VUV Spectroscopy of Atoms, Molecules and Surfaces

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98 Chapter 5. Femtosecond VUV core-level spectroscopy ... saturation coverage of CO. With a sticking probability of 0.8 [106] the 0.5 ML saturation coverage is obtained after only a few seconds of CO dosing at 2×10 −7 Torr but to be on the safe side the dosing was continued for a few minutes. An eventual offset in the monochromator calibration has not been corrected for, as this was of no relevance in the present context. The 4f 5/2 and 4f 7/2 peaks and the CO on-top features towards higher binding energy are clearly visible in the 130 eV spectrum but unfortunately almost buried in the (extrinsic) secondary-electron background at 98 eV. The 130 eV spectrum is shown for comparison in figure 5.6 (b) also, and the background levels are seen to differ by at least a factor of ∼7 despite a difference in the inelastic mean-free path of at most a factor of 2 (cf. the universal curve). On account of the very large background level the use of Pt for the fs desorption studies can be excluded. 5.4.3 Oxidation of Al(111) Following the negative experience with Pt, attention was drawn to Al which has some very well-defined 2p 1/2 and 2p 3/2 core levels at 73.1 and 72.9 eV that have been the subject of numerous studies at Aarhus University [87]. These studies have, however, mainly concentrated on the theoretically relevant alkali/Al systems which tend to form surface alloys and thus are inappropriate for laser-desorption studies [77]. On the other hand, the oxidation process of Al is not yet fully understood in spite of the very large attention that has been given to this technologically important system. Previous core-level spectroscopic measurements of oxygen adsorption on Al have shown pronounced effects of the oxygen adsorption on the appearance of the Al 2p peaks [107, 108]. This would make the system a good candidate for an experiment with a modest energy resolution. In addition, a photo-desorption experiment using synchrotron radiation has demonstrated that the dissociatively adsorbed oxygen atoms can be ejected as O − ions, folllowing the absorption of 8.7 eV photons [109]. This was ascribed to a direct electronic process on account of the 8.7 eV photon energy being in exact resonance with the transition from the ground-state to a repulsive excited-state potential-energy curve of AlO − . No studies have been made with visible ns- or fs laser light but it may be possible to desorb the O atoms by an electron-mediated process despite an activation energy towards desorption of ∼5 eV [110], at least by using 4-5 eV photons. For comparison, in the oxidation reaction of CO on Ru(0001) studied by Bonn et al. (cf. section 5.2.3) the bond breakage of the dissociatively adsorbed oxygen atoms with a 1.8 eV activation energy could be activated with 1.5 eV fs laser pulses by the electron-mediated mechanism. Following these considerations the O/Al(111) was given priority to a further
5.4 Core-level spectroscopic tests at ASTRID 99 investigation at the SGM I beamline. The Al single-crystal surface, in particular the close-packed Al(111), has been the subject of numerous experimental and theoretical investigations as reviewed up to 1984by Batra and Kleinman [111]. The O2 molecule is known to adsorb dissociatively on the Al surface to form a chemisorbed O/Al phase that is converted into a very stable 3–4nm thick amorphous Al2O3 layer, following an increase in the temperature or oxygen coverage. The electrically insulating oxide layer protects the metal against corrosion [95] and makes Al the most widely used metal in semiconductor devices [112] and magnetic tunneling junctions [113]. Recently, the growth of an oxide layer was suggested as a method for the production of reliable single-electron transistors, providing an effective means of confining the electron in an Al quantum dot [112]. Al2O3 is also the most commonly used support material for the production of 1–10 nm catalyst metal particles, which are deposited in carefully prepared micropores of the oxide [1, 114]. In spite of this, the structures of the different crystallographic phases that can be attained by Al2O3 have still not been mapped out [115] and even the initial oxidation behaviour, the dissociative chemisorption of O2 on Al remains a matter of debate [116, 117, 118, 119, 120]. There is general agreement that for the O/Al(111) system adsorption at room temperature initially leads to an O adlayer of randomly distributed, immobile O atoms, developing into (1×1) O islands as the coverage increases [121]. This chemisorbed phase coexists with the oxide phase which starts to develop at a 0.2 ML coverage, long before the surface is fully covered by the O islands. Around this coverage the island density saturates and further oxygen uptake leads to a growth in the island size. The oxide layer preferentially develops at the interface between the O islands and the bare Al surface, initially in the form of small (∼2 nm diameter) grains as observed in STM images by Trost et al. [116]. The grain formation continues until the surface is completely covered with an amorphous oxide layer, growing in thickness with increasing O2 exposure. In the chemisorbed phase the O atoms have been observed to occupy three-fold hollow sites above the Al(111) surface. No sub-surface oxygen, as required for the oxide formation process, was found below a 1 ML coverage [122, 123]. This is in agreement with recent calculations, pointing towards the on-surface site as energetically preferable below a 1 ML coverage, with a sub-surface tetrahedral site becoming equally favourable as the coverage is increased beyond this value [119]. The above observations are supported by measured core-level spectra which have shown the existence of three so-called sub-oxide peaks and one oxide peak adjacent to the 2p peaks on the high binding-energy side [124, 107, 108]. The three sub-oxide peaks were found to be shifted by 0.49, 0.97
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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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46 High-resolution VUV spectroscopy
Page 58 and 59: 48 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 110 and 111: 100 Chapter 5. Femtosecond VUV core
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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.
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148 The following three chapters ar
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