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Dependence of the 3p electron energy loss spectra of nickel on momentum transfer
Applications of Surface Science 11/12 (1982) 385—389 North-Holland Publishing Company
385
DEPENDENCE OF THE 3p ELECTRON ENERGY LOSS SPECTRA OF NICKEL ON MOMENTUM TRANSFER Terrence JACH and C.J. POWELL Surface Science Division, National Bureau of Standards, Washington, DC 20234, USA Received 8 June 1981
The Fano lineshape of the threshold region in nickel 3p electron energy loss spectra is observed to change as the incident electron energy is lowered from 1000 to 150 eV. This change is attributed to changes in momentum transfer over the energy range investigated. The lineshape changes are consistent with a change of Fano’s parameter q from 0.95 to 1.2, with significant deviations from the predicted Fano lineshape at the lowest incident energy. A satellite peak 12 eV above the 3p threshold is observed to decrease in intensity relative to the principal line.
I. Introduction Electron energy loss spectra of nickel in the vicinity of the 3p threshold show changes in lineshape as the incident energy is varied over a range 150 ‘zE~1000 eV [I]. These changes occur in two regions of the energy loss lineshape: the threshold region (E 66 eV), which shows an actual decrease in continuum final-state scattering due to the Fano effect [2], and a broad satellite peak which occurs about 12 eV above threshold. We have studied the threshold dip and the satellite in the energy loss spectra and observe both features to decrease in amplitude as the incident electron energy decreases. We report here that the lineshapes observed, when fitted with the formula for Fano interference, indicate changes in the lineshape parameter q from 0.95 at an incident energy of 1000 eV to q = 1.2 at an incident energy of 150 eV. Significant deviations from the Fano lineshape are observed at the lowest incident energy. The changes in Fano lineshape and satellite amplitude are attributed to changes in momentum transfer for forward scattering as the incident electron energy is lowered. —
2. Experimental The experiment consisted of recording electron energy loss spectra of electrons backscattered from a nickel target using a double pass cylindrical mirror analyzer and a coaxial electron gun. The cylindrical mirror analyzer was 0378-5963/82/0000—0000/$02.75
©
1982 North-Holland
T Jach, C.J. Powell
386
/
3p EELS of Ni and momentum transfer
operated in a constant pass-energy mode to give essentially constant energy resolution. The target was a Ni foil cleaned by argon ion sputtering but not annealed. In addition to conventional N( E) spectra, loss spectra were taken with an incident electron energy offset of 1.3 eV applied on alternate sweeps and were stored in a signal averager alternately additively and subtractively. The resulting dN( E )/d E spectra highlight small changes in lineshape, and Auger lines are eliminated to first order.
3. Results Fig. 1 shows a typical N( E) energy loss spectrum of nickel in the vicinity of the 3p binding energy for an incident energy of E = 1000 eV. Region I includes the dip in inelastic scattering at threshold associated with Fano interference [2]. Region II is the local maximum associated with the satellite excitation centered approximately 12 eV above the 3p threshold. The dN(E)/dE loss spectra were taken for a series of incident electron energies from 1000 eV down to 150 eV and the original data were reported in ref. [1]. To examine changes in lineshape in region I on a systematic basis, we have performed additional background subtraction. A cubic polynomial was least squares fitted to each spectrum with the exclusion of the immediate vicinity of the derivative peak corresponding to the threshold edge 66 eV. The resulting dN(E)/dE loss spectra are shown as solid lines in fig. 2 and have been normalized to constant amplitude of the derivative peak at 66 eV. The dashed lines in fig. 2 are derivatives of the Fano lineshape [3] which have been fitted by variation of the lineshape parameters. The lineshape
N(E) ~
50
60
70
80
90
100
ELECTRON ENERGY LOSS (eV)
Fig. 1. Electron energy loss spectrum of solid nickel for an incident electron energy E= 1000 eV. Region I includes the negative interference portion of the Fano effect. Region Il is an additional broad satellite excitation -~ 12 eV above threshold.
