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Dipole forbidden f-f excitation in ytterbium oxide
Surface
212
Science 251/252
(1991) 272-275 North-Holland
Dipole forbidden f-f excitation in ytterbium oxide A. Gorschliiter,
R. Stiller
Physikolisches
der Uniuersitiit
Received
Institut
I October
1990, accepted
’ and
H. Merz
Miinster.
Wilhelm-Klemm-Stmsse
for publication
5 October
IO. W-4400
Miinster.
Germany
1990
The dipole forbidden f-f excitation in YbaO, has been investigated with EELS. The oxide with its large band gap provides in EEL-spectra a reliable separation of the low lying dipole forbidden 4f excitations from other, optically allowed transitions. For primary energies ranging from 1245 eV down to 10 eV we have performed EELS measurements on thin oxide films. In the loss spectra the intensity of the 1.33 eV loss increases drastically with decreasing primary electron energy. Additional EELS measurements on differently prepared films of ytterbium oxide and on Lu,O, throw light on the nature of the 1.33 eV loss. The origin of a previously reported unexplained 2.5 eV loss is discussed.
1. In~~uction Dipole forbidden excitations in rare earth metals and their compounds have been repeatedly observed with electron energy loss spectroscopy (EELS) in the reflection mode [l-3]. Usually such transitions are very weak but they increase in intensity when the primary energy is close to the excitation threshold [1,3]. In rare earth elements the optically forbidden f-f transitions are of particular importance because the f-levels retain most of their localized character with a narrow energy width so that f-f excitations might show up as sharp structures in EEL-spectra [2,3]. The evaluation of f-f intensity from experimental loss spectra is often complicated by a supe~osition of other intense structures caused by optically allowed transitions. For our experiment we have chosen the insulator Yb,O, with the electronic configuration (Xe)4f’3(5d6s)3 [4]. The low lying f-f excitation with the transition 2F,,2 -+ ‘F5,,? with hE = 1.29 eV [S] shows only a single peak in the EEL-spectra that is to be expected within the broad optical band gap. This enables a reliable separation be-
’ Present address: Hagen 1. Germany.
Hoesch
Hohenlimburg
AG.
W-5800
0039~6028/91/$03.50
~CJ1991 - Elsevier Science Publishers
tween the f-f loss intensity and the weak background. To prove that the 1.33 eV loss in Yb,O, is in fact due to a f-f transition we additionally took EEL-spectra of Lu,O, which has a very similar electronic structure but will not allow any f--f excitation because of its completely filled 4f-shell. Furthermore we tried to get detailed information about a loss structure at 2.5 eV in Yb,O, that remained unexplained in a previous paper 121.
2. Experimental All spectra were taken at a pressure in the 5 X lO--” mbar range. We used a Pierce-type electron gun and a simulated he~spherica~ spectrometer [6] with a four element lens for recording the energy distribution of the scattered electrons. The EEL-spectra were taken at a constant pass energy of E = 30 eV with a total resolution of AE = 250 meV. The angle of incidence usually was set to 6” off-normal, in some cases we also took angle dependent measurements. The samples were prepared by evaporating the rare earth metal onto a thin molybdenum plate in an oxygen atmosphere of about 5 x lo-” mbar: immediately afterwards 5000-20000 L 0, was added. The cleanliness of the samples was checked by AES.
B.V. (North-Holland)
A. Gorschliiter et al. / Dipole forbidden
For the characterization and determination of the valence state we used BI-spectra that clearly show the well-known valence change Yb’++ Yb3’ due to the oxidation of Ytterbium.
3. Results and discussion The EEL-spectra of differently prepared films of ytterbium oxide were studied to get more information about the occurence of the double loss structure at AE = 1.3 eV and AE = 2.5 eV reported previously [ 21. Fig. 1 shows the EEL-spectra of a clean metallic film of ytterbium (a), of an inhomogeneously oxidized film Yb,O,_, (b), of a well oxidized film Yb,O, taken at different .primary energies (cd) and of Lu,O, (e). The spectrum of metallic ytterbium shown for comparison demonstrates the change during oxidation; it agrees well with a
I
0
I
I
20 10 LOSS ENERGY teV1
I
30
1. EEL-spectra of metallic Yb (a), of inhomogeneously oxidized Yb$_, (b), of well prepared Yb*O, taken at different primary energies (c, d) and of Lu,O, (e). The position of the elastic peak is indicated by a vertical bar, primary energies are given at the side.
