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AP-PH (a.u.)
0.4
0.3
0.2
0.1
0.0
0 5 10 15 20 25 30 35 40 45
Ba deposition (min)
Figure 2 The AP-PHs of the Ba(75 eV), BaO(68
eV) and O(510 eV) Auger transition lines as a
function of the Ba deposition on the oxygen
chemisorbed phase O(2×1)/Ni(110).
In Fig. 2, the AP-PHs of the Ba(75 eV), BaO(68 eV) and O(510
eV) Auger transition lines are shown as a function of the Ba deposition
time. The BaO(68 eV) Auger line is attributed to an interatomic
transition involving the Ba 4d, Ba 5p and O 2p atomic levels, while the
Ba(75 eV) is due to the Ba 4d, Ba 5p and Ba 6s levels [8,9]. Both of
these lines are valence band characteristic Auger lines and so are very
sensitive to the chemical changes of the surface, induced by the
oxidation of Ba. It is noteworthy that while the Ba(75 eV) line is
characteristic of pure Ba development on surfaces, it appears also for
BaO growth on surface [10]. This can be explained by the interaction
of some Ba adatoms directly with the Ni atoms. The Ba(75 eV) line
increases until ~14 min, while the BaO(68 eV) one increases until ~8
min and remains almost constant for further deposition. The BaO(68
eV) line declares the formation of BaO as a result of interaction
between the Ba adatoms and the chemisorbed O atoms. On the other
hand, the O(510 eV) line shows a small initial increase until about the
time where the BaO(68 eV) line maximizes. This might be due to an
“upwards” movement of the chemisorbed O atoms reacting with the
Ba ones forming BaO on the surface. This rearrangement of the O atoms might be the cause of the nickel surface
dereconstruction, causing an initial slower rate reduction of the Ni(848 eV) line as observed in Fig. 1 and described above.
When the BaO (68 eV) line stops increasing, we observe a more rapid Ba(75 eV) peak growth up to ~14 min. This is where
the first Ba layer is completed according to the results shown in Fig. 1. At the same time, an almost constant O(510 eV) peak
decrease is observed until high Ba deposition times. Further Ba deposition shows no Ba signal increase. This behavior can be
attributed to the smaller escape depth of the Auger electron than the Ba
overlayer thickness. The fact that the Ba(68 eV) line maximizes before the
completion of the first Ba layer, means that the BaO overlayer is
incomplete, probably interspersed by Ba adatoms reacting directly with Ni
atoms. It is noteworthy that the BaO(68 eV) and Ba(75 eV) Auger lines
show a drift to smaller energies as a function of Ba deposition as it is
shown in Fig. 3. In general, the Auger electron energy shifts (AEES) are
related to changes in the chemical state or environment of the surface
atoms. Also, according to our recent work [11], initial and final state
effects of the three levels involved play an important role to these energy
shifts. By combining the observed AEES and our own previous x-ray
photoemission measurements of low core Ba and O atomic levels, as well
as of the valence band states, we conclude that the AEES are due to the
extra-atomic relaxation effects and electron-electron interactions [12].
Comparing the present AEES, with previous ones of BaO formation by the
oxidation of bariated nickel surface [13], we conclude that the Ba-O
interaction depends on the substrate, because initial state effects such as
the Madelung potential and final state effects such as extra-atomic
relaxation are determine by the substrate.
4. Conclusions
The Ba adsorption on the oxygen chemisorbed O(2×1)/Ni(110) surface, results in an incomplete layer of BaO
interspersed by chemisorbed Ba adatoms reacting directly with Ni atoms. The second layer of Ba seems to approach the
metallic phase. The low energy Auger transition lines of Ba(75 eV) and BaO(68 eV) shift towards lower energy with Ba
coverage due to both initial and final state effects.
References
Ba/O(2x1)/Ni(110)
Ba(75 eV)
O(510 eV)
BaO(68 eV)
66
0 5 10 15 20 25 30 35 40 45
Figure 3 The energy of the Ba(73 eV) and
BaO(68 eV) Auger transition lines for Ba
deposition on the oxygen chemisorbed phase
O(2×1)/Ni(110).
[1] C. Gaertner and D. den Engelsen, Appl. Surf. Sci. 251 (2005) 24 and references therein.
[2] A. Shih, J.E. Yater and C. Hor, Appl. Surf. Sci. 242 (2005) 35 and references therein.
[3] S. Matsumoto, Catal. Today 29 (1996) 43.
[4] P. Stone, M. Ishii and M. Bowker, Surf. Sci. 537 (2003) 179.
[5] D. Vlachos, S.D. Foulias, S. Kennou, C. Pappas and C. Papageorgopoulos, Surf. Sci. 331-333 (1995) 673.
[6] C. Benndorf, B. Egert, C. Nöbl, H. Seidel and F. Thieme, Surf. Sci. 92 (1980) 636.
[7] M. Kamaratos, D. Vlachos and S.D. Foulias, Surf. Rev. & Lett. 12 (2005) 721.
[8] G.A. Haas, C.R.K. Marrian and A. Shih, Appl. Surf. Sci. 16 (1983) 125.
[9] G.A. Haas, R.E. Thomas, A. Shih and C.R.K. Marrian, Appl. Surf. Sci. 40 (1989) 265.
[10] G.A. Haas and A. Shih, Appl. Surf. Sci. 31 (1988) 239.
[11] D. Vlachos, M. Kamaratos and S.D. Foulias, J. Phys.:Cond. Matter. 18 (2006) 6997.
[12] D. Vlachos, S.D. Foulias and M. Kamaratos, to be submitted.
[13] D. Vlachos, N. Panagiotides and S.D. Foulias, J. Phys.:Cond. Matter. 15 (2003) 8195.
Kinetic energy (eV)
75
74
73
72
71
70
69
68
67
Ba deposition time (min)
Ba/O(2x1)/Ni(110)
Ba(75 eV)
BaO(68 eV)
157