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Coverage-dependence of Pb 5d core level binding energies on Al(111) and Ni(111)
Surface
272
Science 152/153 (1985) 272-277 North-Holland. Amsterdam
COVERAGE-DEPENDENCE OF Pb 5d CORE LEVEL BINDING ENERGIES ON Al(111) AND Ni(ll1) K. GURTLER
Received
and K. JACOBI
1 April 1984; accepted
for pubhcation
18 May 1984
The Pb 5d binding energies were measured by UPS in films evaporated onto Ni(ll1) and Al( 111) and characterized by LEED. AES, and TDS; The measured binding energies are in good agreement with values calculated in a model based on a BorwHaber cycle. The desorptmn energies needed for this calculation are taken from the TDS measurements.
1. Introduction In a previous paper [l] we reported on coverage-dependent shifts of the Pb 5d binding energies (BE) in Pb films evaporated onto Ni( 111) and Al(111) substrates. The BE’s were compared with calculated values based on a Born-Haber cycle [2,3] using semiempirical desorption energies for Pb and Bi on Ni(ll1) [4]. The agreement with the experimental values was unexpectedly good. Because of the importance of this model for the understanding of the relaxation mechanism in photoemission we measured the desorption energies of Pb and Bi on Ni(ll1). Having then all four quantities used in the Born-Haber cycle determined experimentally establishes a safe ground for the discussion of the model. 2. Experimental All experiments were performed in a UHV chamber equipped with an Ar-ion gun, a LEED optics, a cylindrical mirror analyser for Auger spectroscopy (AES) and a quadrupole mass spectrometer [5]. Details of sample preparation and film evaporation were the same as described earlier [l]. 3. Results 3.1. UPS measurements Fig. 1 shows the BE of the Pb 5d,,, core level on Ni(l11) as function of the Pb coverage. At low coverage the BE is smaller than the bulk value and 0039-6028/85/$03.30 @j Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
K. Giirtler, K. Jacobi / Pb 5d core level binding energies
213
18.0
Pb 5d5/2
17.8
,2 “LLIrn
0
UPS
x
BHC
17.6
X 17.4 I I
1 COVERAGE
3
2 Q /ML
Fig. 1. Pb 5d,,, binding energies E, referenced to the Fermi level as a function of coverage in units of monolayer (ML). (0) From the UPS experiment (hv = 40.8 eV). (X) Calculated for the Born-Haber cycle model including the desorption energies from the TDS experiment (see text).
SUBSTRATE ADLAYER
WITH OF
Z-ATOMS
Fig. 2. Born-Haber cycle for the calculation of the core level binding energy EC referenced to the Fermi level. EA is the adsorption (or sublimiation) energy, Ex the core level binding energy for the atom referenced to the vacuum level and I, the first ionization potential. The asterisk in Z* indicates a Z atom with a core hole.
continuously shifts reaching the bulk value at about 2 ML. For further details see our previous work [l]. The BE can be calculated by a BornHaber cycle as sketched in fig. 2. The initial state is a layer of Pb on Ni(ll1). To describe the transition into the photoemission final state the following steps are virtually separated. Pb is desorbed by applying the desorption energy E,(Z). Then it is core-ionized (Z* + e) by applying the gas phase value of BE El( Z, gas). approximation and complete final state Assuming the “equivalent-core” screening the ionization energy 1, (Z + 1) is gained. Finally the neutral (Z + 1) atom (Bi) is adsorbed thereby releasing the adsorption energy E,\( Z + 1). Obviously, the coverage dependence of the BE is contained in a coverage dependence of the desorption energies. In the next sections we report on TDS of Pb and Bi on Ni( 111) and a characterization of the overlayer structures by AES and LEED. 3.2. A ES und LEED
measurements
AES intensities were measured as function of evaporation time showing a behaviour attributed to Stranski-Krastanov growth mode, i.e. to three-dimensional growth of crystallites on top of the first monolayer. For Pb and Bi several ordered overlayers were found. Pb showed (43 X fi)R30”, (3 X 3). (7 X 7) and (4 X 4) structures and Bi a (43 X 43 )R30”. (7 x 7) and a (\/7/4 X J7/4 )R19” overlayer structure. These structures indicate hexagonal packing with decreasing next neighbour distance as function of coverage [ 1,6]. 3.3. Thermul desorption spectroscop) Fig. 3 shows TD spectra for Pb/Ni( 111) and Bi/Ni( 111). At low coverages Pb desorbs at about 1040 K. Bi at 1220 K. With increasing coverage the desorption maximum shifts to lower temperature. The peaks shape after the desorption maximum was nearly independent of coverage (indicated by error bars). The reproducibility of the high temperature peak was about 20 K. For higher coverages a strong peak at 670 K for Pb and 685 K for Bi appears. With increasing coverage only this peak grows and the peak shape becomes asymmetric. The shape is characteristic for a zero-order desorption as expected for desorption from a 3-dimensional crystal. For zero-order desorption the activation energy EA was determined by Arrhenius plots as: E,, (Pb) = 188 kJ/mol
= 1.95 eV/atom,
EA(Bi) = 197 kJ/mol
= 2.04 eV/atom.
