Excitation of atoms and molecules

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

Electric dipole radiation is the most important component involved in normal excitation of atoms and molecules. Ttu electric dipole operator has the form TejXf where e is the electronic charge in esu and xt is the displacement vector for the jth electron in the oscillating electromagnetic field. 

The ionization and/or excitation of atoms and molecules when the energies of nuclear particles are absorbed in matter is the basis for the detection of individual particles. Macroscopic collective effects, such as chemical changes and heat evolution, can also be used. The most important of the latter have been described before because of their importance for dose measurements (e.g. the blackening of photographic films and other chemical reactions, excitation of crystals (thermoluminescence), and heat evolved in calorimeters Ch. 7). 

Electronic Excitation of Atoms and Molecules by Electron Impact... 

The powerful VUV generated by resonant frequency mixing is suited for applications which require very intense laser radiation like the multiphoton excitation of atoms and molecules, photodissociation studies of molecules or plasma diagnostics. 

This chapter provides an introduction to different spectroscopic techniques that are based either on the coherent excitation of atoms and molecules or on the coherent superposition of light scattered by molecules and small particles. The coherent excitation establishes definite phase relations between the amplitudes of the atomic or molecular wave functions this, in turn, determines the total amplitudes of the emitted, scattered, or absorbed radiation. 

For many problems in atomic, molecular, and solid-state physics intense sources of tunable X-rays are required. Examples are inner-shell excitation of atoms and molecules or spectroscopy of multiply charged ions. Until now, these demands could only partly be met by X-ray tubes or by synchrotron radiation. The development of lasers in the spectral range below 100 nm is therefore of great interest. 

The estimated maximum probability of Hg dissociation is P 0.1 [295]. It can be concluded from the various electron energies measui ed that the excitation function of the level has a maximum at 8.8 eV, close to the threshold of excitation. This corresponds to the most probable transition according to the Franck-Condon principle (Fig. 39). The threshold of molecular nitrogen dissociation has been found [335] to be 9.6 i 0.05 eV, a value virtually coinciding with the Ng dissociation energy (9.76 eV). (Concerning electron impact excitation of atoms and molecules see e.g. [181, 441].)... 

Ackerhalt, J. R., and Eberly, J. H. (1976). Coherence versus incoherence in stepwise laser excitation of atoms and molecules. Physical Review A, 14, 1705-1710. 

---