VUV Spectroscopy of Atoms, Molecules and Surfaces

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94 Chapter 5. Femtosecond VUV core-level spectroscopy ... Slit carousel Lens elements: Retarding Focusing Grounded e - Crystal V Deflectors X-ray Hemispheres Field termination net MCP Fluorescent screen CCD Figure 5.5: A schematic of the SCIENTA photoelectron spectrometer at the SGM I beamline at ASTRID. From [87]. sputter process, the upper surface layers are removed by bombardment with ∼1 kVAr + ions at a ∼1×10−6 Torr pressure, thereby generating craters and implanting Ar + ions into the surface [91]. The craters are expected to be of ∼nm dimensions for the relatively low sputter energy applied here and may develop into regular patterns, especially for sputtering at an angle near normal incidence [92]. These features are removed by the annealing process which, in turn, may induce diffusion and segregation of bulk contaminants to the surface [93, 94]. As a consequence, several sputter-annealing cycles are required to clean a newly installed, mechanically polished crystal, and the total amount of cleaning required depends on the nature and the prehistory of the crystal. For a newly installed Al crystal, for example, a relatively large effort must be invested initially in order to penetrate a 3–4nm protective oxide layer [95]. Recently, it has been demonstrated that an extremely long sputtering time is required to produce an Al surface that exhibits a reproducible O2 adsorption behaviour, as will be discussed in section 5.4.3. In the present experiments a sputter-annealing cycle typically consisted of 45 min sputtering with 1 kV Ar + ions at 1×10−6 Torr Ar pressure followed by 15 min annealing to ∼600◦ Cand∼450◦ C for Pt and Al, respectively. The Ar + ion beam is incident on the surface at a 45◦ angle with respect to the surface normal. The annealing temperature was chosen with the melting
5.4 Core-level spectroscopic tests at ASTRID 95 temperatures of the surfaces in mind and should be high enough that the craters induced by the sputtering process are smoothened out and low enough that disordering and defect formation at the surface are avoided. The Pt surface was annealed in a 2×10 −8 Torr oxygen atmosphere (which has been proposed as a method for the removal of carbon impurities [96]), followed by a couple of minutes heating to ∼900 ◦ C. The cleanness was checked by looking for core-level peaks from expected contaminants and by checking whether the core-levels to be measured for the clean surface exhibited signatures of contamination (shoulders or additional peaks). The Pt and Al crystals were mounted on Ta and Si sample holders, respectively, and were resistively heated through a couple of wires attached to these holders. The temperature was measured with an n-type thermo-couple attached to the rear side of the sample holder and could be kept at the desired value to within a few degrees by a feedback mechanism. There was, however, problems with several degrees overshoot during initial temperature ramping, especially, for the Si mount where the resistivity undergoes a sudden decrease at a certain ”breakthrough” temperature, corresponding to electronhole pair excitations across the band-gap. The crystals could be cooled down to ∼−110 ◦ C by thermal contact with a liquid N2 reservoir through a flexible copper braid fastened to the rear side of the sample holder. The core-level spectra were recorded at this temperature in order to minimize the phonon broadening, and CO was dosed at this temperature while O2 was dosed at room temperature. Both CO and O2 bind to Pt and Al, respectively, at room temperature, but the CO/Pt(111) system has been most intensively studied in the low-temperature regime. 5.4.2 CO/Pt(111) As outlined in section 5.3, the magnitude of the extrinsic background level of a core-level spectrum exhibitits a minimum if core-level electrons are photoionized with kinetic energies around ∼50 eV. This ideal condition is difficult to fulfil with the ∼100 eV photon energy aimed at for the fs studies. More precisely, a 96.4eV photon energy has been chosen (the 63rd harmonic of the 810 nm Ti:sapphire fs laser), considering the ∼99 eV absorption edge of the Si/Mo multilayer mirror intended for the setup. Since the inelastic mean-free path is still relatively short (
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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
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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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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