Both values agree well with the heats of sublimation for the bulk metals [4]. King and Adams [7,8] showed that repulsive lateral interaction may lower the
275
K. Giirtler, K. Jacobi / Pb Sd core level bindrng energie.s
desorption energy. Lateral interaction should increase with coverage. seems plausible because also LEED shows closer packing of adlayers increasing coverage.
Tabte 1 Desorption
temperatures
Tp and energies
EA for Pb and Bi on Ni(lll)
at different
units of monolayers (ML)
Pb/Ni(lll)
(ML)
Bi/Ni(lll)
Tp
0.1
0.56 0.88 1 >I
F4
W
(eW
1040 980 740 700 670
3.00 2.93 2.19 2.07 1.95
1070 810 750 685
a PblNi(lll1 P=LKls
I
600
1
700
t
800
TEMPERATURE
f
900 I K
i
I
1000
it00
3.13 2.40 2.22 2.05
coverages
This with
B in
b Bi / Ni (111 1 fi=lKls
600
800
700
900
TEMPERATURE Fig. 3. Thermal
1100
1000
f200
1300
I K
desorption spectra for Ph (a) and Bi (h) on NI(I 11). The heating rate is ,8 = 4 K/s.
The observed LEED
patterns are indioated.
For coverages in the monolayer region the EA values shown in table 1 were calcualted by Redhead’s equation [9] assuming first order desorption and a coverage independent frequency factor. This is a strong assumption but it seems probable that the value of Y and any possible coverage dependence of it is very similar for Pb and Bi. For determining the desorption energy Y= 1.7 x 1014 s-’ was used. This value is necessary to obtain the bulk sublimation energy for Pb using Redhead’s equations. Using these experiment~iy determined desorption energies the BH cycle calculation was repeated. In fig. 1 the calculated and experimental EL values are compared. There is excellent agreement between measurement and calculation. 4. Summary and conclusion The desorption energy of Bi/Ni(lll) depends more strongly on coverage than the desorption energy of Pb/Ni(lll). With the measured desorption
K. Giirtler, K. Jacobi / Pb 5d core level binding energies
277
energies a calculation of the Pb 5d BE using a Born-Haber cycle is possible. This model gives correct BE’s for high coverages as well as for coverages in the ML region. Therefore, we conclude that the Pb 5d photohole is completely screened by the free charges of the substrate so that the final state of Pb+ can be approximated by a neutral Bi atom. This is the main contribution to the relaxation in the bulk. Small changes in the bonding (desorption) energy contribute less but are responsible for the small coverage-dependent shift of BE. For Al(111) no shift of BE with coverage was observed. In the light of the BH cycle model this indicates a coverage-independent difference in EA for Pb and Bi on Al(111). Therefore, we expect one single ordered overlayer structure also for Bi/Al( 111) as it was found for Pb/Al( 111).
Acknowledgements
We thank Professor H. Gerischer for his interest and support and P. Geng for valuable technical assistance. This work was supported by the Deutsche Forschungsgemeinschaft (Sfb 6).
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
K. Girtler and K. Jacobi, Surface Sci. 134 (1983) 309. B. Johansson and N. Martensson, Phys. Rev. B21 (1980) 4427. D. Tomanek, P.A. Dowben and M. Grunze, Surface Sci. 126 (1983) 112 A.R. Miedema and J.W.F. Dorleijn, Surface Sci. 95 (1980) 447. D. P&s, W. Ranke and K. Jacobi, Surface Sci. 105 (1981) 77. K. Giirtler and K. Jacobi, to be published. D.L. Adams, Surface Sci. 42 (1974) 12. D. King, Surface Sci. 47 (1975) 384. P.A. Redhead, Vacuum 12 (1962) 203.
