VUV Spectroscopy of Atoms, Molecules and Surfaces

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82 Chapter 5. Femtosecond VUV core-level spectroscopy ... the technique of pico- or femtosecond X-ray diffraction, using X-rays emitted from fs laser-produced plasmas or obtained from ”sliced” or time-gated synchrotron-radiation pulses (cf. chapter 1.4). The non-thermal melting process is characterized by a disordering of the crystal lattice accomplished by the sudden excitation of a large (∼10 %) fraction of the electrons from the valence band to the conduction band of the semiconductor. The lattice instability occurs before a significant amount of the electronic energy has been transferred to the lattice by electron-phonon interactions and is thus purely electronic in nature. The disordering has been observed for different laser fluences encompassing the damage threshold [19, 20, 22], below which it is found to be reversible and accompanied by a band-gap collapse for fluences near the damage threshold [23]. 5.2 Laser-induced desorption The field of photo-induced desorption of molecules from surfaces using pulsed laser light of nanosecond (ns) or femtosecond (fs) duration has undergone a rapid development within the last ten years as reflected by the large amount of review literature available on the subject [24, 25, 26, 27, 28, 29, 30, 31, 32, 33]. A chemical reaction taking place at a surface can be very different from that of the gas-phase, since the molecules are confined to aligned geometries and their chemical properties are modified by the presence of the surface [29]. In addition, a non-insulating surface acts as a heat-reservoir with an almost infinte number of degrees of freedom, interacting with the adsorbate and representing a challenge to theorists. Since the heating- or exciting laser light penetrates several atomic layers into the substrate supporting the adsorbate layer, the desorption process is much more likely the result of an adsorbate-substrate coupling than a direct laser-adsorbate interaction. The emphasis of the present chapter is on the laser-induced desorption of diatomic molecules from metal surfaces, which are the systems that have been most intensively investigated with ns–fs laser pulses. In particular, the interest has concentrated on the catalytically- and theoretically relevant systems of CO, NO or O2 molecules adsorbed on Cu, Ru, Pd or Pt surfaces. The focus of the following discussion will therefore mainly be on the information obtained from previous all-optical studies of such systems, representing a good starting point for the application of the new technique of fs VUV core-level spectroscopy. In most of the all-optical approaches the desorbed molecules have been detected in a mass spectrometer [34, 35] or by the technique of Resonantly Enhanced Multi-Photon Ionization (REMPI) [36, 37, 38]. In this manner information was obtained about the translational-, vibrational- and rotational distributions of the desorbed molecules and the desorption prob-
5.2 Laser-induced desorption 83 ability as a function of wavelength and absorbed fluence. With fs pulses pump-probe experiments have, in addition, become feasible, measuring the desorption yield [39], the reflected probe light [40] or its second harmonic [41] as a function of the pump-probe delay [39]. Before proceeding to a discussion of the results obtained from these measurements it is necessary to consider the mechanisms by which a molecule may be bound to a surface. 5.2.1 The adsorbate-substrate bond The unperturbed adsorbate-substrate system is usually described by a potential-energy curve (PEC), evaluated as a function of the distance of the adsorbate from the surface and exhibiting a minimum at a certain equilibrium distance. The adsorbate may be weakly bound by a van der Waals attraction in which case it is said to be physisorbed and undergoes only a slight distortion of the electronic structure [42]. This type of bonding is usually encountered for noble-gas adsorbates and/or substrates but is possible for all adsorbate-substrate systems if the temperature is sufficiently low that the adsorbate is unable to surpass the energy barrier to a more stable chemisorption state. The activation energy for desorption is at most a few tens of an eV and adsorption of more than one monolayer is possible. At higher temperatures and/or for more reactive systems, the electronic structure of the adsorbate molecule is significantly distorted and it may dissociate to form new bonds with the substrate. The adsorbate is then said to be chemisorbed and the activation energy for desorption may be significantly increased compared with the physisorbed case. The different types of bonding encountered in this case are shown in figure 5.1, illustrating the possibilities of (a) molecular adsorption, (b) activated and (c) non-activated dissociative adsorption. In the latter case the incoming molecule will most likely dissociate upon adsorption but there exists a (small) possibility that it looses some energy before dissociating and is caught in the molecular well. For a physisorbed molecule the bonding will only take place in the outer, molecular, potential well as dictated by figure 5.1 (a). The ground-state PEC of the adsorbate-substrate complex is the common starting point for the discussion of the different mechanisms by which the molecule may desorb when the system is exposed to laser light. Conventionally, a distinction is made between the processes of direct- and indirect desorption as will be outlined in the following [43]. 5.2.2 Direct laser-induced desorption In the direct desorption process the wavelength of the laser light is resonant with a vibrational transition of the adsorbate or an electronic transition of the adsorbate-substrate complex. In the vibrational case, desorption occurs
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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
Page 42 and 43: 32 Negative ions feel the repulsion
Page 44 and 45: 34 Negative ions Figure 2.1: Adiaba
Page 46 and 47: 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
Page 140 and 141: 130 Chapter 6. Two-colour pump-prob
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132 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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