Spectroscopy of Collision Processes

The two main sources of information about atomic and molecular structure and interatomic interactions are provided by spectroscopic measurements and by the investigation of elastic, inelastic, or reactive collision processes. For a long time these two branches of experimental research developed along separate lines without a strong mutual interaction. The main contributions of classical spectroscopy to the study of collision processes have been the investigations of collision-induced spectral line broadening and line shifts (Sect. 3.3). 

The situation has changed considerably since lasers were introduced to this field. In fact, laser spectroscopy has already become a powerful tool for studying various kinds of collision processes in more detail. The different spectroscopic techniques presented in this chapter illustrate the wide range of laser applications in collision physics. They provide a better knowledge of the interaction potentials and of the different channels for energy transfer in atomic and molecular collisions, and they give information that often cannot be adequately obtained from classical scattering experiments without lasers. 

Some techniques of laser spectroscopy, such as the method of separated fields optical Ramsey fringes. Sect. 14.4), coherent transient spectroscopy (Sect. 12.4), or polarization spectroscopy (Sect. 7.4) allow one to distinguish between phase-changing, velocity-changing, or orientation-changing collisions. 

The high time resolution that is achievable with pulsed or mode-locked lasers (Chap. 11) opens the possibility for studying the dynamics of collision processes and relaxation phenomena. The interesting questions of how and how fast the excitation energy that is selectively pumped into a polyatomic molecule by absorption of laser photons, is redistributed among the various degrees of freedom by intermolecular or intramolecular energy transfer can be addressed by femtosecond laser spectroscopy. 

One of the attractive goals of laser spectroscopy of reactive collision processes is the basic understanding of chemical reactions. The fundamental 

The most detailed information on the collision process can be obtained from laser spectroscopy of crossed-beam experiments, where the initial quantum states of the collision partners before the collision are marked, and the scattering angle as well as the internal energy of the reactants are measured. In such an ideal scattering experiment all relevant parameters are known (Sect. 13.6). 

One of the attractive goals of laser spectroscopy of reactive collision processes is the basic understanding of chemical reactions. The fundamental question in laser chemistry of how the excitation energy of the reactants influences the reaction probability and the internal state distribution of the reaction products can be at least partly answered by detailed laser spectroscopic investigations. Section 12.4 treats some experimental techniques in this field. 

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Several examples illustrate the advantages of molecular beams for spectroscopic investigation. The wide, new field of laser spectroscopy of collision processes in crossed molecular beams is discussed in Chap. 8. 

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