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3p EELS of Ni and momentum transfer
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E=l000eV
I \~ N 2.0eV
~i~4~It
50
60
70
80
90
100
ELECTRON ENERGY LOSS (eV)
Fig. 2. Derivative electron energy loss spectra of solid nickel for different incident electron energies E. The dashed lines are fits to the Fano lineshape formula (eq. (I )), with parameters I’, q. All best fits to Region I used a discrete state energy E 0 = 64.8 eV except for E0 = 64.5 eV for E 1000 eV. The feature at 90—100 eV energy loss in the spectrum for E = 150 eV is an uncancelled remnant of the Ni M23VV Auger line.
parameters were varied to optimize the fit to a Fano lineshape in region I (55
T Jach, C.J. Powell / .?p EELS of Ni and momentum transfer
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energies nearest threshold (E = 200 eV, 150 eV) the local minimum of region I progressively disappears. The disappearance of this dip corresponds to the disappearance of the corresponding feature with negative slope in region I of fig. 1 associated with the Fano interference. The fits to the Fano lineshape indicate that q is increasing from 0.95 to 1.2 as E is decreased from 1000 to 200 eV. In addition, there appears to be a narrowing of the peaks in fig. 2 with decreasing incident energy. There are small deviations of the observed spectra from the Fano lineshape for 1000> E> 200 eV. These deviations became more pronounced for E = 150 eV; for this incident energy, the Fano lineshape formula cannot simultaneously fit the 60 and 68 eV energy loss regions of the spectrum. Region II in fig. 2 shows the derivative of the satellite feature in fig. 1. While a positive assignment has not been made for this excitation, satellite features have been observed at corresponding energies in related 3p spectra from nickel. First, optical absorption has been observed at 12 eV above the 3p edge in Ni vapor and metal by Bruhn, Sonntag and Wolff [4]. They attribute this 3p5 ns, absorption in Ni vapor to single-electron transitions of the type 3p6 nd (n > 3). It is possible that the core hole relaxes these levels in solid nickel to the point where they are still quasi-bound states or shape resonances. Second, a 12 eV satellite has been observed in the 3p X-ray photoelectron spectrum of nickel [5] (unlike the well-known 6 eV satellites observed in the 3d, 3s, and 2p photoemission spectra). The 12 eV satellite has been attributed to a two-electron excitation by Martensson and Johansson [6]. As shown in fig. 2, the excitation in region II of the energy loss spectrum is observed qualitatively to disappear as the incident energy is lowered. —~
4. Discussion Since the energy loss range is identical in the spectra shown, we attribute the changes observed in both the edge profile and the satellite excitation to increasing momentum transfer as the incident electron energy is decreased. It is generally assumed that electrons backscattered into the cylindrical mirror analyzer at these energies have undergone an inelastic forward scattering (where the cross section is largest) plus multiple elastic events which are responsible for the large deflection angle [2]. For an energy loss of 66 eV with an incident energy of 1000 eV, there is a minimum momentum transfer of L~k= 0.56 A For an incident ener~yof 150 eV, the forward scattering momentum transfer increases to 1.54 A~. The actual momentum transfer in our experiment could be much larger than these minimum values. Changes with incident energy of the Fano parameter q, corresponding to changes in the branching ratio of transitions into discrete and continuum states, can be expected to vary with momentum transfer ~k. Deviations from the simple atomic model in the Fano theory used may occur in our experiment ‘.
T. Jach. C.J. Powell / 3p EELS of Ni and momentum transfer
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for several reasons: (1) the correct model should actually describe several unresolved discrete states overlapping multiple continua, (2) the excitation in region II may overlap with the Fano profile in the energy loss region about 68—70 eV, and (3) Fano interference may be too simple to describe the physical processes in solid Ni. The qualitative disappearance of the satellite observed in the energy loss spectrum relative to the intensity of the main peak supports the model of a multiple-electron excitation since the single-electron “shape resonance model” should have the same momentum-transfer dependence as the main peak. The variation of the relative intensities of the satellite and threshold 3p loss features may be considered an indication of the transition from “adiabatic” (at low incident energy) to “sudden” approximation which has been observed in similar systems [7]. For E = 150 eV, both the incident electrons and the electrons contributing to our energy loss spectrum have kinetic energies in a range for which the inelastic mean free path is near its minimum value [8]. Our measured energy loss spectra therefore increasingly probe surface atoms as the incident energy is decreased from 1000 to 150 eV. It is possible that the changes of lineshape we have found are due to variations of electronic structure for surface and bulk atoms.
Acknowledgement This work was supported in part by the Office of Environment, US Department of Energy.
References [1] [2] [3] [4] [5] [6] [7] [8]
T. Jach and C.J. Powell, Solid State Commun. 40 (1981) 967. RE. Dietz, E.G. McRae, Y. Yafet and C.W. Caldwell, Phys. Rev. Letters 33 (1974) 1372. W. Fano, Phys. Rev. 124 (1961) 1866. R. Bruhn, B. Sonntag and H.W. Wolff, J. Phys. B12 (1979) 203. S. Hufner and G.K. Wertheim, Phys. Letters 51A (1975) 299. N. Martensson and B. Johansson, Phys. Rev. Letters 45 (1980) 482. J.C. Fuggle, R. Lässer, 0. Gunnarson and K. Schonhammer, Phys. Rev. Letters 44 (1980)1090. C.J. Powell, Surface Sci. 44 (1974) 29.