f-f excitation in Yb,O,
273
previous measurement [7]. We recorded the spectrum of Yb,O,_, at a sample that had been exposed to 1500 L 0, after evaporation. The measurements were carried out within 24 h after preparation. The main structures are a double peak (a,, a2) at 1.3 and 2.5 eV, a shoulder at 10 eV that is due to surface plasmon excitation and a strong loss peak at 15 eV due to bulk plasmon excitation. The broad structure at 32 eV is caused by 5p + 5d transitions. The loss energies of peaks a, and a2 are in good agreement with those reported by Della Valle and Modesti [2]. The different intensity ratio of the peaks a, and a, is caused by the higher primary energy in this work. Such strong features are not to be expected within the energy range of the band gap. EELspectra with primary energies between 180 and 1245 eV and angle dependent measurements show that the intensity of peak a2 is drastically reduced at lower primary energies or at larger angles of incidence. Peak a, shows a similar but less pronounced behaviour. The occurrence and the intensity of both features depend on the procedure of oxidizing the thin film of ytterbium. If ytterbium had been evaporated at a partial pressure of oxygen higher than 5 X 10e6 mbar and additionally exposed to more than 4000 L 0, we recorded EELspectra as shown in fig. lc. The positions and shapes of the main features in fig. lb remain unchanged but the double peak (a,, a2) completely disappears so that the optical gap with Eg = 4.0 eV is clearly visible. We remark that poorly oxidized samples initially gave spectra with an intense double peak (a,, a,). After several days this double peak disappeared and we could take spectra identically with fig. lc. We assume that the double peak is due to an inhomogeneously oxidized film of ytterbium with oxygen deficiencies in its structure and distortions of the lattice [8]. EEL-spectra of well prepared thin films of Yb,O, taken at primary energies E, = 1245 eV and E, = 200 eV are shown in fig. lc and fig Id. respectively. Both spectra reveal strong features due to surface (B) and bulk (C) plasmon excitation and a broad structure (D) due to the ionization of the Sp-shell. Whereas in fig. lc no feature is to be seen inside the optical gap, in fig. Id a
weak loss structure appears at 1.3 eV. This feature is due to a dipole forbidden f-f excitation. For demonstration we recorded EEL-spectra of Lu,O,. Its electronic configuration is very similar to Yb,O,; the main difference indeed is the completefy filled 4f-shell in Lu,O, so that no intrasheli 4f-excitation can occur. Fig. le shows the EEL-spectrum of Lu,O, taken with EP = 200 eV. The main structures are also due to surface (B) and bulk (C) plasmon excitation. The optical gap in Lu,O, has the same width Ep = 4.0 eV as in Yb,Q. Within the gap no loss structure is visible - in contrast to the situation of Yb,U, with its open 4f-shdf. This demonstrates that the weak peak at 1.3 eV in Yb,O, is caused by an intrashell 4f-transition. Fig. 2 shows the low energy loss structures in Yb,O, for different primary energies. All spectra are normalized to equal intensity of the peak of
1leV
:
_..%.“,.+,e2
Fig. 2. EEL-spectra of the low energy region in Yb,& taken with different primary energies. The spectra are normalized to the same intensity of the elastic peak. The position of the elastic peak is indicated by a vertical bar.
4
6
7
I
I
XXI
50
0
PRIMARY
Fig.
3. Relative
ENERGY
1
1
!SO
xx)
I&‘)
intensity of the f-f loss peak primary eaergies.
for different
elastically scattered electrons. Therefore the energy dependence of the transmission of the energy analyzer as well as changes in total resolution are negligible. The sharp loss structure at I.33 f 0.04 eV is clearly resolved. Its intensity increases drastically at low primary excitation energies and it vanishes at high excitation energies. The normalized intensity of the f-f peak for different primary energies is plotted in fig. 3. The loss energy E = 1.33 eV agrees well with the multiplet spfitting of the 4f”-level in Yb determined by Dieke f5]. The small energy width of the f-f peak which is only slightly larger than the total energy resolution of the apparatus indicates a transition between two narrow localized levels. We therefore conclude that the sharp loss peak at 1.33 eV is caused by the dipole forbidden spin flip excitation of the 4fr3-level in Yb,O,. The energy dependence of the normalized 1.33 eV loss intensity supports the interpretatian as a dipole forbidden transition [9]. EELS at high primary energies (high compared to the energy loss) strongly favours dipole allowed transitions [l] whereas at low primary energies excitations via electron exchange gain more and more in intensity so that optically forbidden tr~sitio~ can lead to the most prominent features in EEL-spectra. Notice that the f-f peak in fig. 2 becomes more intense than peak C in fig. 1. The normalized intensity of the f-f transition
A. Gorschliiter et al. / Dipole forbidden f-f excitation in Yb,O,
(fig. 3) shows two weak maxima at 35 and 181 eV. Similar structures have been reported by Della Valle and Modesti [2]. They found that the loss intensity of f-f transitions in rare-earth metals is increased when the primary energy reaches the threshold for the 4d + 4f excitations. This structure was explained by the interference of the direct excitation channel with a second excitation channel (via an intermediate negative ion core state 4d”-’ 4f”+2) which opens at the 4dthreshold. Such a resonant mechanism cannot occur in the case of Yb,O, because the intermediate configuration 4f” + * does not exist. We therefore assume that the weak maxima are due to a decrease of the reflection coefficient of elastically scattered electrons at the threshold of the excitation from the 5p- respectively 4d-level to the conduction band. Similar intensity variations have been reported by Stenborg and Bauer [lo] for ytterbium and samarium. The decrease of the intensity at higher primary energies (fig. 3) is not so pronounced as expected [9]. Therefore it cannot be excluded that mechanisms different from the spin flip exchange scattering process discussed might additionally contribute to the intensity observed.
