VUV Spectroscopy of Atoms, Molecules and Surfaces

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100 Chapter 5. Femtosecond VUV core-level spectroscopy ... and 1.46 eV with respect to the 2p 3/2 level at 72.7 eV while the broad oxide peak was shifted by 2.5–2.7 eV (see also figure 5.7) [124]. The three sub-oxide peaks were observed to show up one after the other, following an increase in oxygen coverage, with the 0.49 eV peak appearing first. The latter was hidden by the 2p 1/2 peak at 73.1 eV while the other peaks were clearly visible. The three sub-oxide peaks have been associated with Al atoms binding to one-, two- and three O atoms, respectively, as can be rationalized from the simple electrostatic picture of section 5.3, given the fact that the oxygen atoms represent an electro-negative environment. This corresponds to Al atoms situated at the edges and in the interior of an O island where the coordination numbers to O atoms are one (or two) and three, respectively [121]. In a single high-resolution (50 meV) study, an additional shoulder on the low bindingenergy side of the Al 2p peaks was observed and ascribed to atoms sitting in a metallic environment, most likely in a (mono-atomic) semi-amorphous Al layer situated at the Al-Al2O3 interface [107]. A similar shoulder was recently resolved for the O/Al(100) system and given a similar assignment [87]. In that experiment the oxidation behaviour of Al(100) was observed to be similar to that of Al(111) apart from the three sub-oxide peaks appearing simultaneously. The above core-level measurements have been performed with photon energies in the 100–110 eV range, so the O/Al(111) system was considered a relatively safe candidate for a 96 eV measurement. The controversy regards the mechanism of initial sticking of the O2 molecules to the surface [120] and the observation that the rates of chemisorption (i.e. the sticking probability) and oxide formation vary considerably among different experiments [117]. The oxygen sticking probability of ∼0.005 is abnormally low considering the ∼5 eV chemisorption energy, and its eventual relation to an observed non-thermal behaviour of the dissociating O atoms remains unclear [120, 125]. The sticking probability has been observed to be independent of temperature by Österlund et al. [126] while an increase was found by the group of Yates [117, 127, 125]. Both groups agree on an activated dissociative adsorption process in the 243–600 K regime which, on account of the temperature dependence of the sticking probability, was ascribed by Yates et al. to the presence of an O2 precursor state prior to dissociation [125, 127]. So far further support for a molecular precursor state has not been given experimentally [124] or theoretically [118, 128] but in the latter case this might be attributed to the approximation applied. The experiments of Yates et al. were performed with an Al(111) surface that had been subjected to an extreme amount of sputtering (≥45 hours) and annealing which was found to be necessary in order to obtain a reproducible adsorption behaviour. With this intensive pre-treatment the sticking probability was observed to be lower than that of an ”ordinarily” cleaned surface
5.4 Core-level spectroscopic tests at ASTRID 101 and the conversion from the metastable chemisorbed phase to the oxide phase occured at a higher temperature compared with previous reports [117]. This was attributed to a reduction in the concentration of defect sites with respect to that of an ordinarily cleaned surface [127]. These defects may arise from, e.g., the crude mechanical polishing of the surface prior to application, in which case several atomic layers must be peeled off by the sputter process in order to reach a depth with a lower defect concentration. Remarkably, however, at least 10 hours of sputtering was necessary after each measurement performed with only a sub-monolayer oxygen coverage for reproducible measurements to be obtained. The most obvious explanation for this would be a diffusion of oxygen atoms into the bulk but no further comments were made about this issue. One could, in principle, imagine defects to be introduced by the rather crude sputter treatment but this was apparently not the case, maybe because of the correspondingly prolonged (to 45–60 minutes) annealing treatments performed with 5–10 hours intervals [129]. The annealing process has, however, by STM measurements been observed to induce defects when performed at temperatures above ∼450 ◦ C [129]. The question about the possible influence of defects on the oxidation behaviour has previously been addressed by Larson and Lauderback for the Al(100) surface and they did, in fact, provide evidence for an enhanced chemisorption and oxide formation probability for Al surfaces with a higher defect concentration [130]. The nature of the defects could not be established but the defect density could be varied by changing the mechanical polishing procedure, and for the most defect-ful surfaces a penetration depth on the order of a µm wasestimated. This was rationalized on account of the persistence of defects even after several sputter-anneal cycles. The recognition that even a low concentration of surface defects or -imperfections may have a large influence on the adsorbtion- and desorption behaviour of a surface is not new [131] but the need for an extremely long (≥45 hours) sputtering time for reliable results to be obtained has not been previously reported. This could be attributed to a particularly large sensitivity of the Al oxidation behaviour to the presence of defects or, more likely, the softness of the clean Al metal, making it more vulnerable to mechanical polishing. The purpose of the core-level measurements on O/Al(111) at the SGM I beamline was partly to check whether the secondary-electron background was still well-behaved at a ∼96 eV photon energy, partly to investigate the effect of the surface pre-treatment on the appearance of the Al 2p core-level spectra. The newly installed Al(111) crystal was initially degassed for 48 hours at 400 ◦ C, and then six times repeatedly Ar + sputtered for half an hour at 1–1.5 kV and annealed at 400 ◦ C for 10 minutes. This was followed by a total of 9 hours sputtering during which the crystal was kept at 440 ◦ C
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VUV Spectroscopy of Atoms, Molecule
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ii CONTENTS 4 .6 Conclusion . . . .
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iv Experimental and theoretical stu
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vi me to the principle of negative-
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viii
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2 VUV light generation: possibiliti
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10 VUV light generation: possibilit
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18 REFERENCES [14 ] C. R. Vidal, in
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20 REFERENCES [55] C. J. G. Uiterwa
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22 REFERENCES [98] Parameters from
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24 Negative ions ing the binding en
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26 Negative ions located energetica
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28 Negative ions available has been
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30 Negative ions assignment will th
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32 Negative ions feel the repulsion
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34 Negative ions Figure 2.1: Adiaba
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36 Negative ions
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38 REFERENCES [16] E. Kopp, Adv. Sp
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40 REFERENCES [63] P. Balling et al
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42 REFERENCES
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44 High-resolution VUV spectroscopy
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Page 60 and 61: 50 High-resolution VUV spectroscopy
Page 70 and 71: 60 REFERENCES [19] J. S. Nielsen an
Page 72 and 73: 62 Lifetimes of molecular negative
Page 86 and 87: 76 REFERENCES [19] V. V. Petrunin e
Page 88 and 89: 78 REFERENCES [66] F. Robicheaux, P
Page 90 and 91: 80 Chapter 5. Femtosecond VUV core-
Page 112 and 113: 102 Chapter 5. Femtosecond VUV core
Page 130 and 131: 120 REFERENCES [17] A. Cavalleri et
Page 132 and 133: 122 REFERENCES [61] R. J. Finlay et
Page 134 and 135: 124 REFERENCES [104]R.W.Joyner,inTh
Page 136 and 137: 126 REFERENCES [152] T.-C. Chiang,
Page 138 and 139: 128 Chapter 6. Two-colour pump-prob
Page 154 and 155: 144 REFERENCES [16] Information ava
Page 156 and 157: 146 REFERENCES [62] S. Baier et al.
Page 158 and 159: 148 The following three chapters ar
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