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Photoemission from Ag Pd
Solid State Communications, Vol. 6, Pp. 649 651, 1968. Pergamon Press. -
Printed in Great Britain
PHOTOE MISSION FROM AgPd C. Norris and P.O. Nilsson Physics Department, Chalmers University of Technology Gothenburg, Sweden (Received 17 June 1968 by S. Lundqvist)
The energy distribution of photoemitted electrons from pure silver and a silver-palladium alloy has been measured. The results confirm the existence of resonant bound states of the Pd 4d electrons lying just below the Fermi level.
1 and Myers, IN RECENT Walldén and articles Karlsson2Abelés have reported the occurrence of an additional absorption band in alloys of Pd and Mn in Ag, Au and Cu. The absorption was attributed to resonant bound states associated with the d electrons of Pd and Mn, confirming the model of Friedel3 and his co-workers, Optical absorption indicates only the difference in energy of the initial and final states and cannot determine with certainty the position of the resonant states. This can be ascertained, however, by photoemission in which two parameters are measured: the energy of the exciting light, and the energy of the ejected electrons.
Figure 1 shows the photoemission measured from silver-palladium compared with the same result for pure silver. The silver curve compares very well with a similar measurement made by Krolikowski4 using an evaporated film. The large peaked structure below 2. 5 eV (the energy is that of the emitted electrons) corresponds to the 4d band and the flatter structure at higher energies to the 5s conduction band; the Fermi level can be seen at 6. 2 eV. The effect of alloying with palladium is to produce a large hump in the energy distribution curve. We associate this hump with the resonant states of the 4d Pd electrons. The energy distribution curves for AgPd obtained with different energies of the exciting light are shown in Fig. 2. The results are referred to the energy of the initial states by plotting them against E-hv -f c~ is the work function. The zero on this scale is the Fermi energy. Clearly the Pd hump in the electron distribution curves does not move for different exciting energies, suggesting it is due to a feature in the density of the initial states. The location of the resonant bound states at about 2. 3 eV below the Fermi energy compares with the maximum of the extra absorption, which according to Myers etal. occurs at 2.6 eV in AgPd films. If it is not due to experimental causes this difference might indicate that the absorption measured by Myers et al. does not correspond to a transition to the Fermi level, but rather to a level slightly above it. The width measured here, 1.4 eV, is less than the value of 1.8 eV given by Myers et al. Little variation in the position of the silver~ffbandrelative to the Fermi edge can be seen by alloying with Pd,
Bulk samples of silver-palladium (15% at. wt. Pd) and pure silver were prepared in a conventional way. They were mechanically polished, finishing with fine alumina powder; no electropolishing was performed. The measurements were made with the samples mounted in an ultra-high vacuum chamber, in which a pressure of 4 x 1O”~mm Hg was readily maintained, Sample surfaces were cleaned prior to measurement with the aid of heat treatment and argon bombardment. In the photoemission technique, monochromatic vacuum u. v. light is made mcident on a clean conducting surface. The distributton in energy of the emitted electrons will, after being corrected for some effects, principally electron-electron scattering and the escape function, reproduce the details of the density of states within the material. The results presented here are the uncorrected electron distribution curves. Nevertheless the main features of the density of states can still be seen, 649
650
PHOTOEMISSION FROM AgPd
Vol. 6, No.9
h~. 101 •V
~A9s~PdIsL~f~
E 0—hy
FIG. 1
•0
.V)
FIG. 2
Energy-distribution curves from pure Ag and Ag,85 Pd.15
Energy-distribution curves from Ag~BEPd, i~ measured using different energies of the exciting light.
.
in agreement with Myers et al. who found no shift in the position of the~~rptionedge in AgPd films.
emission data for a series of alloys of Ag and Pd will be published at a later date. Acknowledgment We wish to thank Dr. Stig Hagström for provision of research facilities and his help with the work. -
A complete description of the photo-
References
1.
ABELES F., Optical Properties and Electronic Structure of Metals and Alloys (Edited by ABELES F.) North Holland Publishing Co. (1965).
2.
MYERS H. P.,
3.
FRIEDEL J.,
4.
KROLIKOWSKI W. F.,
WALLDEN L. and KARLSSON Nuovo Cim. Suppi.,
A.,
Phil. Mag., to be published.
7, 287 (1958).
Ph. D. Thesis, Stanford (1967).
Vol. 6, No. 9
PHOTOEMISSION FROM AgPd La distribution des electrons photo-émis de l’argent pur et d’un alliage de l’argent et du palladium fut mésurêe. Les résultats confirment l’êxistence des états fi.xés resonant des electrons 4d du Pd, localisés juste au-dessous du niveau Fermi.