215
An elaborated experiment with polarized electrons could yield direct information about this aspect. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References [l] F.P. Netzer, G. Strasser and J.A.D. Matthew, Phys. Rev. Lett. 51 (1983) 211. [2] F. Della Valle and S. Modesti, Phys. Rev. B 40 (1989) 933. [3] E. Bauer and J. Kolaczkiewicz, Phys. Status Sohdi 131 (1985) 699.. [4] L.I. Johansson, J.W. Allen, I. Lindau, M.H. Hecht and S.B.M. Hagstrom, Phys. Rev. B 21 (1980) 1408. [5] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals (Wiley, New York, 1968). [6] K. Jost, J. Phys. E 12 (1979) 1006. [7] E. Bertel, G. Strasser, F.P. Netzer and J.A.D. Matthew, Surf. Sci. 118 (1982) 387. [S] B.D. Padaha, J.K. Gimzewski, S. Affrossman, W.C. Lang, L.M. Watson and D.J. Fabian, Surf. Sci. 61 (1976) 468. [9] J. Glazer and E. Tosatti. Solid State Commun. 52 (1984) 905. [lo] A. Stenborg and E. Bauer, Solid State Commun. 66 (1988) 561.
212
Science 251/252
(1991) 272-275 North-Holland
Dipole forbidden f-f excitation in ytterbium oxide A. Gorschliiter,
R. Stiller
Physikolisches
der Uniuersitiit
Received
Institut
I October
1990, accepted
’ and
H. Merz
Miinster.
Wilhelm-Klemm-Stmsse
for publication
5 October
IO. W-4400
Miinster.
Germany
1990
The dipole forbidden f-f excitation in YbaO, has been investigated with EELS. The oxide with its large band gap provides in EEL-spectra a reliable separation of the low lying dipole forbidden 4f excitations from other, optically allowed transitions. For primary energies ranging from 1245 eV down to 10 eV we have performed EELS measurements on thin oxide films. In the loss spectra the intensity of the 1.33 eV loss increases drastically with decreasing primary electron energy. Additional EELS measurements on differently prepared films of ytterbium oxide and on Lu,O, throw light on the nature of the 1.33 eV loss. The origin of a previously reported unexplained 2.5 eV loss is discussed.
1. In~~uction Dipole forbidden excitations in rare earth metals and their compounds have been repeatedly observed with electron energy loss spectroscopy (EELS) in the reflection mode [l-3]. Usually such transitions are very weak but they increase in intensity when the primary energy is close to the excitation threshold [1,3]. In rare earth elements the optically forbidden f-f transitions are of particular importance because the f-levels retain most of their localized character with a narrow energy width so that f-f excitations might show up as sharp structures in EEL-spectra [2,3]. The evaluation of f-f intensity from experimental loss spectra is often complicated by a supe~osition of other intense structures caused by optically allowed transitions. For our experiment we have chosen the insulator Yb,O, with the electronic configuration (Xe)4f’3(5d6s)3 [4]. The low lying f-f excitation with the transition 2F,,2 -+ ‘F5,,? with hE = 1.29 eV [S] shows only a single peak in the EEL-spectra that is to be expected within the broad optical band gap. This enables a reliable separation be-
’ Present address: Hagen 1. Germany.
Hoesch
Hohenlimburg
AG.
