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On the coverage dependence of chemisorption characteristics on metals
A372 468
Surface
ON THE COVERAGE DEPENDENCE CHARACTERISTICS ON METALS G.M. GAVRILENKO, R. CARDENAS Lahoratoty of Theoretical Physics, JINR Received
30 September
1988; accepted
Science 217 (1989) 468-488 North-Holland, Amsterdam
OF CHEMISORPTION and V.K. FEDYANIN
P. 0. B. 79, 101000 Moscow, for publication
10 January
USSR 1989
Within the composite Anderson-I@ Hamiltonian the coverage dependence of chemisorption characteristics like the chemisorption energy, the electron charge and magnetic moment localized at impurities (adsorbed atoms) and the impurity electron density of states are investigated. Calculations are carried out within the self-consistent unrestricted Hartree-Fock scheme for the electron component and within the Bragg-Williams approximation for the ionic adsorbate component. The dependence of the magnetic phase transition on the coverage is obtained. It is interpreted as a second-order phase transition with critical exponent s =1/2. The crossover problem in the phase space of the model parameters is also discussed.
Surface Science 217 (1989) 4899510 North-Holland, Amsterdam
489
STUDY OF HIGH COVERAGES ON THE Pt(ll1) SURFACE Deborah
Holmes
PARKER
OF ATOMIC
*, Michael
OXYGEN
E. BARTRAM
and Bruce E. KOEL
Department of Chemisiry and Biochemistry and Cooperative Institute for Research in Environmental Scrences (CIRES), University of Colorado, Boulder, CO 80309.449, USA Received
8 July 1988; accepted
for publication
22 February
1989
Atomic oxygen coverages of up to 0.75 ML may be adsorbed cleanly on Pt(ll1) surfaces under UHV conditions by exposure to NO, at 400 K. We have studied this adsorbed oxygen layer by using AES, LEED, UPS, HREELS, TPD, and work function (A+) measurements. The (2x2)-0 structure formed at So = 0.25 ML is still apparent at 0, = 0.60 ML and a faint (2 X 2) pattern persists even up to 0, = 0.75 ML. AES and A+ measurements show no evidence for chemically distinct species in the adlayer as a function of oxygen coverage. HREELS spectra clearly rule out the presence of molecular oxygen and oxide species over the range of oxygen coverage studied. UPS also shows no shift in binding energy of the oxygen-derived peak as the coverage is increased. These spectroscopic probes indicate that all oxygen is present as atomic oxygen with no indication of oxide formation or molecular oxygen at any coverage. Multiple 0, desorption peaks observed in TPD are interpreted as arising largely from kinetic effects rather than a result of multiple, distinctly different chemical species, even though large changes in the Pt-0 bond energy are determined from the TPD data. The activation energy for 0, desorption reflects the sum of the heat of dissociative adsorption of 0, and the activation energy for 0, dissociation. The structure in the 0, TPD spectrum is due to large changes in the activation energy for 0, desorption resulting from increases in the barrier to dissociative 0, chemisorption and decreases in the Pt-0 bond energy. These barriers arise from strong repulsive interactions between adsorbed oxygen adatoms that cause sharp reductions in the Pt-0 bond strength at these coverages. Finally, we note that our spectroscopic probes are quite insensitive to the changes in the Pt-0 bond strength over the entire range of oxygen coverage studied.
Surface
ON THE COVERAGE DEPENDENCE CHARACTERISTICS ON METALS G.M. GAVRILENKO, R. CARDENAS Lahoratoty of Theoretical Physics, JINR Received
30 September
1988; accepted
Science 217 (1989) 468-488 North-Holland, Amsterdam
OF CHEMISORPTION and V.K. FEDYANIN
P. 0. B. 79, 101000 Moscow, for publication
10 January
USSR 1989
Within the composite Anderson-I@ Hamiltonian the coverage dependence of chemisorption characteristics like the chemisorption energy, the electron charge and magnetic moment localized at impurities (adsorbed atoms) and the impurity electron density of states are investigated. Calculations are carried out within the self-consistent unrestricted Hartree-Fock scheme for the electron component and within the Bragg-Williams approximation for the ionic adsorbate component. The dependence of the magnetic phase transition on the coverage is obtained. It is interpreted as a second-order phase transition with critical exponent s =1/2. The crossover problem in the phase space of the model parameters is also discussed.
Surface Science 217 (1989) 4899510 North-Holland, Amsterdam
489
STUDY OF HIGH COVERAGES ON THE Pt(ll1) SURFACE Deborah
Holmes
PARKER
OF ATOMIC
*, Michael
OXYGEN
E. BARTRAM
and Bruce E. KOEL
Department of Chemisiry and Biochemistry and Cooperative Institute for Research in Environmental Scrences (CIRES), University of Colorado, Boulder, CO 80309.449, USA Received
8 July 1988; accepted
for publication
22 February
1989
Atomic oxygen coverages of up to 0.75 ML may be adsorbed cleanly on Pt(ll1) surfaces under UHV conditions by exposure to NO, at 400 K. We have studied this adsorbed oxygen layer by using AES, LEED, UPS, HREELS, TPD, and work function (A+) measurements. The (2x2)-0 structure formed at So = 0.25 ML is still apparent at 0, = 0.60 ML and a faint (2 X 2) pattern persists even up to 0, = 0.75 ML. AES and A+ measurements show no evidence for chemically distinct species in the adlayer as a function of oxygen coverage. HREELS spectra clearly rule out the presence of molecular oxygen and oxide species over the range of oxygen coverage studied. UPS also shows no shift in binding energy of the oxygen-derived peak as the coverage is increased. These spectroscopic probes indicate that all oxygen is present as atomic oxygen with no indication of oxide formation or molecular oxygen at any coverage. Multiple 0, desorption peaks observed in TPD are interpreted as arising largely from kinetic effects rather than a result of multiple, distinctly different chemical species, even though large changes in the Pt-0 bond energy are determined from the TPD data. The activation energy for 0, desorption reflects the sum of the heat of dissociative adsorption of 0, and the activation energy for 0, dissociation. The structure in the 0, TPD spectrum is due to large changes in the activation energy for 0, desorption resulting from increases in the barrier to dissociative 0, chemisorption and decreases in the Pt-0 bond energy. These barriers arise from strong repulsive interactions between adsorbed oxygen adatoms that cause sharp reductions in the Pt-0 bond strength at these coverages. Finally, we note that our spectroscopic probes are quite insensitive to the changes in the Pt-0 bond strength over the entire range of oxygen coverage studied.