272
Science 152/153 (1985) 272-277 North-Holland. Amsterdam
COVERAGE-DEPENDENCE OF Pb 5d CORE LEVEL BINDING ENERGIES ON Al(111) AND Ni(ll1) K. GURTLER
Received
and K. JACOBI
1 April 1984; accepted
for pubhcation
18 May 1984
The Pb 5d binding energies were measured by UPS in films evaporated onto Ni(ll1) and Al( 111) and characterized by LEED. AES, and TDS; The measured binding energies are in good agreement with values calculated in a model based on a BorwHaber cycle. The desorptmn energies needed for this calculation are taken from the TDS measurements.
1. Introduction In a previous paper [l] we reported on coverage-dependent shifts of the Pb 5d binding energies (BE) in Pb films evaporated onto Ni( 111) and Al(111) substrates. The BE’s were compared with calculated values based on a Born-Haber cycle [2,3] using semiempirical desorption energies for Pb and Bi on Ni(ll1) [4]. The agreement with the experimental values was unexpectedly good. Because of the importance of this model for the understanding of the relaxation mechanism in photoemission we measured the desorption energies of Pb and Bi on Ni(ll1). Having then all four quantities used in the Born-Haber cycle determined experimentally establishes a safe ground for the discussion of the model. 2. Experimental All experiments were performed in a UHV chamber equipped with an Ar-ion gun, a LEED optics, a cylindrical mirror analyser for Auger spectroscopy (AES) and a quadrupole mass spectrometer [5]. Details of sample preparation and film evaporation were the same as described earlier [l]. 3. Results 3.1. UPS measurements Fig. 1 shows the BE of the Pb 5d,,, core level on Ni(l11) as function of the Pb coverage. At low coverage the BE is smaller than the bulk value and 0039-6028/85/$03.30 @j Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
K. Giirtler, K. Jacobi / Pb 5d core level binding energies
213
18.0
Pb 5d5/2
17.8
,2 “LLIrn
0
UPS
x
BHC
17.6
X 17.4 I I
1 COVERAGE
3
2 Q /ML
Fig. 1. Pb 5d,,, binding energies E, referenced to the Fermi level as a function of coverage in units of monolayer (ML). (0) From the UPS experiment (hv = 40.8 eV). (X) Calculated for the Born-Haber cycle model including the desorption energies from the TDS experiment (see text).
SUBSTRATE ADLAYER
WITH OF
Z-ATOMS
Fig. 2. Born-Haber cycle for the calculation of the core level binding energy EC referenced to the Fermi level. EA is the adsorption (or sublimiation) energy, Ex the core level binding energy for the atom referenced to the vacuum level and I, the first ionization potential. The asterisk in Z* indicates a Z atom with a core hole.
continuously shifts reaching the bulk value at about 2 ML. For further details see our previous work [l]. The BE can be calculated by a BornHaber cycle as sketched in fig. 2. The initial state is a layer of Pb on Ni(ll1). To describe the transition into the photoemission final state the following steps are virtually separated. Pb is desorbed by applying the desorption energy E,(Z). Then it is core-ionized (Z* + e) by applying the gas phase value of BE El( Z, gas). approximation and complete final state Assuming the “equivalent-core” screening the ionization energy 1, (Z + 1) is gained. Finally the neutral (Z + 1) atom (Bi) is adsorbed thereby releasing the adsorption energy E,\( Z + 1). Obviously, the coverage dependence of the BE is contained in a coverage dependence of the desorption energies. In the next sections we report on TDS of Pb and Bi on Ni( 111) and a characterization of the overlayer structures by AES and LEED. 3.2. A ES und LEED
measurements
AES intensities were measured as function of evaporation time showing a behaviour attributed to Stranski-Krastanov growth mode, i.e. to three-dimensional growth of crystallites on top of the first monolayer. For Pb and Bi several ordered overlayers were found. Pb showed (43 X fi)R30”, (3 X 3). (7 X 7) and (4 X 4) structures and Bi a (43 X 43 )R30”. (7 x 7) and a (\/7/4 X J7/4 )R19” overlayer structure. These structures indicate hexagonal packing with decreasing next neighbour distance as function of coverage [ 1,6]. 3.3. Thermul desorption spectroscop) Fig. 3 shows TD spectra for Pb/Ni( 111) and Bi/Ni( 111). At low coverages Pb desorbs at about 1040 K. Bi at 1220 K. With increasing coverage the desorption maximum shifts to lower temperature. The peaks shape after the desorption maximum was nearly independent of coverage (indicated by error bars). The reproducibility of the high temperature peak was about 20 K. For higher coverages a strong peak at 670 K for Pb and 685 K for Bi appears. With increasing coverage only this peak grows and the peak shape becomes asymmetric. The shape is characteristic for a zero-order desorption as expected for desorption from a 3-dimensional crystal. For zero-order desorption the activation energy EA was determined by Arrhenius plots as: E,, (Pb) = 188 kJ/mol
= 1.95 eV/atom,
EA(Bi) = 197 kJ/mol
= 2.04 eV/atom.
