Weinberg (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Technol, 10 (l), Jan/Feb 1973, 89-94. 335
Editor’s
on this page
note
The labelimmediately following the title of each item denotes country of origin of publication, that at the end of each abstract indicates country of origin of work (where known).
I. General vacuum science and engineering 10. VACUUM
SCIENCE AND TECHNOLOGY
10 553. The many faceta of vacuum safety. (USA) M Tom Thomas, Abstract. (Proc 19th Nat Symp Am Vat Sot) J Vnc Sci Tech&, 10 (I), JanjFeb 1973,219. 16. GASES AND SOLIDS 16 554. Studies of adsorption kinetics by means of molecular lkw experiments. (Germany) An UHV-apparatus is described for the study of transient molecular flow of adsorbable gases through a single capillary. The simultaneous recording of different components of a gas mixture is achieved by means of a quadrupole mass-spectrometer. The application of the method is demonstrated for a CH4-CD4 gas mixture. The observed adsorption isotope effects are discussed. B A Gottwald et al, Vakuum-Tech, 22 (l), Feb 1973, 6-9 (in German). 16 555. Carbon monoxide adsorption on Ni(ll0).
(USA) The chemisorotion of CO on Ni(l10) has been studied at temoeratures down to--- 145”C, by means of LEED, Auger and work function measurements, and flash desorption. Since electron impact caused dissociation of the adsorbed molecules, precautions had to be taken to minimize spurious effects. The LEED patterns showed considerable disorder but could be described approximately by the sequence c(2 x 2) + (4 x 2) -+ ~(2 x 1). These structures suggest that both linear and bridged bonding of CO occurs on Ni(llO), and flash desorption revealed two bindings states separated by -0.2 eV. The isosteric heat of adsorption for the lower energy state was found to be 1.1 eV. The maximum work function change was 1.6 eV. T N Taylor and P J E&up, (Proc 19th Nat Symp Am Vat Sot) J Vat Sci Tech&, 10 (l), Jan/Feb 1973,26-30. 16 556. Orbital energy spectra of CO and Hg adsorbed on Ni(100). (USA) Orbital energy spectra of CO and Hg adsorbed individually and coadsorbed on Ni(100) have been determined by ion-neutralization spectroscopy (INS) and ultraviolet photoelectron spectroscopy (UPS) at 45” incidence. In the case of CO, two orbitals derived from the 50 and lr orbitals of free CO are observed 7.8 and 11.l eV below the Fermi level, respectively, by UPS, with INS revealing only the 7.8-eV orbital since 11.l eV is outside its accessible range. In the case of Hg, UPS and INS both reveal orbital peaks 7.8 and 9.7 eV below EF, identified, respectively, with the 5d,lz and 5d,,z states of Hg. The Hg(6s) orbital is not in evidence in the energy spectra obtained by either method. We have determined energy shifts of the observed orbitals with respect to their positions in the free molecule or atom and have discussed these shifts in terms of the effects of bonding and electric charge shifts in the metal-adsorbate complexes. This work permits a fairly detailed intercomoarison of INS and UPS. It also bears on the problem of peak shift‘s due to interference between bulk and surface photoemission and possible perturbation by the probing ion in the case of ion neutralization. G E Becker and H D Hagstrum (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Technol, 10 (l), Jrm/Feb 1973, 31-34. 16 557. The adsorption and decomposition of CO on Pt(ll1). (USA) A cylindrical mirror electron energy analyzer has been used to study the adsorption and decomposition of CO on an initially clean Pt(ll1) surface. The observed rate of adsorption and fractional surface coverage as a function of CO exposure are identical to those observed previously in this laboratory by flash filament adsorption technique. This result provides an absolute calibration of the Auger
and
spectrometry system for carbon and oxygen. Beam-induced decomposition of CO was observed at high incident fluxes. The surface oxygen coverage decreased exponentially to zero during electron bombardment, while surface carbon decreased and then levelled off at a finite value. CO adsorption studies on this carbon contaminated surface showed decreasing CO adsorption rate and decreasing saturation coverage with increasing carbon contamination. The adsorption rate approached zero at high carbon concentration. J M Mart&x and J B Hudson, (Proc 19th Nat Symp Am Vat Sot) J Vat Sci Tech&, 10 (l), JanjFeb 1973, 35-38. 16 558. Time-of-tight analysis of ions and excited neutrals released by ESD of CO on (100) W. (USA) The release of ions and excited neutrals by the interaction of low energy electrons with CO on (100) W is studied by a time-of-flight technique. The flight time distributions, approximate threshold energies, adsorption kinetics, and dependence behavior of the desorbed particle currents upon heating and heavy electron bombardment are presented for the excited neutrals, CO + and 0 + ions in the temperature range 110-600 K. The fact that the binding state producing the excited neutrals is identical with the state releasing CO + ions suggests that the excited neutrals are CO molecules. T G Newsham and D R Sandstrom (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Tecbnol, 10 (l), JanlFeb 1973, 39-42. 16 559. Auger line shape comparison of N and S in two different chemical environments. (USA) Line shapes of Auger electron spectra have been analyzed in the derivative mode dN/dE to obtain evidence of chemisorption bond characteristics of sulphur and nitrogen in two different chemical environments. Sulphur in a monolayer surface structure on nickel, Ni(llO)-c(2 x 2)s is characterized as having a residual d-band and two (molecular oribtal) resonances, as first depicted by ion-neutralization spectroscopy; while sulphur in a cleaved surface (1120) of CdS displays only filled band properties with no resolvable resonance structure. Nitrogen adsorbed on Ni(ll0) is characterized by a residual d-band and two resonances : one at N 12 eV and the other at N 25 eV below the vacuum zero. The analysis is consistent with data published earlier on a characteristic energy-loss description of Ni(llO)-(disordered)N. Nitrogen adsorbed on Si(ll1) appears to have similar resonances at 13 eV and 27 eV below the vacuum zero with no other intervening band structure. E N Sickafos and F Stelnrlsser (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Technof, 10 (l), Jan/Feb 1973,43-46. 16 560. The bond-energy bond-order (LEBO) model of chemisorption. (USA) The bond-energy bond-order (BEBO) model of chemisorption allows an estimate to be made of the interaction energy between a gaseous specie and a solid surface as a function of either bond length or bond order, ie, the length or order of either the gas-surface bond being formed or the bond of the gaseous molecule being broken. The relationship between bond energy and either bond length or bond order is deduced from spectroscopic correlations for gaseous molecules, and a linear relationship between bond energy and bond order is assumed for the surface-adsorbate interaction. The geometry of the surface orbitals is taken to be that predicted by the crystal field model. The model allows a prediction of several relevant quantities in gassurface interactions, namely: (1) binding energies for molecular adsorbed species, (2) binding energies to atomically adsorbed species, (3) activation energies to chemisorption, and (4) activation energies to dissociative chemisorption. The model is illustrated for the adsorption of HZ, CO, NO and O2 on Pt, W and Ni surfaces. W H Weinberg (Proc 19th Nat Symp Am Vat Sot), J Vat Sci Technol, 10 (l), Jan/Feb 1973, 89-94. 335