[en] Supperradiance, the idea that the spontaneous emission rate of an assembly of atoms (or molecules) can be much greater than that of the same number of isolated atoms, has been the subject of much interest since it was originally proposed by DICKE in 1954. In the process of superradiant emission the atoms are coupled together by their common radiation field, and so decay cooperatively. The intensity emitted by N atoms in this case is proportional to N² instead of N, as in ordinary emission. Thus, superadiance is a fundamental effect. (orig.)

[en] In this chapter, we are dealing with the processes, where transitions between discrete levels of atoms are produced by the siumultaneous absorption of n photons. They are resonant in the sense that the sum of the energies of the n photons is equal to the energy difference between the two concerned levels. But we exclude the case where an intermediate level would be resonantly excited during the process (its energy difference from the ground state beeing equal to the sum of the energies of n-1 photons, or n-2 photons, ...). Many experiments have been done with two such successive resonant processes: these involve, for example, the well-known stepwise excitation or the three-level resonance technique. Nevertheless, the detailed study of these two successive processes raises quite different problems; and we restrict our topic to resonant n-photon transitions without a resonant intermediate step. (orig./WL)

[en] In this chapter we review the theoretical tools and concepts needed for a first-principles quantum-mechanical study of the coherent laser excitation of polyatomic molecules under collision-free conditions, and illustrate these methods with selected numerical results. (orig./WL)

[en] Such well-known techniques as radiation echo, the method of quantum beats, and superradiance are widely used in studying relaxation processes both in gases and in condensed media. These techniques are based on coherent processes. The advent of lasers extended extremely the potentialities of these methods in the optical band. Over the last years new spectroscopic methods were developed in optics on the basis of narrow nonlinear optical resonances. The methods proved to be very useful in examining low-pressure gases as they permitted obtaining Doppler free resonances with a homogeneous width. This increased to a great extent a spectroscopic resolution in the optical band and simultaneously permitted studies into relaxation phenomena. A lot of methods to obtain Doppler free narrow resonances have been developed in optics by now. The following methods are widely used in nonlinear laser spectroscopy: 1) Method of saturated absorption. 2) Two-photon resonances. 3) Saturation resonances of stimulated Raman scattering. 4) Method of separated optical fields. (orig./WL)

[en] The advent of a coherent light source, the laser, marks the beginning of coherent experiments in the optical part of the electromagnetic spectrum. Numerous coherent phenomena have been elucidated in recent years, some of which are discussed in various chapters of this volume. Most of these experiments were performed with metal vapors or molecular gases at low pressure where the dephasing times are long, of the order of 10^-9 s or more. In these cases, commercial electronic detection systems may be used even for the study of transient processes. In the condensed phases at room temperature fast interactions occur between the closely packed molecules. It is not surprising that coherently excited systems lose the phase relation quite rapidly at 300 K. In fact, new experimental methods working in the time domain of picoseconds (10^-12 s) were required to observe directly the vibrational dephasing time T₂ of molecules in liquids and solids. (orig.)