Laser Processing and Chemistry
Laser Processing and Chemistry gives an overview of the fundamentals and applications of laser-matter interactions, in particular with regard to laser material processing. Special attention is given to laser-induced physical and chemical processes at gas-
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Springer-Verlag Berlin Heidelberg GmbH
Physics and Astronomy
ONLINE LIBRARY
http://www.springer.de/phys/
Advanced Texts in Physics This program of advanced texts covers a broad spectrum of topics which are of current and emerging interest in physics. Each book provides a comprehensive and yet accessible introduction to a field at the forefront of modern research. As such, these texts are intended for senior undergraduate and graduate students at the MS and PhD level; however, research scientists seeking an introduction to particular areas of physics will also benefit from the titles in this collection.
Dieter Bauerle
laser Processing and Chemistry Third, Revised and Enlarged Edition With 314 Figures and 13 Tables
~ Springer
Professor Dr. Dieter Băuerle Institut fiir Angewandte Physik Johannes-Kepler- Universităt Linz Altenbergerstrasse 69 4040 Linz, Austria E-Mail: dieter. baeuerle 1 I 2 and I7,L> 2 I2 , where 7,b; are vibrational wave functions for the lower and upper
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2 Thermal, Photophysical, and Photochemical Processes
electronic states. If the excited electronic state is unstable (Fig. 2.3.1a), excitation results in direct dissociation within times of, typically, 10- 14 to 10- 13 s. Clearly, relaxation and energy transfer between gas-phase molecules is unlikely within such short times. If the excited electronic state is stable, dissociation only occurs for photon energies hv ~ E~ (Fig. 2.3.1b). However, in many cases dissociation of isolated molecules is even observed for hv ::;; E~ (Fig. 2.3.lc,d). This phenomenon is termed spontaneous predissociation. It is related to transitions from the initially excited electronic state to an unstable state (Fig. 2.3.1c) or to a stable electronic state whose dissociation energy is below the originally excited state (Fig. 2.3.1d). The final state can also be the electronic ground state itself; then, the molecule dissociates if hv ~ ED. Such intramolecular radiationless transitions result from the mixing of states near crossings of potential curves. They are therefore more common in polyatomic molecules than in diatomic molecules. The typical times for predissociation are between 10- 12 and 10- 6 s. Radiationless transitions are also termed in-
e)
COORDINATE
f)
Q-
Fig. 2.3.1a-f. Potential energy curves for the electronic ground state and excited states of molecules, showing different cases of optical excitation and dissociation. ED and E~ are dissociation energies. Vibrational energy levels are only indicated. Rotational levels are not shown at all. Cases (a) to (d) show single-photon excitations. (e) Coherent two-photon excitation. (f) Sequential two-photon excitation. The energies of photons in cases (e) and (f) are not necessarily equal
2.3 Selective Excitations of Molecules
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ternal conversion and intersystem crossing [Avouris et al. 1977; Bixon and Jortner 1968] or as Landau-Zener transitions [Levine and Bernstein 1987]. The main limitation of single-photon excitation/dissociation processes relevant to laser-chemical processing is the lack of
