VUV Spectroscopy of Atoms, Molecules and Surfaces

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104 Chapter 5. Femtosecond VUV core-level spectroscopy ... Intensity (arb. units) 77 430 o 250 o 50 o 20 o 76 75 74 Binding energy (eV) Figure 5.9: From below: Al 2p core-level spectra at 95 eVof the clean Al(111) surface following a long-term sputtering, the surface after 20 L oxygen exposure at room temperature (20 ◦ ) and the surface after ∼10 minutes heating to 50 ◦ , 250 ◦ and 430 ◦ . 5.8. The background level is seen to be considerably lower than that of figure 5.7 and the Al 2p peaks are no longer broadened. The 95 eV spectrum is depicted in figure 5.9 together with spectra measured after 20 L of O2 dosing. The 20 ◦ spectrum was recorded (at −110 ◦ C) immediately after O2 dosing while the following spectra were measured (at −110 ◦ C) after the surface had been heated to the indicated temperature for ∼10 minutes. The 20 ◦ spectrum does not look like anything which has previously been observed at 20 L coverage, but the well-known characteristics of the chemisorbed phase (cf. figure 5.7) are recovered in the 50 ◦ spectrum. The 250 ◦ spectrum again 73 72
5.5 The VUV light source of high-order harmonics 105 looks very different from what has previously been observed while the 430 ◦ spectrum exhibits the characteristic behaviour of an oxide-covered Al surface. The 20 ◦ spectrum looks so peculiar that one might think that something has gone wrong during the measurement, for example that the sample has not been properly grounded. This issue was checked at a subsequent beamtime where a long-term sputtering of a new crystal was attempted. Unfortunately, the sputter gun was not sufficiently stable that the long-term pre-treatment could be done but the grounding issue was checked and the spectra found to be identical, independent of the apparent gounding conditions. The interesting features of the 20 ◦ C spectrum are the significant suppresion of all of the peaks despite the very low coverage and the appearance of a peak at the low binding-energy side, shifted by ∼0.5 eV with respect to the 2p 3/2 peak at 72.7 eV. A shift in this direction can only be attributed to Al atoms which are entirely Al-coordinated since the electro-negative environment of O atoms would work in the opposite direction. As described above, a shoulder at the low binding-energy side of the of the 2p 3/2 peak was previously observed by Berg et al. and tentatively assigned to a (mono-atomic) semi-amorphous Al layer at the Al-Al2O3 interface. The peak responsible for this shoulder represented a chemical shift of only 0.14eV with respect to the 2p 3/2 peak and it is at present unclear whether the present peak could be attributed to this layer also. This issue will hopefully be clarified by the peak fitting which remains to be done, and eventual input from theorical calculations. Also, the experiment should be repeated on a new long-term sputtered surface to see if the spectra are reproducible, and spectra should be recorded at a couple of additional, more surface-sensitive, photon energies to be able to estimate the order of the crystal ”layers” responsible for the different peaks. In addition, it would be interesting to investigate the characteristics of the core-level peaks as a function of oxygen coverage and to investigate how long sputtering time is needed to obtain reproducible results when changing between different coverages, i.e. to clarify whether diffusion of oxygen into the bulk plays arole. 5.5 The VUV light source of high-order harmonics The VUV light source of high-order harmonics to be used in the fs VUV CLS setup has been optimized and characterized in order to demonstrate that the 63rd harmonic (96.4eV) of a 810 nm, 100 fs pulsed Ti:Sapphire laser could be generated. In addition, the reflectivities of a collection of homemade multilayer mirrors optimized for this photon energy have been tested.
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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 64 and 65: 54 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
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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VUV Spectroscopy of Atoms, Molecules and Surfaces

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