VUV Spectroscopy of Atoms, Molecules and Surfaces

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90 Chapter 5. Femtosecond VUV core-level spectroscopy ... Figure 5.3: A schematic of the relation between the energy-level diagram of the solid and the kinetic-energy spectrum of the photoelectrons emitted in core-level spectroscopy, using a VUV light source with photon energy �ω. The work function of the solid and the electronic density of states evaluated as a function of electronic energy E are denoted by φ and N(E), respectively. From [70]. the surface is illuminated with a VUV- or X-ray beam of fixed photon energy and a kinetic-energy spectrum of the emitted photoelectrons is recorded. The kinetic energies of the photoelectrons are given (with reference to the vacuum level) by Ekin = �ω − EB − φ where �ω is the photon energy, EB the binding energy of the electron with respect to the Fermi level and φ the work function of the surface. Ideally, this results in a translation of the energy spectrum of the adsorbate-substrate complex into the continuum, as shown in figure 5.3 [70]. If �ω is sufficiently large this spectrum will include some of the core-levels of the elements present at the surface, thus facilitating a sensitive chemical analysis of the surface. Furthermore, the core-level binding energy of the adsorbate- or substrate atom will be affected by its chemical surroundings, signified by a meV–eV ”chemical shift”. The shift
5.3 The technique of core-level spectroscopy 91 Figure 5.4: A schematic of the valence-band narrowing at the bulk-surface boundary, which implies (a) an increase in the core-level binding energy for a less than half-filled valence band and (b) a decrease in the core-level binding energy for a more than half-filled valence band. From [81]. arises from the redistribution of the electronic charge in the valence orbitals due to the chemical bonding. By the many-body nature of the surface, the calculations of these shifts represent a challenge to theorists which resort to density-functional theory [79] or the (MC)SCF methods [80] (cf. chapter 2). Often, use is made of the equivalent cores approximation by which the finalstate energy of a Z-electron atom with a core-hole is replaced by the more easily calculated ground-state energy of a Z + 1-electron atom [71, 80]. Correctly, the core-level binding energy is given by the energy difference between the total initial- and final-state energies, i.e. the electronic energies before and after the ionization. Most difficult is the final-state calculation where relaxation effects must be taken into account, arising from the rearrangements of the outer electrons to screen the core-hole. By a simple electrostatic consideration, the core-level binding energy can be expected to be higher for an atom sitting in an electro-negative environment: the removal of an electronic charge δe from the valence shell with radius r causes the binding energy of an inner electron with elementary charge e to increase by the amount eδe/r [82]. For an atom bound in the surface layer there will be a spill-out of valence charge into the vacuum (or air), i.e. the vacuum acts as an electro-negative environment, and an increase in core-level binding energy should be expected. For a metal, the narrowing of the valence band at the surface with respect to that of the bulk will give rise to an additional contribution as shown in figure 5.4. Since the electronic charge must be (approximately) conserved across the bulk-surface region, the surface valence band must adjust to align its Fermi energy with that of the bulk, resulting in down- and up-ward shifts for less- and more than half-filled valence bands, respectively [81]. This shift will be partly followed by the core-levels, im-
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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
Page 50 and 51: 40 REFERENCES [63] P. Balling et al
Page 52 and 53: 42 REFERENCES
Page 54 and 55: 44 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
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
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140 Chapter 6. Two-colour pump-prob
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142 Chapter 6. Two-colour pump-prob
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144 REFERENCES [16] Information ava
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146 REFERENCES [62] S. Baier et al.
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148 The following three chapters ar
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VUV Spectroscopy of Atoms, Molecules and Surfaces

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