W-5800
0039~6028/91/$03.50
~CJ1991 - Elsevier Science Publishers
tween the f-f loss intensity and the weak background. To prove that the 1.33 eV loss in Yb,O, is in fact due to a f-f transition we additionally took EEL-spectra of Lu,O, which has a very similar electronic structure but will not allow any f--f excitation because of its completely filled 4f-shell. Furthermore we tried to get detailed information about a loss structure at 2.5 eV in Yb,O, that remained unexplained in a previous paper 121.
2. Experimental All spectra were taken at a pressure in the 5 X lO--” mbar range. We used a Pierce-type electron gun and a simulated he~spherica~ spectrometer [6] with a four element lens for recording the energy distribution of the scattered electrons. The EEL-spectra were taken at a constant pass energy of E = 30 eV with a total resolution of AE = 250 meV. The angle of incidence usually was set to 6” off-normal, in some cases we also took angle dependent measurements. The samples were prepared by evaporating the rare earth metal onto a thin molybdenum plate in an oxygen atmosphere of about 5 x lo-” mbar: immediately afterwards 5000-20000 L 0, was added. The cleanliness of the samples was checked by AES.
B.V. (North-Holland)
A. Gorschliiter et al. / Dipole forbidden
For the characterization and determination of the valence state we used BI-spectra that clearly show the well-known valence change Yb’++ Yb3’ due to the oxidation of Ytterbium.
3. Results and discussion The EEL-spectra of differently prepared films of ytterbium oxide were studied to get more information about the occurence of the double loss structure at AE = 1.3 eV and AE = 2.5 eV reported previously [ 21. Fig. 1 shows the EEL-spectra of a clean metallic film of ytterbium (a), of an inhomogeneously oxidized film Yb,O,_, (b), of a well oxidized film Yb,O, taken at different .primary energies (cd) and of Lu,O, (e). The spectrum of metallic ytterbium shown for comparison demonstrates the change during oxidation; it agrees well with a
I
0
I
I
20 10 LOSS ENERGY teV1
I
30
1. EEL-spectra of metallic Yb (a), of inhomogeneously oxidized Yb$_, (b), of well prepared Yb*O, taken at different primary energies (c, d) and of Lu,O, (e). The position of the elastic peak is indicated by a vertical bar, primary energies are given at the side.
f-f excitation in Yb,O,
273
previous measurement [7]. We recorded the spectrum of Yb,O,_, at a sample that had been exposed to 1500 L 0, after evaporation. The measurements were carried out within 24 h after preparation. The main structures are a double peak (a,, a2) at 1.3 and 2.5 eV, a shoulder at 10 eV that is due to surface plasmon excitation and a strong loss peak at 15 eV due to bulk plasmon excitation. The broad structure at 32 eV is caused by 5p + 5d transitions. The loss energies of peaks a, and a2 are in good agreement with those reported by Della Valle and Modesti [2]. The different intensity ratio of the peaks a, and a, is caused by the higher primary energy in this work. Such strong features are not to be expected within the energy range of the band gap. EELspectra with primary energies between 180 and 1245 eV and angle dependent measurements show that the intensity of peak a2 is drastically reduced at lower primary energies or at larger angles of incidence. Peak a, shows a similar but less pronounced behaviour. The occurrence and the intensity of both features depend on the procedure of oxidizing the thin film of ytterbium. If ytterbium had been evaporated at a partial pressure of oxygen higher than 5 X 10e6 mbar and additionally exposed to more than 4000 L 0, we recorded EELspectra as shown in fig. lc. The positions and shapes of the main features in fig. lb remain unchanged but the double peak (a,, a2) completely disappears so that the optical gap with Eg = 4.0 eV is clearly visible. We remark that poorly oxidized samples initially gave spectra with an intense double peak (a,, a,). After several days this double peak disappeared and we could take spectra identically with fig. lc. We assume that the double peak is due to an inhomogeneously oxidized film of ytterbium with oxygen deficiencies in its structure and distortions of the lattice [8]. EEL-spectra of well prepared thin films of Yb,O, taken at primary energies E, = 1245 eV and E, = 200 eV are shown in fig. lc and fig Id. respectively. Both spectra reveal strong features due to surface (B) and bulk (C) plasmon excitation and a broad structure (D) due to the ionization of the Sp-shell. Whereas in fig. lc no feature is to be seen inside the optical gap, in fig. Id a
weak loss structure appears at 1.3 eV. This feature is due to a dipole forbidden f-f excitation. For demonstration we recorded EEL-spectra of Lu,O,. Its electronic configuration is very similar to Yb,O,; the main difference indeed is the completefy filled 4f-shell in Lu,O, so that no intrasheli 4f-excitation can occur. Fig. le shows the EEL-spectrum of Lu,O, taken with EP = 200 eV. The main structures are also due to surface (B) and bulk (C) plasmon excitation. The optical gap in Lu,O, has the same width Ep = 4.0 eV as in Yb,Q. Within the gap no loss structure is visible - in contrast to the situation of Yb,U, with its open 4f-shdf. This demonstrates that the weak peak at 1.3 eV in Yb,O, is caused by an intrashell 4f-transition. Fig. 2 shows the low energy loss structures in Yb,O, for different primary energies. All spectra are normalized to equal intensity of the peak of
1leV
:
_..%.“,.+,e2
Fig. 2. EEL-spectra of the low energy region in Yb,& taken with different primary energies. The spectra are normalized to the same intensity of the elastic peak. The position of the elastic peak is indicated by a vertical bar.
