VUV Spectroscopy of Atoms, Molecules and Surfaces

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84 Chapter 5. Femtosecond VUV core-level spectroscopy ... Figure 5.1: Adsorbate-substrate potential-energy curves corresponding to (a) molecular adsorption, (b) activated dissociative adsorption and (c) unactivated dissociative adsorption. From [42]. when enough energy has been transferred from the intra-molecular vibrational mode to the adsorbate-substrate bond by anharmonic couplings. The direct resonant excitation of an adsorbate-substrate vibration is considered a less likely desorption channel, the required ladder-climbing of the vibrational levels being hindered by the anharmonicity of the PEC [28]. In the electronic case, the excited state may be bound, representing, e.g., the binding of an adsorbate positive- or negative ion to its image charge, or it may be repulsive, corresponding to the promotion of an electron from a bonding to an anti-bonding orbital [24]. In either case the excitation is assumed to proceed instantaneously via a Frank-Condon transition, followed by the down-hill motion of the adsorbate towards the equilibrium distance of the excited-state PEC. Eventually the excited state will be quenched and the adsorbate brought back to the ground-state PEC by another Frank-Condon transition, as shown in figure 5.2. If the kinetic energy E ′ k gained by the adsorbate from its motion on the excited-state PEC exceeds the binding energy of the ground state evaluated at the same adsorbate-substrate distance, the adsorbate will be released with a positive kinetic energy Ek. An ionic excited state can be quenched by the tunneling of an electron to or from a substrate state, corresponding to a lifetime in the 1–100 fs range. The shortest lifetime is obtained if a substrate state happens to be resonant with the adsorbate state [28]. A neutral excited state can be quenched via charge exchange or electric-field coupling with the substrate, resulting in a lifetime of a few fem-
5.2 Laser-induced desorption 85 Figure 5.2: A schematic of the mechanism of desorption induced by electronic transitions. M and A denote the surface and adsorbate respectively. The excited-state potential-energy curve may (but need not) be ionic as indicated in the figure. From [24]. toseconds which is comparable to the time-scale required for bond-breaking on a repulsive PEC [28]. The electronic process outlined above is, obviously, called Desorption Induced by Electronic Transitions (DIET) and its description in terms of the excursion to an excited-state PEC is named the MGR-model after Menzel, Gomer and Redhead. It should be noted that the one-dimensionality of the MGR-model not always applies, as illustrated by the example of ammonium where the cross section for NH3 desorption has been found to be four times larger than that of ND3 [44]. From the MGR-model applied to the adsorbate-substrate coordinate, the desorption probability would be expected to be largest for the lightest particle which is accelerated faster on the excited-state PEC, becoming more energetic before the quenching occurs. The measured value of the cross section ratio was, however, much larger than predicted from the MGR-model and could only be accounted for by an additional desorption coordinate corresponding to an intra-molecular vibrational motion of the NH3 molecule. This has become known as the inversionor umbrella vibration and gives rise to a fascinating desorption mechanism: the three H ”legs” of the NH3 molecule, initially pointing away from the surface, are pushed towards the surface, implying an inversion of the molecular
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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
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
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134 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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