Both values agree well with the heats of sublimation for the bulk metals [4]. King and Adams [7,8] showed that repulsive lateral interaction may lower the
275
K. Giirtler, K. Jacobi / Pb Sd core level bindrng energie.s
desorption energy. Lateral interaction should increase with coverage. seems plausible because also LEED shows closer packing of adlayers increasing coverage.
Tabte 1 Desorption
temperatures
Tp and energies
EA for Pb and Bi on Ni(lll)
at different
units of monolayers (ML)
Pb/Ni(lll)
(ML)
Bi/Ni(lll)
Tp
0.1
0.56 0.88 1 >I
F4
W
(eW
1040 980 740 700 670
3.00 2.93 2.19 2.07 1.95
1070 810 750 685
a PblNi(lll1 P=LKls
I
600
1
700
t
800
TEMPERATURE
f
900 I K
i
I
1000
it00
3.13 2.40 2.22 2.05
coverages
This with
B in
b Bi / Ni (111 1 fi=lKls
600
800
700
900
TEMPERATURE Fig. 3. Thermal
1100
1000
f200
1300
I K
desorption spectra for Ph (a) and Bi (h) on NI(I 11). The heating rate is ,8 = 4 K/s.
The observed LEED
patterns are indioated.
For coverages in the monolayer region the EA values shown in table 1 were calcualted by Redhead’s equation [9] assuming first order desorption and a coverage independent frequency factor. This is a strong assumption but it seems probable that the value of Y and any possible coverage dependence of it is very similar for Pb and Bi. For determining the desorption energy Y= 1.7 x 1014 s-’ was used. This value is necessary to obtain the bulk sublimation energy for Pb using Redhead’s equations. Using these experiment~iy determined desorption energies the BH cycle calculation was repeated. In fig. 1 the calculated and experimental EL values are compared. There is excellent agreement between measurement and calculation. 4. Summary and conclusion The desorption energy of Bi/Ni(lll) depends more strongly on coverage than the desorption energy of Pb/Ni(lll). With the measured desorption
K. Giirtler, K. Jacobi / Pb 5d core level binding energies
277
energies a calculation of the Pb 5d BE using a Born-Haber cycle is possible. This model gives correct BE’s for high coverages as well as for coverages in the ML region. Therefore, we conclude that the Pb 5d photohole is completely screened by the free charges of the substrate so that the final state of Pb+ can be approximated by a neutral Bi atom. This is the main contribution to the relaxation in the bulk. Small changes in the bonding (desorption) energy contribute less but are responsible for the small coverage-dependent shift of BE. For Al(111) no shift of BE with coverage was observed. In the light of the BH cycle model this indicates a coverage-independent difference in EA for Pb and Bi on Al(111). Therefore, we expect one single ordered overlayer structure also for Bi/Al( 111) as it was found for Pb/Al( 111).
Acknowledgements
We thank Professor H. Gerischer for his interest and support and P. Geng for valuable technical assistance. This work was supported by the Deutsche Forschungsgemeinschaft (Sfb 6).
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
K. Girtler and K. Jacobi, Surface Sci. 134 (1983) 309. B. Johansson and N. Martensson, Phys. Rev. B21 (1980) 4427. D. Tomanek, P.A. Dowben and M. Grunze, Surface Sci. 126 (1983) 112 A.R. Miedema and J.W.F. Dorleijn, Surface Sci. 95 (1980) 447. D. P&s, W. Ranke and K. Jacobi, Surface Sci. 105 (1981) 77. K. Giirtler and K. Jacobi, to be published. D.L. Adams, Surface Sci. 42 (1974) 12. D. King, Surface Sci. 47 (1975) 384. P.A. Redhead, Vacuum 12 (1962) 203.