4
6
7
I
I
XXI
50
0
PRIMARY
Fig.
3. Relative
ENERGY
1
1
!SO
xx)
I&‘)
intensity of the f-f loss peak primary eaergies.
for different
elastically scattered electrons. Therefore the energy dependence of the transmission of the energy analyzer as well as changes in total resolution are negligible. The sharp loss structure at I.33 f 0.04 eV is clearly resolved. Its intensity increases drastically at low primary excitation energies and it vanishes at high excitation energies. The normalized intensity of the f-f peak for different primary energies is plotted in fig. 3. The loss energy E = 1.33 eV agrees well with the multiplet spfitting of the 4f”-level in Yb determined by Dieke f5]. The small energy width of the f-f peak which is only slightly larger than the total energy resolution of the apparatus indicates a transition between two narrow localized levels. We therefore conclude that the sharp loss peak at 1.33 eV is caused by the dipole forbidden spin flip excitation of the 4fr3-level in Yb,O,. The energy dependence of the normalized 1.33 eV loss intensity supports the interpretatian as a dipole forbidden transition [9]. EELS at high primary energies (high compared to the energy loss) strongly favours dipole allowed transitions [l] whereas at low primary energies excitations via electron exchange gain more and more in intensity so that optically forbidden tr~sitio~ can lead to the most prominent features in EEL-spectra. Notice that the f-f peak in fig. 2 becomes more intense than peak C in fig. 1. The normalized intensity of the f-f transition
A. Gorschliiter et al. / Dipole forbidden f-f excitation in Yb,O,
(fig. 3) shows two weak maxima at 35 and 181 eV. Similar structures have been reported by Della Valle and Modesti [2]. They found that the loss intensity of f-f transitions in rare-earth metals is increased when the primary energy reaches the threshold for the 4d + 4f excitations. This structure was explained by the interference of the direct excitation channel with a second excitation channel (via an intermediate negative ion core state 4d”-’ 4f”+2) which opens at the 4dthreshold. Such a resonant mechanism cannot occur in the case of Yb,O, because the intermediate configuration 4f” + * does not exist. We therefore assume that the weak maxima are due to a decrease of the reflection coefficient of elastically scattered electrons at the threshold of the excitation from the 5p- respectively 4d-level to the conduction band. Similar intensity variations have been reported by Stenborg and Bauer [lo] for ytterbium and samarium. The decrease of the intensity at higher primary energies (fig. 3) is not so pronounced as expected [9]. Therefore it cannot be excluded that mechanisms different from the spin flip exchange scattering process discussed might additionally contribute to the intensity observed.
215
An elaborated experiment with polarized electrons could yield direct information about this aspect. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References [l] F.P. Netzer, G. Strasser and J.A.D. Matthew, Phys. Rev. Lett. 51 (1983) 211. [2] F. Della Valle and S. Modesti, Phys. Rev. B 40 (1989) 933. [3] E. Bauer and J. Kolaczkiewicz, Phys. Status Sohdi 131 (1985) 699.. [4] L.I. Johansson, J.W. Allen, I. Lindau, M.H. Hecht and S.B.M. Hagstrom, Phys. Rev. B 21 (1980) 1408. [5] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals (Wiley, New York, 1968). [6] K. Jost, J. Phys. E 12 (1979) 1006. [7] E. Bertel, G. Strasser, F.P. Netzer and J.A.D. Matthew, Surf. Sci. 118 (1982) 387. [S] B.D. Padaha, J.K. Gimzewski, S. Affrossman, W.C. Lang, L.M. Watson and D.J. Fabian, Surf. Sci. 61 (1976) 468. [9] J. Glazer and E. Tosatti. Solid State Commun. 52 (1984) 905. [lo] A. Stenborg and E. Bauer, Solid State Commun. 66 (1988) 561.