VUV Spectroscopy of Atoms, Molecules and Surfaces

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116 Chapter 5. Femtosecond VUV core-level spectroscopy ... spectrum of the surface for evidence of laser-assisted photoionization (LAPI). As stated in section 1.3 of chapter 1, LAPI may be recognized in the kineticenergy spectrum as side-peaks, separated from the core-level peak by an integral multiple of the photon energy of the pump. These peaks are the result of the stimulated emission or -absorption of one or more laser photons taking place during the VUV photoionization process. LAPI has so far only been studied with gaseous media [144, 145], but could also be worth a study at the surface. Denoting by σn the differential cross section for coupling of an ionized electron with momentum �p with n optical photons from the laser field with frequency ωl and assuming that � n σn = σ◦ with σ◦ being the differential cross section in the absence of the laser field, the probability for coupling with n photons is given by [145] � � Pn = σn � n σn = σn 2 = Jn σ◦ −e � A◦l · �p mc�ω1 , −Up 2�ω1 . (5.2) Here, Jn is a generalized Bessel function, � A◦l the amplitude of the vector potential of the laser field, Up the ponderomotive energy of a free electron in the laser field, e the elementary charge and m the electronic mass. In the above it has been assumed that �p is independent of n, equivalenttoωVUV ≫ ωl, whereωVUV is the VUV frequency. This expression has been derived by Glover et al. for the 1s state of the hydrogenic atom [145] but can be shown to be valid for any initial state which can be written in a basis of products of single-electron orbitals [146]. Pn is seen to depend on the angle between �A◦l and �p, beingmaximumfor� A◦l � �p (n �= 0). Since photoelectrons are preferentially emitted in the direction of polarization of the VUV light, � AVUV, the total amount of electrons in the sidepeaks is maximum for � A◦l � � AVUV, and emitted in that direction. The probability Pn of generating sidepeaks increases with intensity since A◦l ∝ √ I and Up ∝ I and VUV frequency since p ∝ ω2, withIbeing the laser peak intensity. When studying LAPI at surfaces, the intensity must be chosen with the damage threshold of the surface in mind. For Al the damage intensity for 100 fs pulses is on the order of ∼1012 W/cm2 (100 mJ/cm2 fluence [147]), yielding coupling efficiencies of P0 = 0.19, P1 = 0.33, P−1 = 0.27, P2 =0.12andP−2 = 0.07 for 100 eV photons. At least the first-order sidepeaks should be clearly visible in the Al 2p core-level spectrum even for fluences well below the damage threshold. Apart from the O/Al(111) system there are various other potential candidates for surface dynamical studies using the fs VUV CLS setup. Since most adsorbate-substrate systems should be expected to be amenable to laserinduced, substrate-mediated desorption, the limitations to the fs VUV CLS technique are imposed by the deviations of the actual kinetic energies of the
5.6 Future prospects 117 core-ionized photoelectrons from the optimum ∼50 eV. Since the growth of the inelastic mean-free path is faster towards lower kinetic energies when going away from this value (staying below 10 ˚A in the 20–250 eV region), one should probably concentrate on systems with more loosely bound corelevels than has been done here. The 4f core-levels of the metals to the left of Pt in the third transition-metal group, i.e. Ta [148], W [149], Re [150], Os, and Ir [151] are all in the 20–60 eV range—increasing from the left of the periodic table—and have been clearly resolved with ≤100 eV photons. These systems could like-wise act as important model systems for desorption dynamics, although catalytically not quite as relevant as Pt. The semiconductor elements Ga, Ge, As, Se, In and Sb have 3d or 4d corelevels in the 20–50 eV range which are known to behave properly for ≤100 eV incident photons [152, 153, 154, 155]. In addition, the photo-induced reaction dynamics of adsorbates on semiconductors—in particular Si, Ge and GaAs—is technologically highly relevant and has been the subject of numerous studies using either continuous-wave light [156, 157], synchrotron radiation [158] or ns laser pulses [159, 160, 161]. These studies have included the passivating oxidation-, nitridation- and sulfidication reactions using O2 [157], NO [156], NH3 [161] and H2S [158] adsorbates, and the photo-assisted etching by adsorbed halogen molecules (Cl2) [160]. All of these systems have been the subjects of core-level spectroscopic studies and pronounced chemical shifts induced by the presence of the adsorbate have been observed (see, e.g., [162, 163, 164, 165]). Departing from the dynamics associated with the desorption or reaction of adsorbed species, one could follow the foot steps of the fs X-ray diffraction experiments mentioned in the introduction and study the evolution in a semiconductor core-level spectrum following the excitation of the bare surface with a visible pump pulse. The use of a completely different technique might shed some new light on, e.g., the non-thermal melting process observed in InSb [19, 21], Ge [20] and GaAs [22, 23], and the nature of the band-structure collapse (equivalent to a semiconductor-to-metal transition) which has been observed in GaAs but apparently not in InSb [166]. Another experiment could address the amorphous-to-crystalline phase-transition in GeSb of relevance for optical data storage [167] or a non-specified phase transition recently observed in fs laser-excited Al in an all-optical reflectivity measurement [168].
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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
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62 Lifetimes of molecular negative
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64 Lifetimes of molecular negative
Page 76 and 77: 66 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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