385
DEPENDENCE OF THE 3p ELECTRON ENERGY LOSS SPECTRA OF NICKEL ON MOMENTUM TRANSFER Terrence JACH and C.J. POWELL Surface Science Division, National Bureau of Standards, Washington, DC 20234, USA Received 8 June 1981
The Fano lineshape of the threshold region in nickel 3p electron energy loss spectra is observed to change as the incident electron energy is lowered from 1000 to 150 eV. This change is attributed to changes in momentum transfer over the energy range investigated. The lineshape changes are consistent with a change of Fano’s parameter q from 0.95 to 1.2, with significant deviations from the predicted Fano lineshape at the lowest incident energy. A satellite peak 12 eV above the 3p threshold is observed to decrease in intensity relative to the principal line.
I. Introduction Electron energy loss spectra of nickel in the vicinity of the 3p threshold show changes in lineshape as the incident energy is varied over a range 150 ‘zE~1000 eV [I]. These changes occur in two regions of the energy loss lineshape: the threshold region (E 66 eV), which shows an actual decrease in continuum final-state scattering due to the Fano effect [2], and a broad satellite peak which occurs about 12 eV above threshold. We have studied the threshold dip and the satellite in the energy loss spectra and observe both features to decrease in amplitude as the incident electron energy decreases. We report here that the lineshapes observed, when fitted with the formula for Fano interference, indicate changes in the lineshape parameter q from 0.95 at an incident energy of 1000 eV to q = 1.2 at an incident energy of 150 eV. Significant deviations from the Fano lineshape are observed at the lowest incident energy. The changes in Fano lineshape and satellite amplitude are attributed to changes in momentum transfer for forward scattering as the incident electron energy is lowered. —
2. Experimental The experiment consisted of recording electron energy loss spectra of electrons backscattered from a nickel target using a double pass cylindrical mirror analyzer and a coaxial electron gun. The cylindrical mirror analyzer was 0378-5963/82/0000—0000/$02.75
©
1982 North-Holland
T Jach, C.J. Powell
386
/
3p EELS of Ni and momentum transfer
operated in a constant pass-energy mode to give essentially constant energy resolution. The target was a Ni foil cleaned by argon ion sputtering but not annealed. In addition to conventional N( E) spectra, loss spectra were taken with an incident electron energy offset of 1.3 eV applied on alternate sweeps and were stored in a signal averager alternately additively and subtractively. The resulting dN( E )/d E spectra highlight small changes in lineshape, and Auger lines are eliminated to first order.
3. Results Fig. 1 shows a typical N( E) energy loss spectrum of nickel in the vicinity of the 3p binding energy for an incident energy of E = 1000 eV. Region I includes the dip in inelastic scattering at threshold associated with Fano interference [2]. Region II is the local maximum associated with the satellite excitation centered approximately 12 eV above the 3p threshold. The dN(E)/dE loss spectra were taken for a series of incident electron energies from 1000 eV down to 150 eV and the original data were reported in ref. [1]. To examine changes in lineshape in region I on a systematic basis, we have performed additional background subtraction. A cubic polynomial was least squares fitted to each spectrum with the exclusion of the immediate vicinity of the derivative peak corresponding to the threshold edge 66 eV. The resulting dN(E)/dE loss spectra are shown as solid lines in fig. 2 and have been normalized to constant amplitude of the derivative peak at 66 eV. The dashed lines in fig. 2 are derivatives of the Fano lineshape [3] which have been fitted by variation of the lineshape parameters. The lineshape
N(E) ~
50
60
70
80
90
100
ELECTRON ENERGY LOSS (eV)
Fig. 1. Electron energy loss spectrum of solid nickel for an incident electron energy E= 1000 eV. Region I includes the negative interference portion of the Fano effect. Region Il is an additional broad satellite excitation -~ 12 eV above threshold.
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3p EELS of Ni and momentum transfer
387
E=l000eV
I \~ N 2.0eV
~i~4~It
50
60
70
80
90
100
ELECTRON ENERGY LOSS (eV)
Fig. 2. Derivative electron energy loss spectra of solid nickel for different incident electron energies E. The dashed lines are fits to the Fano lineshape formula (eq. (I )), with parameters I’, q. All best fits to Region I used a discrete state energy E 0 = 64.8 eV except for E0 = 64.5 eV for E 1000 eV. The feature at 90—100 eV energy loss in the spectrum for E = 150 eV is an uncancelled remnant of the Ni M23VV Auger line.