651
Printed in Great Britain
PHOTOE MISSION FROM AgPd C. Norris and P.O. Nilsson Physics Department, Chalmers University of Technology Gothenburg, Sweden (Received 17 June 1968 by S. Lundqvist)
The energy distribution of photoemitted electrons from pure silver and a silver-palladium alloy has been measured. The results confirm the existence of resonant bound states of the Pd 4d electrons lying just below the Fermi level.
1 and Myers, IN RECENT Walldén and articles Karlsson2Abelés have reported the occurrence of an additional absorption band in alloys of Pd and Mn in Ag, Au and Cu. The absorption was attributed to resonant bound states associated with the d electrons of Pd and Mn, confirming the model of Friedel3 and his co-workers, Optical absorption indicates only the difference in energy of the initial and final states and cannot determine with certainty the position of the resonant states. This can be ascertained, however, by photoemission in which two parameters are measured: the energy of the exciting light, and the energy of the ejected electrons.
Figure 1 shows the photoemission measured from silver-palladium compared with the same result for pure silver. The silver curve compares very well with a similar measurement made by Krolikowski4 using an evaporated film. The large peaked structure below 2. 5 eV (the energy is that of the emitted electrons) corresponds to the 4d band and the flatter structure at higher energies to the 5s conduction band; the Fermi level can be seen at 6. 2 eV. The effect of alloying with palladium is to produce a large hump in the energy distribution curve. We associate this hump with the resonant states of the 4d Pd electrons. The energy distribution curves for AgPd obtained with different energies of the exciting light are shown in Fig. 2. The results are referred to the energy of the initial states by plotting them against E-hv -f c~ is the work function. The zero on this scale is the Fermi energy. Clearly the Pd hump in the electron distribution curves does not move for different exciting energies, suggesting it is due to a feature in the density of the initial states. The location of the resonant bound states at about 2. 3 eV below the Fermi energy compares with the maximum of the extra absorption, which according to Myers etal. occurs at 2.6 eV in AgPd films. If it is not due to experimental causes this difference might indicate that the absorption measured by Myers et al. does not correspond to a transition to the Fermi level, but rather to a level slightly above it. The width measured here, 1.4 eV, is less than the value of 1.8 eV given by Myers et al. Little variation in the position of the silver~ffbandrelative to the Fermi edge can be seen by alloying with Pd,
Bulk samples of silver-palladium (15% at. wt. Pd) and pure silver were prepared in a conventional way. They were mechanically polished, finishing with fine alumina powder; no electropolishing was performed. The measurements were made with the samples mounted in an ultra-high vacuum chamber, in which a pressure of 4 x 1O”~mm Hg was readily maintained, Sample surfaces were cleaned prior to measurement with the aid of heat treatment and argon bombardment. In the photoemission technique, monochromatic vacuum u. v. light is made mcident on a clean conducting surface. The distributton in energy of the emitted electrons will, after being corrected for some effects, principally electron-electron scattering and the escape function, reproduce the details of the density of states within the material. The results presented here are the uncorrected electron distribution curves. Nevertheless the main features of the density of states can still be seen, 649
650
PHOTOEMISSION FROM AgPd
Vol. 6, No.9
h~. 101 •V
~A9s~PdIsL~f~
E 0—hy
FIG. 1
•0
.V)
FIG. 2
Energy-distribution curves from pure Ag and Ag,85 Pd.15
Energy-distribution curves from Ag~BEPd, i~ measured using different energies of the exciting light.
.
in agreement with Myers et al. who found no shift in the position of the~~rptionedge in AgPd films.
emission data for a series of alloys of Ag and Pd will be published at a later date. Acknowledgment We wish to thank Dr. Stig Hagström for provision of research facilities and his help with the work. -
A complete description of the photo-
References
1.
ABELES F., Optical Properties and Electronic Structure of Metals and Alloys (Edited by ABELES F.) North Holland Publishing Co. (1965).
2.
MYERS H. P.,
3.
FRIEDEL J.,
4.
KROLIKOWSKI W. F.,
WALLDEN L. and KARLSSON Nuovo Cim. Suppi.,
A.,
Phil. Mag., to be published.
7, 287 (1958).
Ph. D. Thesis, Stanford (1967).
Vol. 6, No. 9
PHOTOEMISSION FROM AgPd La distribution des electrons photo-émis de l’argent pur et d’un alliage de l’argent et du palladium fut mésurêe. Les résultats confirment l’êxistence des états fi.xés resonant des electrons 4d du Pd, localisés juste au-dessous du niveau Fermi.
651