VUV Spectroscopy of Atoms, Molecules and Surfaces

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88 Chapter 5. Femtosecond VUV core-level spectroscopy ... approach it is implicitly assumed that the electrons can be designated a temperature which may not be justified within the first ps after the presence of the desorbing laser pulse [46]. Since the electron temperature depends on the pulse duration and the absorbed fluence but not the wavelength, the wavelength dependence of the desorption yield observed by the group of Mazur has been attributed to the influence of non-thermalized substrate electrons [35]. Attempts have been made to deduce some of the time scales involved in the electron-mediated desorption proces by means of pump-probe measurements. Most of these have applied a two-pulse correlation scheme where the desorption yield is monitored as a function of pump-probe delay [39, 47, 51, 63, 64]. These measurements have revealed a peak with a ∼1 ps width (FWHM) on top of a broad plateau with a decay time of ∼100 ps. The ∼100 ps decay time has been attributed to equilibriation of the laserheated region with the ambient substrate [51] but the assignment of the ∼1ps spike is less clear. Bonn et al. have performed two-pulse correlation measurements for CO and CO2, generated from the oxidation of CO, on Ru(0001) yielding correlation times of 20 ps and 3 ps, respectively [47]. These were taken as evidence for phonon- and electron-mediated processes, respectively, with reference to the much longer cooling time of the lattice compared with the electron gas. The short-lived component was attributed to the electronmediated mechanism on account of a comparison with the sub-picosecond electron-phonon coupling time which determines the cooling time of the hotelectron gas [39, 47]. This was supported by the fact that the energy barrier to CO desorption is significantly lower than that for CO2 formation, the latter being given by the activation energy for breaking of the Ru–O bond of the dissociatively adsorbed O atoms [47]. It remains unclear what the correlation time really means [35], i.e. with which physical proces it should be identified. An upper limit to the actual desorption time, i.e. the time required for the breakage of the adsorbatesubstrate bond, has been set to 325 fs from a measurement of the change in second-harmonic signal of the probe as a function of pump-probe delay by Prybyla et al. [41]. The second-harmonic signal depends on the degree of adsorbate coverage and thus allows a more unambiguous interpretation of the observed time scale. An even more direct interpretation will be allowed by the technique of fs VUV core-level spectroscopy where the size of a substrate core-level shift depends on its chemical surroundings, in this case the degree of adsorbate coverage.
5.3 The technique of core-level spectroscopy 89 5.3 The technique of core-level spectroscopy The most common techniques in use today for structural analysis of surfaces at the atomic level are Low-Energy Electron Diffraction (LEED) [65], Surface-Extended X-ray Absorption Fine Structure (SEXAFS) [66], X-ray Photo-Electron Diffraction (PED) [67], X-ray Core-Level Spectroscopy (CLS) [68, 69, 70, 71] and Scanning-Tunneling Microscopy (STM) [72]. Apart from STM, the surface sensitivity of these techniques is accomplished by the 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
Page 48 and 49: 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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138 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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