VUV Spectroscopy of Atoms, Molecules and Surfaces

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92 Chapter 5. Femtosecond VUV core-level spectroscopy ... plying an increase in the core-level binding energy for a metal with a less than half filled valence-band and vice versa. For the transition metals with a partly filled d-shell the valence-band narrowing is dominating while for the simple sp metals the two contributions are almost equal, resulting in a small (∼100 meV) shift towards lower binding energy for Al(100) and no shift for Al(111) [83]. The natural width of a core-level depends on the probability that the corehole will be filled by an electron decaying from an outer (valence) shell. This probability is, in general, larger the larger the number of electrons occupying the outer shells but this picture may be modified by correlation effects and particularly strong decay channels [84]. Typically, the core-level widths vary in the 0.1–2 eV range between different elements with comparable core-level binding energies [85]. For example, the Al 2p level with a core-level binding energy of ∼73 eV has an unusually narrow width of ∼50 meV, as reflected by the presence of only three electrons in higher-lying shells. At the other extreme, the widths of the 3p and 4p levels of the first- and second-row transition elements, spanning a 30–90 eV binding-energy range, are 2–4eV, with the lowest value attained at the left of the periodic table [85, 86]. The natural Lorentzian width is often masked by Gaussian contributions from phonon broadening and the experimental resolution of the electron-energy analyzer (each contributing at least ∼50 meV) [87]. In addition, there will be a tail on the low-kinetic energy side of the core-level peak arising from intrinsic- and extrinsic excitation processes taking place during- and after the photoionization process, respectively. The most important intrinsic losses are due to electron-hole pair excitations which give rise to an asymmetry of the core-level peak. The extrinsic losses arise from inelastic scatterings of the photoelectrons travelling towards the surface and their contribution will be minimum when the escape depth is smallest possible, i.e. for a ∼50 eV kinetic energy of the core-ionized electrons (which contribute the most to the spectrum). Considering all these contributions, a measured core-level peak is most often fitted with the so-called Doniach-Sunjic line-shape [88] D(E) = E−EB Γ(1 − α)cos[πα/2+(1−α)arctan(−γ )] (1 + (E−EB)2 γ2 ) (1−α)/2 (5.1) convoved with a Gaussian representing the combined effects of the phonon broadening and experimental resolution, and added to a polynomial background representing the intrinsic- and extrinsic losses. Here, EB is the core-level binding energy, γ the natural width (HWHM), α an asymmetry parameter accounting for electron-hole pair excitations and Γ the Gamma function.
5.4 Core-level spectroscopic tests at ASTRID 93 5.4 Core-level spectroscopic tests at ASTRID 5.4.1 Experimental setup The core-level spectroscopic tests of the CO/Pt and O/Al systems were performed at the SGM I synchrotron-radiation beamline at ASTRID. SGM refers to the Spherical Grating Monochromator of the beamline which can be operated in the 25–300 eV photon-energy range, using three sets of spherical gratings and plane mirrors [89]. For the present experiments the monochromator was operated with entrance/exit-slit settings of 50/50 µm and 100/100 µm, corresponding to 30 meV and 50 meV resolutions (FWHM), respectively, for a ∼100 eV photon energy. A target chamber is installed at the end of the beamline and the sample can be moved to a pre-chamber with standard equipment for surface preparation and -characterization. This includes an ion sputter gun, a quadropole mass spectrometer (QMS) and LEED apparatus. The horizontally polarized synchrotron-radiation beam is incident on the surface at 40 ◦ with respect to the surface normal and photoelectrons emitted within a ∼0.1 steradian space angle normal to the surface are colleced with a SCIENTA electron spectrometer [90], as shown in figure 5.5. The electrons are focused onto the entrance slit of the spectrometer by a lens system and are dispersed according to their kinetic energy by the radial electrostatic field between the hemispheres. The lens system in addition retards the electrons to a pre-determined pass energy, Epass, which is proportional to the voltage difference between the two concentric spheres. Due to the spherical symmetry of the spectrometer, the entrance slit is imaged onto the detector system, consisting of a Microchannel-Plate Detector (MCP) and a Charged-Coupled Device (CCD) camera. There is thus no need for an exit slit and energies within a window of size Epass/10 can be simultaneously detected. The energy resolution ∆Espec (FWHM) of the spectrometer is given by the width ∆r of the entrance slit and the pass energy as ∆Espec =∆r/(Epass2r) wherer = 20 cm is the mean radius of the hemispheres. The ultimate energy resolution that can be achieved is ∼5 meV but for the present purpose the spectrometer was operated with a resolution in the 40–150 meV range. The base pressure of the target chamber is ∼1×10 −10 Torr and has to be at least that good if the surface is to be kept clean during a few hours of measurement. Gas doses are usually given in Langmuir, with 1 L = 1×10 −6 Torr·s, which is approximately the amount required for a monolayer coverage of rest gas (N2), assuming a sticking probability of unity. This means that if every (rest) gas molecule hitting the surface stays there, a monolayer coverage is obtained in one second at a pressure of 1×10 −6 Torr! A surface therefore has to be cleaned every day before use, which was done by Ar + sputtering followed by annealing to elevated temperatures. By the
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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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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
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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
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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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