VUV Spectroscopy of Atoms, Molecules and Surfaces

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112 Chapter 5. Femtosecond VUV core-level spectroscopy ... Intensity (arb. units) Intensity (arb. units) 3 1 1.5 1.0 0.5 5 H61 85 90 95 100 Photon energy (eV) Figure 5.12: (a) A survey of the harmonic spectrum in the vicinity of the 63rd order before (grey) and after (black) reflection from a multilayer mirror. (b) The multilayermirror spectrum from (a) together with theoretical predictions for the reflectivity for Mo/Si layer ratios of 0.2 (grey) and 0.25 (black), respectively, assuming a 7.3 nm bilayer thickness. for the reflectivity for R = 0.2 (grey) and 0.25 (black), respectively, assuming a bilayer thickness of 7.3 nm. The theoretical curves, exhibiting peak reflectivities of 65 % and 70 %, respectively, have been scaled to fit the height of the measured spectrum which has been calibrated to the actual wavelength of the 61st harmonic. The bandwidth is seen to increase by ∼0.5 eV towards higher photon energies when going from R =0.25toR = 0.2, in principle leaving space for an adjacent harmonic order below the envelope. (a) (b)
5.6 Future prospects 113 5.6 Future prospects Following the completion of the preparations described in the preceeding three sections the next step towards fs VUV core-level spectroscopy is the test of a newly constructed electron spectrometer for kinetic-energy analysis of the core-ionized photoelectrons. Due to the modest photon flux expected from the VUV light source (see chapter 1), the electron spectrometer should have the highest possible collection efficiency, favouring a parabolic-mirror time-of-flight spectrometer. This spectrometer consists of a 1 m long flight tube and a pair of closely spaced paraboloid-shaped grids made of thin metalwire mesh [142]. The inner- and outer grids are at ground- and negative potentials, respectively, such that photoelectrons emitted from the (approximately common) focal point of the paraboloids will be reflected into a parallel beam directed towards the flight tube. By this method all electrons emitted within ∼ π steradians are collected and recorded by an MCP detector terminating the flight tube. A schematic of the electron spectrometer mounted on the UHV target chamber is shown in figure 5.13 as it is visualized for the future setup including the fs VUV light source. The paraboloids are made of wowen stainless-steal mesh with the highest available wire density and transmission (100×100 wires/inch and 81 %, respectively, from Unique Wire), stretched into paraboloidal shape and stiffened with a gold coating. A focal length of 10 mm of the paraboloids has been found to fit the diameter of the flight tube, leaving space enough for the sample holder which is attached to a manipulator extending from the top of the UHV chamber. The spacing between the grids should be small compared to the curvature of the grids in order to minimize the velocity dispersion of the photoelectrons [142], and is 1 mm in the present case. The paraboloids are mounted on a manipulator which can be retracted in order to ease the rotation of the sample towards the sputter gun. The time-of-flight tube is shielded with µ metal to avoid eventual magnetic field-induced deflection of the electrons, which will be retarded to a few eV and slightly focused by an electrostatic lens system positioned at the entrance to the flight tube. The retardation improves the energy resolution of the spectrometer and is maintained along the tube by means of a grid fastened to the hindmost of the three circularly apertured plates constituting the lens system. The electron spectrometer has very recently been mounted on the UHV chamber and will be tested in the near future—initially with 266 nm laser light incident on an Al surface and later with the 63rd harmonic. Assuming ∼10 4 VUV photons/pulse incident on the surface, an electron mean-escape depth to VUV absorption-depth ratio of 1 to ∼25 (Al at 96 eV) and a quantum efficiency of 100 %, a count rate of ∼2×10 3 photoelectrons per second (10 Hz) is expected which should provide
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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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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.
Page 158 and 159: 148 The following three chapters ar
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