parameters were varied to optimize the fit to a Fano lineshape in region I (55
T Jach, C.J. Powell / .?p EELS of Ni and momentum transfer
388
energies nearest threshold (E = 200 eV, 150 eV) the local minimum of region I progressively disappears. The disappearance of this dip corresponds to the disappearance of the corresponding feature with negative slope in region I of fig. 1 associated with the Fano interference. The fits to the Fano lineshape indicate that q is increasing from 0.95 to 1.2 as E is decreased from 1000 to 200 eV. In addition, there appears to be a narrowing of the peaks in fig. 2 with decreasing incident energy. There are small deviations of the observed spectra from the Fano lineshape for 1000> E> 200 eV. These deviations became more pronounced for E = 150 eV; for this incident energy, the Fano lineshape formula cannot simultaneously fit the 60 and 68 eV energy loss regions of the spectrum. Region II in fig. 2 shows the derivative of the satellite feature in fig. 1. While a positive assignment has not been made for this excitation, satellite features have been observed at corresponding energies in related 3p spectra from nickel. First, optical absorption has been observed at 12 eV above the 3p edge in Ni vapor and metal by Bruhn, Sonntag and Wolff [4]. They attribute this 3p5 ns, absorption in Ni vapor to single-electron transitions of the type 3p6 nd (n > 3). It is possible that the core hole relaxes these levels in solid nickel to the point where they are still quasi-bound states or shape resonances. Second, a 12 eV satellite has been observed in the 3p X-ray photoelectron spectrum of nickel [5] (unlike the well-known 6 eV satellites observed in the 3d, 3s, and 2p photoemission spectra). The 12 eV satellite has been attributed to a two-electron excitation by Martensson and Johansson [6]. As shown in fig. 2, the excitation in region II of the energy loss spectrum is observed qualitatively to disappear as the incident energy is lowered. —~
4. Discussion Since the energy loss range is identical in the spectra shown, we attribute the changes observed in both the edge profile and the satellite excitation to increasing momentum transfer as the incident electron energy is decreased. It is generally assumed that electrons backscattered into the cylindrical mirror analyzer at these energies have undergone an inelastic forward scattering (where the cross section is largest) plus multiple elastic events which are responsible for the large deflection angle [2]. For an energy loss of 66 eV with an incident energy of 1000 eV, there is a minimum momentum transfer of L~k= 0.56 A For an incident ener~yof 150 eV, the forward scattering momentum transfer increases to 1.54 A~. The actual momentum transfer in our experiment could be much larger than these minimum values. Changes with incident energy of the Fano parameter q, corresponding to changes in the branching ratio of transitions into discrete and continuum states, can be expected to vary with momentum transfer ~k. Deviations from the simple atomic model in the Fano theory used may occur in our experiment ‘.
T. Jach. C.J. Powell / 3p EELS of Ni and momentum transfer
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for several reasons: (1) the correct model should actually describe several unresolved discrete states overlapping multiple continua, (2) the excitation in region II may overlap with the Fano profile in the energy loss region about 68—70 eV, and (3) Fano interference may be too simple to describe the physical processes in solid Ni. The qualitative disappearance of the satellite observed in the energy loss spectrum relative to the intensity of the main peak supports the model of a multiple-electron excitation since the single-electron “shape resonance model” should have the same momentum-transfer dependence as the main peak. The variation of the relative intensities of the satellite and threshold 3p loss features may be considered an indication of the transition from “adiabatic” (at low incident energy) to “sudden” approximation which has been observed in similar systems [7]. For E = 150 eV, both the incident electrons and the electrons contributing to our energy loss spectrum have kinetic energies in a range for which the inelastic mean free path is near its minimum value [8]. Our measured energy loss spectra therefore increasingly probe surface atoms as the incident energy is decreased from 1000 to 150 eV. It is possible that the changes of lineshape we have found are due to variations of electronic structure for surface and bulk atoms.
Acknowledgement This work was supported in part by the Office of Environment, US Department of Energy.
References [1] [2] [3] [4] [5] [6] [7] [8]
T. Jach and C.J. Powell, Solid State Commun. 40 (1981) 967. RE. Dietz, E.G. McRae, Y. Yafet and C.W. Caldwell, Phys. Rev. Letters 33 (1974) 1372. W. Fano, Phys. Rev. 124 (1961) 1866. R. Bruhn, B. Sonntag and H.W. Wolff, J. Phys. B12 (1979) 203. S. Hufner and G.K. Wertheim, Phys. Letters 51A (1975) 299. N. Martensson and B. Johansson, Phys. Rev. Letters 45 (1980) 482. J.C. Fuggle, R. Lässer, 0. Gunnarson and K. Schonhammer, Phys. Rev. Letters 44 (1980)1090. C.J. Powell, Surface Sci. 44 (1974) 29.